Designing effective Tunnel Ventilation System (TVS) for Underground Metro stations in densely populated urban environments presents multiple challenges which includes space optimisation, energy efficient design, lifecycle cost and fire life safety standard compliance which actually necessitates innovative Tunnel Ventilation system solutions. This paper presents a case study of the Chennai Metro Phase 2 tunnel ventilation system design, showcasing an innovative approach that eliminates the Under-Platform Exhaust (UPE) system which is traditionally employed in metro stations.
Conventional trackway exhaust configurations utilize both the Over-Track Exhaust (OTE) and the UPE system to capture heat generated by stationary trains. The OTE captures heat generated by rooftop air-conditioners and overhead equipment at high level, while the UPE extracts heat generated beneath the train due to braking and propulsion losses.
In Chennai Metro Phase 2, the UPE system has been eliminated owing to the adoption of modern rolling stock equipped with regenerative braking and the absence of onboard brake resistors. Regenerative braking allows the kinetic energy during train deceleration to be converted into electrical energy and fed back into the overhead traction supply, effectively removing under-car heat sources during station dwell periods. Furthermore, the limited clearance between the train underframe and platform ducting restricts effective air extraction, rendering the UPE system of negligible benefit.
The space originally designated for the UPE duct has been reconfigured as an air transfer path, enabling air to be pushed or pulled across the station length using fans, dampers, and nozzles. This approach facilitates a single-sided Tunnel Ventilation Room (TVR) arrangement, thereby requiring only two tunnel ventilation fans per station. A series of Subway Environment Simulation (SES) and Computational Fluid Dynamics (CFD) analyses were conducted to evaluate the performance of the OTE-only system under normal, congestion, and emergency modes of operation as per NFPA 130 & NFPA 502. The results confirm that the OTE configuration satisfies all design criteria without necessitating a UPE system.
This case study demonstrates elimination of UPE for a cost-effective, energy-efficient, and spatially optimized tunnel ventilation solution utilising software tools, system integration and evolving train technology for modern and next generation metro applications.

Road tunnels use ventilation systems to reduce the pollutant levels generated by vehicles inside the tunnel; to exhaust vitiated air to atmosphere; and to manage and control smoke during a fire event. To perform all the functions required during normal operations, and to have the flexibility to cater for all traffic flows and conditions, the fans selected must have sufficient capacity and pressure ratings. Typically, the same fans are used during fire (emergency) operations. This adds another constraint to the fans selected as, in most instances, the hot smoke flows through the fans necessitating that the fans be adequately fire rated. Meeting these disparate criteria add significant complexity to the ventilation plant arrangement and the fan construction, limiting its efficiency and translating into high capital and operational costs.

This paper describes a method to overcome these disadvantages by use of a Venturi nozzle located within the ventilation plantroom. In the proposed arrangement, the fan plant is located above ground, away from the confines of the tunnel. Fresh air is drawn in from the external environment is used, rather than tunnel air, with the primary function of facilitating the Venturi effect as a means of tunnel ventilation and smoke exhaust. With this arrangement, the requirement for the fans to be fire-rated is removed and fan sizes can be reduced, resulting in capital and operational cost savings. Capital costs may also be reduced where the fan plant is to be located below ground due to reduced excavation costs.

The objective of this experimental work is to investigate the behavior of a fire in a tunnel under construction. To this end, a small-scale tunnel, 6 m long with a 40 × 40 cm² cross-section, was built, corresponding to a 1/22nd scale model of a real tunnel. The setup (shown in fig 1) is mounted on an adjustable frame that allows the slope to be varied, and one end of the tunnel is sealed to reproduce the conditions of a tunnel during the construction phase. The experimental analysis primarily focuses on characterizing the smoke layer in terms of thickness, velocity, and mean temperature. A detailed study based on the Ellison & Turner model [1] is used to estimate the entrainment coefficient (E), which quantifies the mixing between smoke and ambient air as a function of the local Richardson number Ri, an indicator of flow stability. Our results show that the entrainment coefficient follows the scaling: E∝ Ri^(-1). In parallel with this experimental work, numerical simulations were performed using the FDS computational code. The objective is to investigate the smoke flow in greater depth in order to determine its characteristics with improved accuracy. Stack effect, buoyancy, smoke-air mixing and related impacts on smoke flow (Fig. 2), will be analyzed in details in the full paper.

Underground metro station smoke control design confronts engineers with an early-phase challenge: establishing the extraction airflow rates that govern fan sizing, ductwork configuration, spatial allocation, and electrical capacity while station geometry remains undefined and subject to frequent modification. Engineers require these specifications rapidly as designs iterate, yet Computational Fluid Dynamics (CFD) analysis demands substantial time and computational resources, creating a fundamental mismatch between design timeline constraints and available analytical tools.
This study addresses this gap by developing a semi-empirical model enabling rapid preliminary dimensioning of smoke confinement systems. Through systematic CFD parametric investigation spanning three platform lengths (16 m, 32 m, and 64 m), all equipped with full-height platform screen doors, we characterized smoke confinement behavior across fire heat release rates ranging from 0.1 kW to 10 MW. The research establishes the quantitative relationships between confinement airflows and design variables: platform geometry, ceiling height, containment screen dimensions and air inlet surface area.
Numerical simulations revealed that smoke confinement success depends on maintaining critical velocity at air inlet interfaces beneath containment screens. Building upon this physical insight and dimensional analysis, we derived a semi-empirical correlation that predicts required extraction flows as a function of preliminarily available design parameters. Validation against sixteen targeted configurations, including realistic geometric layouts with staircases and varied opening distributions, demonstrates within 10% prediction accuracy relative to full CFD analysis.
This methodology reduces early-phase smoke control dimensioning from computationally intensive multi-day CFD simulations to rapid analytical calculations. While not replacing detailed CFD analysis for final design verification, it provides engineers with a robust pre-dimensioning capability that reduces design uncertainty, accelerates project timelines, and establishes a framework for smoke control system sizing based on fundamental building geometry and fire scenario parameters.

A large majority of studies of micro-pressure waves (MPWs) emitted from railway tunnel portals focus exclusively on wavefronts generated when a train nose enters the opposite end of the tunnel. The subsequent evolution of the wavefront along the tunnel is simulated using suitable theoretical methods, and the MPW amplitudes are inferred from the predicted amplitude and shape of its leading step as it approaches the exit. This is an entirely satisfactory approach when it is undertaken by 3D analyses, using sufficiently detailed numerical grid structures and sufficiently detailed representations of the tunnel geometry. However, meeting these criteria can be highly onerous, especially in the case of long tunnels or in tunnels with complex geometrical features. Both of these complications exist for proposed long tunnels with continuous arrays of air chambers – beneath walkways, for instance – and this inevitably reduces confidence in predicted behaviour regardless of its actual validity.

To assist in the assessment of predictions for such cases, there is obvious merit in having prior knowledge of what outcomes are physically possible, regardless of how they have evolved. The author recently published a theoretical method of achieving this by starting at the leading tip of the wavefront and working backwards along it. Since this method begins at the tip, where the pressure amplitude is nominally infinitesimal, an infinity of possible outcomes exists. However, it is known that the wavefronts evolve towards an asymptotic condition, and when this condition is imposed, only one possible outcome exists. The proposed paper will explore the dependence of this outcome on the geometry of the air chambers, thereby (i) guiding the selection of geometries to be studied in conventional analyses of wavefront propagation along complete tunnels, and (ii) providing benchmarks against which the outcomes of such analyses can be compared.

Ground Support Equipment (GSE) tunnels play an important role in airport operations by enabling efficient movement of service vehicles beneath airside infrastructure. Unlike conventional road tunnels, GSE tunnels present unique design challenges due to limited space, ventilation, multiple-entries and exits, and stringent safety requirements. This paper presents an integrated approach to GSE tunnel ventilation design, addressing allowable vertical clearance, cross-sectional layout, and pressurization requirements for smoke control.
The proposed approach focuses on three key aspects: (1) geometric design parameters, including allowable vertical clearance and cross-sectional layout to accommodate diverse GSE traffic; (2) ventilation and smoke control strategies that ensure compliance with international standards such as NFPA 502; and (3) emergency egress planning supported by performance-based fire safety analysis. Fire scenarios are evaluated using the Available Safe Egress Time (ASET) and Required Safe Egress Time (RSET) methodology to validate occupant safety under worst-case conditions. Pressurization concept is explored to maintain tenable conditions in escape routes and prevent smoke ingress into protected zones.
Preliminary ventilation sizing is conducted using one-dimensional analysis in IDA Tunnel, followed by three-dimensional CFD simulations to assess airflow distribution, smoke propagation, and pressurization performance. Evacuation modeling is performed using Pathfinder to quantify egress times and verify compliance with ASET/RSET benchmarks. The results demonstrate that these tunnels are inherently unique, presenting complex challenges for ventilation design. Unlike conventional underground spaces, their geometry and functional requirements often impose strict spatial limitations.
This study offers practical insights for designers, engineers and airport planners seeking to enhance operational efficiency and fire safety in constrained underground environments.

Road tunnels serve the public for long periods, typically close to a century. These are delivered through design & construction (D&C) projects that last between three to ten years (depending on length and complexity), with design assumptions and construction quality having a major impact on the Operations & Maintenance (O&M) needs. This impact often manifests as gap between the intended and real performance of the functional systems that make up the road tunnel.
This paper discusses such gaps with a focus on fire and life safety systems, given their role in managing health and safety risks for motorists, O&M staff and emergency services. The discussion centres on the validation of initial design assumptions—particularly those concerning the impact of single points of failure, theoretical equipment availability, and integrated system performance. When these are tested against real-world operational conditions, the results might diverge from the specified performance requirements, introducing safety and business continuity risks that in turn can lead to significant unforeseen expenditure.
Based on this discussion, a framework is proposed to better identify and validate key design assumptions for fire detection, fire suppression, and smoke management systems throughout the O&M phase. Rooted in process safety and systems engineering principles, the framework aims to enable iterative design changes that help reduce or close the gaps identified after construction. It is put forward to raise awareness among key stakeholders, particularly those involved in developing the reference design and resulting technical requirements for new projects.

The implementation of Platform Screen Doors (PSDs) in modern metro systems is increasingly adopted to enhance passenger safety and improve station climate control. However, the installation of PSDs significantly alters the aerodynamic and thermodynamic equilibrium of the tunnel-station network. This paper presents a comprehensive evaluation of the effects of PSDs on critical environmental parameters, including tunnel temperatures, air velocities, pressure transients, station heat loads, and ventilation fan requirements.
This study conducts a comparative analysis between two scenarios: with and without PSDs, applied to a specific planned metro network using a validated numerical simulation model SES (Subway Environment Simulation). The study focuses on the decoupling of the station environment from the tunnel environment.
The simulation results indicate that while PSDs significantly reduce the station air-conditioning heat load by isolating the platform from tunnel heat but necessitate revised tunnel cooling and ventilation strategies for the tunnel sections. The study quantifies the increase in tunnel ambient temperature and the corresponding adjustments required in Tunnel Ventilation System (TVS) fan sizing. Furthermore, the paper analyses pressure transients acting on the PSD structure and throughout the tunnel wayside equipment. The findings provide critical guidelines for optimizing Environmental Control System (ECS) designs, understanding behaviour of tunnel temperatures and pressure transients in enclosed tunnel networks.
Keywords: Platform Screen Doors (PSD), Tunnel Aerodynamics, Station Heat Load, Piston Effect, Environmental Control System (ECS), Metro Ventilation.

 Australia’s vehicle fleet is evolving with the gradual uptake of alternative fuel vehicles and the
establishment of the national New Vehicle Efficiency Standard in 2025. Furthermore, the New South
Wales and Federal Governments are investing billions of dollars in the transition to renewable energy.
All tunnels in Sydney, New South Wales, constructed after the M5 East tunnel (2001) are required
under their Environmental Conditions of Approval to have zero vehicle emissions from the tunnel
portal. Achieving portal emissions requires ongoing operation of energy-intensive, costly major
ventilation fans.
Both Brisbane and Melbourne have at least two tunnels each which commenced operations with zero
portal emissions but have successfully introduced portal emissions following a consultative process
with the state regulatory authorities, resulting in substantial environmental, sustainability and
commercial benefits.
The objective of this article is to:
1) Demonstrate technical feasibility and precedence for portal emissions in Australia and
overseas,
2) Quantify commercial and decarbonisation benefits of adopting portal emissions in Sydney
and,
3) Identify a regulatory pathway to enable the New South Wales Government to activate road
tunnel portal emissions, whilst meeting community expectations.
The research identified an annual cost saving of approximately $4,880,000 in electricity expenditure
for a single Sydney tunnel case study if nighttime portal emissions were to be adopted.
The research also outlined a complex regulatory pathway to implement portal emissions, with a
potential time frame exceeding three years. Opportunities to fast track the process are also identified.

Tunnel fires can severely impair the structural integrity of concrete linings through thermal degradation, explosive spalling, and thermally induced stresses. This study employs the finite element software SAFIR to conduct coupled thermal–mechanical analyses of steel fibre reinforced concrete (SFRC) tunnel linings subjected to realistic fire scenarios, with particular emphasis on the influence of surrounding ground conditions and lining thickness.

Transient heat transfer simulations first establish temperature distributions within the SFRC lining under standardized fire curve using the ISO 834. These thermal fields inform subsequent nonlinear mechanical analyses that account for temperature-dependent SFRC properties—enhanced tensile capacity and crack control from steel fibres, degradation of stiffness and strength at elevated temperatures and thermal expansion—together with initial ground-induced stresses and permanent loadings.

A comprehensive parametric investigation varies two key factors: (i) the stiffness of the surrounding rock mass, modelled via Winkler compressive-only springs to span weak to very strong rock conditions, and (ii) SFRC lining thickness. Results show that weaker ground permits greater inward deformation, elevating compressive stresses on the intrados and causing bending stresses to dominate over axial stresses during heating. Stiffer rock, by contrast, restrains thermal expansion more effectively, generating higher compressive stresses but markedly reduced bending moments. For thinner linings, the zone of thermal expansion is closer to the section centroid because the intrados fibres at high temperature lose compressive capacity. This promotes higher axial compressive stresses while reducing the eccentricity of thermal loading and thus lowering the overall bending moment.

The analyses highlight the critical interplay between fire exposure, SFRC material advantages, ground stiffness, and lining thickness. These findings support the development of refined, performance-based fire safety design guidelines for SFRC-lined tunnels in diverse geological settings, enabling optimisation of lining thickness, steel fibre dosage, and overall resilience while balancing structural performance, material efficiency, and construction costs.

Over the past 15 years, variable-speed controlled jet fans in road tunnels have gradually but steadily become popular in Japan, because of the benefits of energy saving in normal operation and improved safety in incident response. The energy saving principle detailed by Bopp [1] more than three decades ago [1] was expanded upon by Nakahori et al [2]. The proposed paper will present 3D simulations that give more information than those early 1D studies, enabling better understanding of the dependence of jet fan efficiencies on their speeds of rotation. The authors are not aware of any such previous 3D study even though various excellent studies have enhanced understanding of jet fan operation at full speed in demanding applications – e.g. Kato et al [3].

The paper is primarily concerned with energy saving and presents a series of 3D CFD analyses using the software CONVERGE. In the simulations, jet fans exist at both ends of a tunnel and are operated in opposing directions. At one end, a single fan operates at a fixed rotational speed, usually 100%. At the other end, two or more fans operate at reduced speed. Attention is focussed on cases in which the net flowrate midway between the two sets of fans is zero. In this condition, the opposing thrusts are equal and the required powers can be compared meaningfully. As shown by Kato et al [3], the performance of fans operating against imposed counter-forces can differ from that in the absence of such forces, so all of the fans are operating in a realistic environment in which conventional 1D analyses can be misleading. The paper will include comparisons of the inferred power requirements with corresponding predictions using 1D representations. Comparisons will also be made with measurements of energy consumption characteristics measured in full-scale tests in the past. Overall, the paper will strengthen understanding and will contribute to international recognition of the potential energy-saving benefits for tunnel owners and for the planet.

[1] R Bopp (1994) “Energy-optimized ventilation of road tunnels by speed control of jet fans”, Proc 1st int conf on Tunnel Control and Ventilation, ITC Ltd, 219-229.

[2] I Nakahori, T Ato, K Murakami, D Araki, T Kanatani, A Vardy (2009) “The use of inverter-driven jet-fans to reduce tunnel ventilation costs”, 13th ISAVVT, BHR Group, 69-80.

[3] N Kato, S Ito, A Mizuno, T Chihara & S Hashimoto (2019) “Boosting pressure generated by jet fans when operated against the longitudinal airflow in road tunnels, 18th ISAVFT, BHR Group, 499-511.

The aerodynamic and ventilation design of high-speed rail (HSR) tunnels requires a robust understanding of unsteady pressure phenomena, micro-pressure wave generation, piston-effect-driven airflow, and the overall performance of ventilation systems under both normal and emergency operating scenarios. As train speeds increase, these coupled aerothermal processes impose stringent demands on numerical modeling accuracy, operational safety, and passenger comfort. This paper presents a comprehensive verification and validation framework for assessing the applicability, strengths, and limitations of several widely used tunnel aerodynamics simulation tools—including IDA Tunnel, ThermoTun, Numsta, and STAR-CCM+—when deployed in the design of HSR systems.
The proposed methodology integrates analytical models with transient one-dimensional and fully three-dimensional computational dynamics (CFD) simulations to create a multi-resolution modeling environment. This approach enables efficient screening of design options while ensuring high-fidelity assessment of critical aerodynamic challenges such as compression-wave formation, transient pressure loading, and complex train–tunnel and train–train interactions. Particular emphasis is placed on evaluating the interplay between numerical fidelity, computational cost, and the practical requirements of largescale HSR infrastructure projects.
The paper further demonstrates the application of this integrated methodology to two HSR project studies: the Espoo–Salo high-speed line in Finland and the Czech Republic’s RS1 (rychlá spojení) fast connection corridor. Both projects involve multiple tunnels with varying lengths, cross-sections, and operational conditions. Assessments are conducted in accordance with RATO 18 and EN 14067-5 guidelines, with detailed analyses of train-induced pressure transients, ventilation performance, and passenger aural comfort.
Results from these studies are presented, including predicted pressure development inside tunnels and rolling stock, evaluation of aural comfort criteria, and analysis of micro-pressure wave emissions and potential sonic boom formation at tunnel portals.

 The main railway station in Luzern, located in central Switzerland, is one of the country’s major transport hubs, serving nearly 100,000 passengers per day. Currently operating as a terminal station, it represents a critical bottleneck in the regional and national rail network. To address this limitation and enhance long-term capacity and operational flexibility, a new cross-rail link is planned. At the core of this project is a newly constructed underground station featuring four platforms. One end of the station will connect to the 3.5 km “Dreilinden Tunnel,” which includes a 400 m underwater section beneath Lake Luzern, while the other end will interface with the 2 km “Neustadt Tunnel.” The project is presently in the development phase.
This paper presents the emergency ventilation design for the underground station and summarizes the conceptual, analytical, and numerical studies conducted to verify the viability of the proposed system. It discusses the measures required to meet defined safety objectives and identifies optimization potential within the ventilation and smoke-management strategy.
An additional aspect under investigation is the possibility of allowing freight trains to pass through the underground station. Such operations are uncommon in Switzerland due to elevated safety risks and significant construction and operational constraints. The paper outlines the quantitative risk-analysis framework applied to evaluate this scenario.
Overall, the study provides a real-world example of the challenges associated with designing an underground station connected to tunnels at both ends, incorporating large and interlinked mezzanine halls, direct openings to above-ground platforms, and the absence of platform screen doors—conditions that together present a demanding environment for achieving stringent safety goals.

 This study investigates performance of a cross-flow vertical axis wind turbine driven by piston wind generated by subway trains traveling through tunnels. Due to the high computational costs of three-dimensional Computational Fluid Dynamics (CFD) simulations, two-dimensional models are assessed as potential alternatives for preliminary design and performance evaluation. A dynamic mesh using layering technique is employed to accurately model the train’s movement within the tunnel. Simulations based on Unsteady Reynolds Averaged Navier Stokes Equations (URANS) are conducted to analyze the aerodynamic behavior, energy potential of the turbine and its possible effects on train drag. The computational results analyze furthermore the performance, accuracy, and limitations of two-dimensional approaches, aiming to identify a computationally efficient modelling strategy. The findings suggest that 2D modeling can offer valuable insights into turbine and train drag behavior with acceptable tradeoffs in accuracy.

High-speed trains moving through tunnels generate strong pressure pulses (both positive and negative) commonly known as the piston effect. The severity of these pressure loads depends mainly on train speed and direction, as well as the geometry of both the train and the tunnel. In longitudinal ventilation systems, jet fans installed inside the tunnel are directly exposed to these transient aerodynamic forces, which can induce uncontrolled free spinning of the fan impeller when the fan is idle. If left unprotected, such conditions may lead to mechanical failure due to excessive rotational speeds.
To mitigate this risk, jet fans can be equipped with circular butterfly dampers that close when the fan is not in operation. These dampers significantly reduce the airflow through the fan when a train is passing, which helps protect the impeller and motor from potential overspeed damage. While this configuration provides a practical solution for safeguarding fans in high-speed rail tunnels, it also introduces certain challenges, such as added aerodynamic resistance and increased maintenance requirements.
This paper explores the performance of jet fans with integrated butterfly dampers under various loading scenarios, based on both computational simulations and experimental testing. Key aspects evaluated include aerodynamic behavior, damper response time and the overall effectiveness of the fan – damper assembly.
The study contributes to a better understanding of how to integrate jet fans into high-speed railway tunnel ventilation systems to ensure reliable and resilient performance.

Underground transit systems present significant challenges related to thermal management, air pollution control, and energy-efficient ventilation due to confined spaces, high passenger density, frequent train operations, and limited natural air exchange. Heat generated from rolling stock, traction equipment, lighting, and occupants can lead to thermal discomfort if not effectively dissipated. In parallel, ingress of urban ambient pollution, tunnel dust, and internally generated contaminants such as particulate matter and carbon dioxide necessitates robust air quality management strategies. These issues are central to the scope of ISAVFT, where ventilation performance, airflow dynamics, and thermal behaviour of underground environments are critical research areas. National Capital Region Transport Corporation Ltd. (NCRTC) is developing India’s first Regional Rapid Transit System, the Namo Bharat Corridor between Delhi and Meerut, designed for a maximum speed of 180 km/h and an operating speed of 160 km/h. As part of this corridor, four underground stations viz. Anand Vihar, Meerut Central, Bhaisali, and Begumpul—have been designed with a strong emphasis on Indoor Environmental Quality (IEQ), integrating global best practices for passenger comfort, safety, and energy efficiency. This paper presents the integrated approach adopted by NCRTC to address thermal and pollution-related challenges in underground stations through advanced ventilation design,
continuous air quality monitoring, and intelligent control systems. Key parameters such as PM₂.₅, PM₁₀, CO₂, and ozone are continuously monitored, enabling demand-controlled ventilation through BMS-SCADA platforms. High-efficiency filtration and air treatment technologies are employed to enhance pollutant removal. CFD-based airflow and thermal simulations were used during the design stage to optimize air distribution, validate thermal comfort conditions as per ASHRAE 55, and minimize ventilation energy consumption. The study demonstrates how performance-based ventilation design can effectively balance air quality, thermal comfort, and energy efficiency, offering a scalable and replicable model for underground transit systems in dense urban environments.

In complex modern underground metro such as Doha Metro network, the tunnel ventilation system (TVS) should operate with precision and minimal intervention from human operators (automated) to ensure passenger safety and comfort during revenue service. This paper presents case study on the integration between TVS and Supervisory Control and Data Acquisition (SCADA) system emphasizing the reduction of manual operator intervention. One of the major challenges to human operator is the transitions of TVS between “Normal mode” to “Congestion mode” to “Maintenance mode” or “Emergency mode”, furthermore choosing the optimal evacuation route during tunnel evacuation is one of the key challenges to the operator hence this paper will highlight how automated logic can assists human operator into taking the appropriate decision under high pressure scenarios. In addition, this paper will illustrate the main interface points between SCADA system and TVS and the criteria to alternate between various TVS modes. The transition between mode can be fully automatic, semi-automatic or manual. Also, this paper addresses in a high level the interface between Signalling system and TVS which occurs through SCADA layer to ensure synchronized response along the network.

The paper presents an analysis of the operational data from the Luboń Mały road tunnel on the S7 expressway in Poland, collected throughout 2024. The study aims to evaluate the effectiveness of the tunnel’s ventilation and monitoring systems under varying traffic and environmental conditions. The analysis is based on a comprehensive dataset, including continuous measurements of pollutant concentrations; airflow velocity and direction within both tunnel tubes; traffic volume and vehicle classification; and external meteorological conditions at the portals.
The paper investigates the correlations between traffic intensity, external weather, and the resulting air quality inside the tunnel. It assesses the performance of the ventilation system in maintaining pollutant levels below safety thresholds, identifying potential patterns of pollutant accumulation during peak traffic or specific weather scenarios. Furthermore, the study explores the energy efficiency of the ventilation operation in relation to real-time demand. The findings provide valuable, data-driven insights for optimizing tunnel ventilation control strategies, enhancing traffic safety, and improving energy management. This case study demonstrates the significant potential of utilizing existing monitoring infrastructure for evidence-based performance assessment, contributing to the development of smarter and more sustainable tunnel operations.

The development of high-speed regional rail systems introduces complex challenges for tunnel ventilation design, particularly in underground environments characterized by high train speeds, stringent fire safety requirements, and increasing energy efficiency demands. The Namo Bharat corridor, India’s first Regional Rapid Transit System (RRTS), is being developed by the National Capital Region Transport Corporation Ltd. (NCRTC) between Delhi, Ghaziabad, and Meerut, with a maximum design speed of 180 km/h and an operating speed of 160 km/h. The corridor comprises two underground sections. The first underground section includes the Anand Vihar station. The second underground section accommodates three underground stations viz. Meerut Central, Bhaisali, and Begumpul—together representing India’s first implementation of a comprehensive tunnel and station ventilation system for high-speed regional rail operations in an underground environment. Tunnel ventilation for the Namo Bharat underground sections was designed using a
performance-based approach with tunnel diameter of 6.5 meter and considering a 20 MW design fire in the tunnel and 1 MW baggage fire in station areas. The system configuration was developed based on the criterion of one train occupying a single ventilation zone, ensuring effective smoke control, safe passenger evacuation, and reliable emergency response. The design follows international standards, including NFPA 130, with a strong focus on life safety and operational robustness. The ventilation design, ensuring that the air distribution and smoke control systems are optimized for the tunnel’s dimensions. A push–pull ventilation strategy was adopted using tunnel ventilation fans to control longitudinal airflow during emergency scenarios, enabling smoke confinement within the incident zone and preventing its migration to adjacent tunnel sections and stations. Distinct ventilation strategies were developed for normal ventilation, Night cooling ventilation, Congestion (with/without tunnel cooling) and emergency operating modes, optimizing thermal comfort and air quality during routine operations while enabling high-capacity smoke control during emergencies. One dimensional simulations and three dimensional Computational Fluid Dynamics (CFD) analyses were carried out to evaluate multiple fire locations, train positions, and operational scenarios. These simulations were used to optimize fan capacities, airflow directions, activation sequences, and control philosophies for both tunnel and station environments. Commissioning tests and system validations confirmed close correlation between simulated and actual system performance, validating the robustness of the adopted design methodology. This paper provides a scalable and replicable reference for tunnel ventilation design and validation in high-speed underground rail corridors in India and similar dense urban environments.

Tunnel ventilation design and analyses rely heavily on computational modelling to evaluate system performance and operational strategies under various scenarios. In practice, designers invest substantial time in constructing base models, generating parametric cases, validating input parameters, and post-processing simulation outputs. These workflows are typically manual, project-specific, and highly iterative, requiring careful handling of complex input structures and querying simulation output datasets. As a result, modelling tasks can become time-intensive, repetitive, and susceptible to human error. This paper presents a practical architecture for integrating large language model (LLM) assistance into engineering simulation workflows to support and streamline the modelling process. The proposed system enables designers to interact with simulation models using natural-language commands expressed in professional engineering terminology. User requests are translated into structured, validated operations that modify simulation input files or execute targeted post-processing tasks on model outputs. All transformations are governed by deterministic rules to preserve model integrity and reproducibility. By bridging natural-language interaction with controlled file transformation and structured result interrogation, the framework reduces manual effort and accelerates analyses across design iterations. Applied to large-scale tunnel projects, this methodology demonstrates a scalable pathway for augmenting computational design workflows with AI assistance without compromising reliability or technical rigour.

In recent years, the effectiveness of water spray systems (WSS) has been recognized, and their application to road tunnels based on risk analysis is progressing.

WSS are not intended to be used for extinguishing fire during their peak. Instead, they are designed to form water curtains around the fire source, providing thermal protection for the tunnel structure and limiting the spread of the fire until firefighters can begin firefighting operations.

In Japan, WSS have been installed in many road tunnels since the late 1960s. Through long term operational experience, unique design concepts and operational practices have been established. Previous studies have reviewed the development history of WSS in Japan, their functions, and installation standards of WSS in Japan, as well as domestic and international history and implementation history.

However, the circumstances surrounding tunnel fires have been changing in recent years. These changes include increasing difficulties in traffic management during fire incidents, the growth in scale of fires caused by large freight vehicles and car carriers, changes in combustion materials and combustion characteristics due to the spread of electric vehicles, and differences in traffic conditions and smoke exhaust operations on urban expressways.
In these situations, certain aspects of the conventional design and operational assumptions for WSS may require reconsideration, highlighting the need for a reevaluation of their appropriate use.
This paper reviews the original design intent and evolution of WSS, identifies current challenges under changing tunnel fire conditions, and discusses the fundamental performance requirements, expected effects, and desirable operational approaches for future WSS applications.

Keywords：Water Spray System(WSS)， Tunnel fire safety， Water spray system operation，Traffic management，Combustion materials，Combustion characteristics，electric vehicles， traffic conditions

Eglinton Crosstown (referred as Line 5) is a new 19 km transit line in Toronto which includes a 10km underground portion with 12 underground in-line stations, three underground interchange stations and ten at-grade stops. One of these three interchange stations integrates with the existing Line 1 at a single aerodynamically integrated Eglinton station. The upgrade of existing Eglinton station (Line 1) and the tunnel to the south of the station were included in the project. Upgrade of the tunnel portion included addition of two axial fans and associated dampers installed in a TVS fan room west of the Berwick portal located south of the station, as well as two axial fans and associated dampers installed north of the existing station. An important design element was the use of portal doors at each tunnel entrance to prevent excessive flow bypass and ensure precise control of smoke direction during emergency scenarios.

This paper presents a comprehensive engineering analysis of TVS and portal door integration. A primary focus is the development of a calibrated loss factor to account for air leakage at the door-track interface. This was achieved through an iterative coupling of 1D network modeling and 3D CFD analysis, ensuring that the mechanical ventilation could overcome “short-circuiting” at the portals to maintain tenability.
The paper further describes the operational coordination required between portal door and fan sequences across various fire scenarios. Comparison of the modeling predictions versus outcomes of Testing and commissioning are discussed. By comparing predicted leakage rates with field-measured performance, this case study offers valuable insights into the practical challenges of using portal doors as active aerodynamic boundaries in complex urban rail environments.

“Slab support hangers” represent a widely used solution for suspending intermediate slabs in road tunnels and are increasingly applied in the structural restoration of ageing tunnel infrastructure. However, their performance under fire exposure remains a critical safety concern. Failure can compromise slab integrity, reduce evacuation safety and critically hinder emergency response operations. Ensuring their reliability is therefore essential for both life safety and operational resilience.
Within the renovation and refurbishment project of the Tunnel de la Vue des Alpes, “Schweizer Riegel” type hangers were installed to reinforce the intermediate slab. Safety analyses were carried out to quantify hanger’s thermal exposure and resulting component temperatures. For this purpose, two reference fire scenarios were considered: a 30 MW heavy goods vehicle fire in accordance with Directive 13001 of the Federal Roads Office, which also forms the basis for the ventilation system design, and a 100 MW extreme semi-trailer fire derived from large-scale experimental results of the EUREKA research program.
This paper presents a methodological framework for assessing the thermal loads on slab support hangers using a combined CFD approach with IDA Tunnel (1D) and STAR CCM+ (3D) and employing the results for typical analytical methods for assessing the influence of insulation thickness. The work presented in this paper captures both the longitudinal distribution of hot gases governed by the emergency ventilation as well as the local heat transfer mechanisms of convection, radiation, and conduction through the intermediate slab, that control hanger temperature rise. Hanger temperatures in proximity to the fire are evaluated, as well as the temperature profile within the intermediate slab and practical recommendations are derived to support the safe and robust implementation of such slab support hangers in tunnel refurbishment projects.

This paper presents a performance-based case study of a 2.7 km underground rail guideway featuring two below-grade open-trench stations. A unique feature for the ventilation system design was to bring together mechanical ventilation requirements within the guideways (all above 305m in length) combined with the inherent aerodynamic advantages of the two open station geometries.
The proposed strategy leverages the open-trench configuration to facilitate passive smoke management via natural buoyancy, allowing heat and smoke to disperse directly into the atmosphere through above platform openings. To complement this passive effect and mitigate the risk of smoke migration into the adjacent enclosed guideways, a hybrid ventilation approach was implemented. This system utilizes jet fans located within the tunnel sections for smoke management within the tunnel and to maintain a longitudinal pressure barrier during a station fire incident.
In this paper, the station opening configurations, the interaction between natural ventilation and mechanical longitudinal flow within the tunnel, and the operational strategies for smoke management are presented. CFD modelling and tenability analyses are used to validate the system’s performance for different modes of operation. The results demonstrate how life safety requirements and regulatory compliance were achieved through an optimized, cost-effective ventilation design.

Retrofitting smoke exhaust systems within operational underground transportation infrastructure presents unique engineering challenges: delivering life-safety upgrades while the facility remains occupied and connected with active tunnel and building systems. This paper presents a case study involving the upgrade of an existing smoke extraction system serving a subsurface public concourse linked to railway tunnels, ancillary underground spaces, and adjacent high-rise developments, where maintaining safe operation during construction became a primary design driver.
A performance-based and risk-informed design approach was adopted to reconcile legacy conditions with current code requirements while maintaining operational continuity, using scenario-based smoke modelling and tenability assessments to demonstrate equivalent life-safety performance where prescriptive compliance was impractical due to existing spatial and operational constraints. The project required selective replacement of ductwork, dampers, and mechanical equipment within severe spatial and access constraints while preserving fire-rated compartmentation and maintaining indoor air quality during phased system outages.
A key technical challenge was implementing the upgrade in a live environment with limited redundancy, requiring temporary risk mitigation measures, staged system isolation, and careful coordination with active building and tunnel interfaces. Constructability planning focused on minimizing downtime while ensuring the modified system could be safely tested, accessed, and adapted for future operational needs.
A structured testing and commissioning plan has been developed to verify performance targets and ensure integration with existing life-safety systems. The project also evaluates lifecycle and energy implications associated with retrofit versus full system replacement and incorporates future-proofing strategies to accommodate evolving safety standards and operational demands.
The project demonstrates how performance-based ventilation retrofit strategies can safely modernise critical underground infrastructure without operational shutdown, offering transferable lessons for resilient, future-ready upgrades in complex urban transport environments.

 A methodology for rigorous evaluation and verification of primary jet fan operational parameters, based on the application of Newton’s laws in thermo-mechanical Open Systems, was proposed at the previous ISAVFT symposium. The comprehensive evaluation of pressure and shear forces together with interface momentums provided direct access to in-situ Thrust and Overall Installed Efficiency in tunnel ventilation design solutions. This approach removed the uncertainties in design analysis for such fundamental jet fan operation parameters.
As part of the development of this methodology, a more accurate and individual quantification of jet fan efficiency parameters for Off-Loading and Installation Efficiency (or Eccentricity Coefficient) is explored in this work. This has been investigated by application of the jet fan in-situ thrust model in conjunction with an explicit 3D jet fan representation including the rotating fan blades, which allows for a direct and separate/ individual quantification of the stated jet fan efficiency parameters from the design analyses.
Additionally, with the application of the approach to engineering design tasks, the impacts of encountered flow instabilities in design applications, on the accuracy of analysis solutions is explored and discussed from project experience.

“Metro Madrid’s digital twin fuses real-time sensor data with SES normal-operation simulations; real data reflect actual tunnel conditions, while SES simulations predict airflow and heat behavior—together enabling smart, predictive ventilation control.”
Metro de Madrid has developed GIV (Gestor Inteligente de Ventilación), an advanced digital twin platform that integrates real operational data with SES (Subway Environment Simulation) modeling to manage and optimize tunnel ventilation system’s energy consumption. This system represents a breakthrough in real-time predictive ventilation management for large underground transport networks.
The operational layer of GIV continuously ingests real data from the metro environment, including CO₂ concentration, temperature, humidity, air velocity, pressure, fan and damper positions, energy consumption, and train positions—capturing the dynamic piston effect of moving trains and ambient outdoor conditions. This ensures the digital twin remains synchronized with the tunnel’s current state.
Complementing this, the predictive layer employs SES simulations, to calculate airflow, heat balance, fan performance, and tunnel–station interactions. SES offers rapid results for forecasting short-term airflow behavior, testing ventilation control strategies, and assessing energy-saving opportunities.
The fusion of live sensor data and SES simulated behavior produces a true digital twin —an evolving, data-driven model that both mirrors and anticipates system conditions. This enables GIV to forecast air quality trends, proactively recommend ventilation settings, detect operational anomalies, and optimize energy distribution.
Operating on a continuous learning and adaptation cycle using Artificial Bee Colony Technique (ABC), GIV updates system’s control every eight hours based on updated inputs. Through this dynamic synergy of perception, prediction, and control, “Metro Madrid” has achieved a ventilation management system that enhances passenger comfort, energy efficiency, and operational resilience across its extensive underground network (350 stations and tunnels) reducing 40% energy consumption.

This paper presents a comprehensive thermal assessment and full scale fire testing program for passive fire protection systems applied to steel tunnel segments located at cross passage interfaces. The steel segments comprise a backplate with welded stiffeners arranged in a rectangular configuration, forming enclosed pockets. Multiple passive fire protection options—including fire mortar, fire board systems and concrete infill—are evaluated with respect to their ability to satisfy specified fire resistance requirements.
A dedicated testing methodology was developed to quantify the influence of these protection systems on the fire performance of the steel segments. Finite Element Method (FEM) analyses were conducted to simulate the thermal response of protected segments subjected to the RABT ZTV (rail) fire curve, with full scale furnace tests undertaken to validate the numerical predictions.
The investigation includes an assessment of spalling behaviour to determine the necessity of polypropylene fibre reinforcement. Thermal gradients within the protection materials and maximum steel temperatures are analysed to verify that critical temperature thresholds associated with reductions in structural steel yield strength are not exceeded.
The outcomes of this study establish a structured framework for the selection, evaluation and experimental validation of passive fire protection systems for steel tunnel segments.

Water-mist fire suppression systems are increasingly used in environments where rapid heat reduction, smoke control, and minimal collateral damage are critical. Despite their growing adoption, many projects lack a structured approach to evaluate mist system performance for both fire mitigation and structural protection. This paper introduces a scenario-driven qualitative evaluation method that integrates Activity Hazard Analysis (AHA) and reliability/availability assessments to guide system design and configuration.

The framework addresses task-level hazards throughout the system lifecycle—commissioning, maintenance, and emergency operations—emphasizing human-factor controls and operational dependencies. Reliability considerations include component failure modes, redundancy, maintainability, and readiness metrics, with design heuristics aimed at eliminating single-point failures and ensuring robust system response.

A key focus is on optimizing mist spray density and distribution for tunnels with large cross-sectional areas and high aspect ratios, where conventional test protocols may underestimate required mist delivery. The framework provides guidance on scaling droplet flux, nozzle spacing, and spray overlap to maintain effective suppression and thermal attenuation in complex geometries. Evaluation also highlights proper system sizing-encompassing pump selection, pressure control, and distribution-line design is essential for ensuring the system can maintain required performance across all operating modes.

Operationally, the evaluation supports coordinated fire department response, highlighting features such as zoned discharge, manual overrides, and compatibility with ventilation systems to enhance responder safety and tactical control. The result is a repeatable, evidence-based approach that enables engineers to compare mist system options, justify configurations, and implement operational controls that improve fire mitigation, responder safety, and structural resilience.
The initial development of this framework was to compare mist-based fire protection systems by their ability to absorb the heat energy of a fire through the evaporation of the mist droplets. This initial product comparison utilized a basic heat transfer analysis to determine the amount of heat absorbed as the mist droplets experience a temperature increase from ambient to boiling before evaporating completely. This heat transfer model will require future refinement to account for the variables that exist in a real-life tunnel fire scenario, but it can be used currently to roughly evaluate the performance of different product offerings for water-mist fire protection systems.

Many aging rail tunnels are in need of refurbishment, replacement, or entirely new installation of some fire and life safety systems features, including elements which were not available at the time or not required to the same extent as those required in current standards and best engineering practice. This paper presents a case study on the replacement of the tunnel ventilation system for a 1,500 m twin tube rail tunnel built in the early 20th century.
Studies present the proposed means to replace the original Saccardo nozzle ventilation system which has far exceeded its service life while also improving the system’s performance to meet the intent of current standards for fire and life safety. Providing an adequate replacement ventilation system is complicated by the tunnel’s inclusion of open cross passages spaced every 30 m along the length of the tunnel. In the design of new twin tube tunnels, cross passage doors are typically provided to prevent ingress of smoke to the non incident tube; however, operational constraints and cross passage doors operations and maintenance needs prevent installation of cross passage doors in this exiting tunnel. This case study presents the design of a ventilation system which considers the open cross passages between the two tubes their impact to requirements of the ventilation system to maintain smoke control while also ensuring safety for evacuees using non-incident tunnel. The effects of fire size and fire origin on potential ingress of smoke to the non-incident tunnel are also discussed.

The increasing complexity of tunnel ventilation systems, combined with rising expectations for safety, resilience and sustainability, is accelerating the adoption of digital monitoring and analytics throughout the tunnel lifecycle. This paper presents a real-world case study from the A55 Conwy road tunnel in Wales, where a severe vehicle fire in June 2025 provided a full-scale test of emergency ventilation performance and digital asset intelligence capabilities.
During the incident, a truck-mounted crane caught fire within the westbound bore, generating extreme thermal and smoke conditions. Seventy-two jet fans were automatically activated, enabling safe evacuation and effective smoke clearance prior to the arrival of emergency services. In parallel, real-time data from a cloud-connected monitoring platform provided continuous visibility of fan operation, temperatures and vibration behaviour throughout the event.
Post-incident analysis combined time-series data, thermal and vibration shock metrics, exposure analysis, and frequency-domain spectral techniques to assess potential degradation mechanisms arising from thermal shock and sustained operation under fire conditions. Comparative analysis between eastbound and westbound fans enabled prioritisation of inspection and maintenance activities, while statistical evaluation of spectral changes highlighted emerging risks such as mechanical looseness, misalignment or thermally induced shaft deformation.
The paper demonstrates how digitalisation during operation and commissioning can extend beyond compliance verification, supporting evidence-based decision-making during emergency response, accelerating safe recommissioning, and reducing unnecessary asset replacement. Lessons learned from the Conwy Tunnel incident are discussed in the context of smart tunnels, predictive maintenance, and performance-based fire safety management, offering practical guidance for operators and designers seeking to improve resilience and lifecycle value in tunnel ventilation systems.

Most highway tunnels in Japan are longitudinal ventilated. In ventilation design, NEXCO employs Meidinger’s formula to calculate boosting pressure. Since the jet flows of fans installed in tunnels suffer losses due to wall friction and equipment drag, it is necessary to multiply the calculated boosting pressure by the boosting coefficient (Kj) in the design. This coefficient has been calculated by measuring the boosting pressure generated when operating jet fan in tunnel and comparing the values to theoretical value.
To deal with expansion of roadway space in recent years, studies to reduce the diameter and weight of jet fans started in 2019. When measuring the low-pressure rise generated by a single small diameter prototype jet fan in a tunnel, conventional measurement methods were susceptible to disturbances, and it was difficult to ensure data reliability.
To counter this problem, a new multi-point measurement system that synchronously acquires static pressure data at one-second intervals. For measurement, pitot tubes are placed at 20 m intervals in a section 460 m distributed before and after the jet fan, and a unit integrating a small differential pressure gauge, power supply, and communication device is connected. The measured data is transmitted to the host PC via wireless serial communication to achieve simultaneous measurement at each pressure measurement point. It was also found that there were pressure fluctuations due to the tunnel’s air column oscillations. These fluctuations were larger than the pressure boosting pressure of the jet fan, averaging the time series data over the 300 seconds measured allowed us to eliminate disturbance factors and obtain reliable data.
As a result, a method for measuring minute boosting pressure was established, and the measurement time per case was reduced from one hour to five minutes. This paper reports on the new jet fan in situ performance measurement method.

 Tunnel internal structures are an important part of a highly complex system that makes transport tunnels function appropriately. These structures include architectural panels, traffic signs, fire walls, cameras, etc. Often these systems are repeatable and replicated along the longitudinal length of the tunnel providing opportunity for detailed engineering assessments to deliver significant costs Capex and Opex savings when the loads are analysed and estimated to a high level of confidence. This paper looks at how the industry currently estimates the vehicle induced loadings on tunnel internal structures and services. The vehicle-induced aerodynamic pressure is investigated by numerical and theoretical methods. The methods are validated against experimental data. A sensitivity study of the vehicle-induced aerodynamic pressure to the main design parameters like vehicle velocity, blockage ratio, tunnel cross section shape is finally presented and a formula for estimation of pressure loads in a variety of arrangements is proposed.

GM3 is demonstrating full scale mitigation of ventilation air methane (VAM) emissions at its Appin coal mine. Using regenerative thermal oxidation for oxidation of methane concentrations between 0.2 to 0.5 %v (4 to 10 % of the lower explosive limit) introduces risk of flashback of a deflagration into the mine if a gas outburst in the mine led to an explosive atmosphere between the mine and the regenerative thermal oxidiser (RTO). Design of the VAM abatement plant to Australian regulatory requirements and expectations has required:
– Identification of risks
– Identification of potential control measures
– Adoption of control measures so far as is reasonably practicable
This has required detailed consideration of:
– The likelihood of an explosive atmosphere connecting the mine to the RTO
– Potential control measures
– The practicality of control measures
The safety systems of that abatement plant design are accordingly bespoke to:
– The particular mine vent at which the abatement plant is located
– The safety systems that were practical for the project risks
This project is funded by the Department of Primary Industries and Regional Development through the Coal Innovation NSW Fund, which is administered by the Minister for Natural Resources (the Minister). Any views expressed herein do not necessarily reflect the views of Coal Innovation NSW, the Department of Primary Industries and Regional Development, the Minister for Natural Resources, or the NSW Government.

ewer networks, often containing deep tunnel networks, present distinctive ventilation phenomena associated with the effects of sewage flows on ventilation flows.
Newton’s method, for single variables, and in matrix implementations, has been found to be broadly useful for predicting ventilation flow rates and pressure distributions in sewer networks across varying flow regimes.
These predictions have been found pivotal to enabling progression of ventilation scheme designs and have been implemented in bespoke analyses to suit project requirements, avoiding restrictions of dedicated fluid network modelling platforms.
Case Studies:
– Predicting vented siphonic overflow behaviour including air and sewage flow rates and pressures using a matrix implementation of Newton’s method via partial derivatives (with respect to air and sewage flow rates when combined).
– Predicting entrained air flow rates and pressures for mitigation of downstream transients using single variate implementation of Newton’s method at each entry into a deep tunnel network.
– Predicting air pressure distributions and air intake and tributary sewer air flow rates in stratified flow sewer networks based on transient hydraulic model inputs using a matrix implementation of Newton’s method.

 As hydrogen fuel cell vehicles (FCEVs) emerge alongside battery‑electric transportation, tunnel systems, many of which were constructed decades before alternative fuels were envisioned, face new safety and operational considerations. This paper presents practical insights gained from the current hydrogen vehicle safety landscape, focusing on how hydrogen behavior, existing infrastructure conditions, and evolving codes intersect in real tunnel environments. The study evaluates hydrogen’s unique hazard profile in comparison with conventional and other alternative fuels, including rapid buoyancy-driven accumulation near ceilings, low ignition energy, and potential exceedance of lower explosive limits under certain leak scenarios. The discussion also recognizes lower‑frequency but high‑consequence hazards, such as jet flames and vapor cloud explosions, that define the upper bound of tunnel risk. Particular attention is given to challenges associated with older tunnels, where steep grades, low clearances, stagnant zones, and legacy ventilation arrangements introduce additional pathways for gas accumulation or delayed detection.
A structured review of relevant codes and standards – including NFPA 2, NFPA 502, CFR 49, GTR No. 13, and regional authority requirements – highlights areas where hydrogen considerations are well integrated, as well as gaps in prescriptive guidance for sloped tunnels, longitudinal airflow regimes, and emergency response protocols specific to gaseous fuels. The paper outlines practical mitigation concepts that transportation agencies can apply today, including enhanced detection strategies, ceiling‑zone ventilation management, equipment siting, maintenance practices, and operational planning that reflects hydrogen’s physical properties.
Drawing from technical analyses, preliminary calculations, and discussions with relevant stakeholders, this work provides a balanced view of where existing designs remain robust, where adjustments are warranted, and how agencies can adopt a risk‑informed framework without requiring extensive redesign or exhaustive modeling. By synthesizing lessons learned from real infrastructure conditions, this paper offers a forward‑looking but actionable path for integrating hydrogen vehicles safely and efficiently into legacy tunnel networks.

Nordhavnsvej Tunnel (NHV), a 700 m twin-tube road tunnel commissioned in 2018, is now being extended by 1,400 m into the Nordhavn Tunnel (NHT), which is planned to be operational in 2028. When the original NHV design was developed, low pressure water mist (LPWM) technology was still in an early stage of adoption, supported by limited fullscale testing and with a recommended design density of 2.5 L/min/m² for road tunnel applications. Over the past decade, the watermist industry has matured significantly, with extensive fire testing programmes, improved performance evidence, and updated international standards.

The paper focuses on following major design and integration challenges that shaped the final solution.

The design integrated the new LPWM system for NHT (4 L/min/m²) with the existing NHV LPWM installation (2.5 L/min/m²) while maintaining full hydraulic compatibility. Only the main pumps at the existing pump station were replaced, with all remaining system components retained and adapted to support higher‑density operation.

A six‑zone activation philosophy was developed to address the wider ramp areas, enabling larger‑area coverage and improving fire‑service accessibility during an incident.

The water‑mist ring main was embedded beneath the tunnel slab using PE‑100 pipework with integrated leakage detection. This approach reduced material footprint, preserved space at the tunnel crown for future upgrades, and maintained system resilience through a looped configuration.

As the existing pump station could not serve the full tunnel length, a new pump station (PSN) was introduced. Together, the two pump stations operates in a duty–standby arrangement, providing redundancy, enabling future tunnel extensions, and allowing removal of the existing diesel pumps.

This paper presents the engineering approach, integration strategy, and key design decisions adopted to extend and integrate the existing and new fire protection systems.

The construction of flyovers to mitigate busy road inter-sections generates open, partially enclosed or fully enclosed roadways beneath it. The creation of such underpasses implicates specific fire and life safety requirements under NFPA 502. Short underpasses between 50 to 300 m in length are classified as Category A or X, which would not generally require mechanical ventilation for fire emergencies. However, given the requirement in NFPA 502 to demonstrate means of egress through engineering analysis, an ASET vs RSET using CFD tends to demonstrate the difficulty in achieving compliance without additional mitigating measures. Underpass 3 of the King Khalid Road upgrade in Riyadh presents a set of site-specific constraints— multiple traffic bores, architectural restrictions prohibiting vertical shafts, high traffic volumes and lanes, the presence of fuel tanker vehicles, and the absence of cross passages due to differing vertical road profiles—that made it particularly challenging to demonstrate egress tenability.
Based on CFD results, an ASET–RSET analysis concluded that vertical shaft openings would be required to keep the evacuation route tenable for the required duration of escape. Where high-level openings were not practicable, natural ventilation remained adequate for lower‑HRR fire scenarios (e.g., passenger cars or buses) but failed to maintain tenability for high HRR HGV and tanker fire. Therefore, the solution included a water-mist system and jet fans so that the required conditions for escape could be met for high HRR scenarios.
This paper discusses the challenges faced by designers on providing effective smoke control and tenable conditions for short tunnels. The KKR underpass tunnel is used as an example, but the author draws on other project examples and mitigations that have been used to resolve such scenarios.

For cable and utility tunnels, there is a current absence of directly applicable prescriptive guidance available in the fire and life safety design. This issue was found to be prevalent in a number of projects. Attempts to align these tunnels with the other more generally relevant standards and guidance do not always result in effective designs.
For the Oxagon Village Utility Tunnel project, a performance-based design was adopted for ventilation and fire safety. The Oxagon tunnel is approximately 2.3 km shallow cut and cover twin tunnel carrying up to 30 Medium Voltage cables within each of the cells.
There were project requirements to limit the number of surface facilities due to constraints within the public realm, while maintaining adequate temperature for the cables to operate within the tunnel as well as safe evacuation provisions. Constraints within the project were the elevated temperature from cable heat loss and the requirement to address maintenance personnel in different locations making simple ventilation responses impracticable.
This led to a solution with compartmentation and relatively long distances between escapes shafts to surface. The extended distance was justified by extensive CFD and evacuation analysis to inform a design that more accurately reflects the nature of the tunnel and the cables.
In this paper, the framework used in the project to undertake this analysis is comprehensively detailed. This included analysis of the properties and fire dynamics of the cables, the occupant characteristics of maintenance staff within these tunnels and the geometry and general layout of the tunnel structure. Data from cable fire tests formed the basis of the design fire.
This paper sets out the approach to the design, the analysis undertaken and how this led to the resulting design. The factors underlying general approach will be set out for application to similar utility tunnels.

 In these days of easy international travel, the world seems to be becoming more homogenous. You can get good sushi in Graz and Brisbane, good steak in Osaka and Graz, and schnitzel everywhere including Osaka and Brisbane. Why then are our approaches to tunnel fire safety so different? For real cultural experiences when you travel, forget the food – go and look at the tunnels. This paper notes differences in approaches to ventilation, fire suppression and tunnel operation and, within the limits of our knowledge, tries to rationalise them. Simply thinking about the differences may assist advancements in any of the jurisdictions, perhaps in a direction not identical to any of them.

Hanshin Expressway Co., Ltd. has worked for achieving carbon neutrality by promoting energy conservation in equipment operations. Among all energy-consuming equipment in Hanshin expressway, tunnel ventilation systems account for a particularly large share-around 50%- of total electricity consumption. As a result, energy conservation in tunnel ventilation operations has become a key issue.
This paper focuses on the longitudinal ventilation systems (central exhaust + jet fans) of existing tunnels that have reached the time for renewal. Recent traffic conditions were analyzed to understand the daily traffic flow characteristics, and the life cycle cost was estimated based on 24-hour ventilation operation patterns under different control methods for jet fans and axial flow fans.
As a result, it was confirmed that adopting an inverter-driven variable speed control system can significantly reduce electricity consumption compared to the current ventilation control system. Additionally, introducing rotational speed control for jet fans improved the controllability of airflow in long and complex tunnel with multiple ramp tunnels, such as those examined in this study. This improvement also reduced the required exhaust flow rate for axial fans in central exhaust systems.
Furthermore, this modification of the control method enabled more precise control of the longitudinal velocity inside the tunnel at the initial phase of fire incidents, thereby enhancing safety.
The findings of this study highlight that optimizing tunnel ventilation systems with consideration of control methods is essential for achieving energy conservation, cost reduction, and improved safety.

A reduced-scale longitudinal ventilation tunnel has been constructed at Zitrón with the objective of experimentally evaluating the aerodynamic behavior of different jet fan configurations under controlled conditions. The configurations tested include standard fans, standard fans with deflectors, slanted silencers, shaped nozzles, and other systems. The experimental campaign included variations in fan installation height relative to the ceiling, distances between fans within the same array, blade angles of deflectors, connection-piece angles used to configure the slanted silencers, fan rotational speeds, and the presence of different tunnel obstacles to simulate traffic blockage.
To characterize the flow field, pressure measurements were taken at six sections along the tunnel, complemented by air velocity measurements, environmental conditions, and fan power consumption in each test. A dedicated electronic acquisition system was developed for the project, enabling the simultaneous and accurate recording of multiple data points during every trial. In addition, a smoke generator was employed to visualize the dynamic behavior of the airflow. The effect of traffic proved to be a significant factor in the aerodynamic response and is systematically included in the study.
Future investigations will also consider alternative fan arrangements with the objective of optimizing the aerodynamic performance in the tunnel.

Ensuring safe conditions for passengers during fire emergencies in above ground rail stations presents significant challenges because external wind can strongly influence how smoke develops and moves. Unlike fully enclosed underground stations, open or semi open above ground environments allow natural wind to interact with buoyancy driven smoke. This interaction can redirect flow paths, disrupt smoke layering, and alter the exposure conditions faced by passengers who may remain on the platform while awaiting emergency response.
As modern transit systems increasingly adopt sustainable and energy efficient approaches, passive ventilation strategies are becoming more pivotal to fire safety planning for above ground stations. This study investigates the extent to which wind driven airflow affects smoke transport and occupant tenability during a station fire. It also highlights how certain instinctive evacuation behaviors, such as relying on visual cues that may prompt passengers to cross tracks, can unintentionally increase risk compared with using designated egress paths.
To explore these issues, a set of transient CFD simulations was conducted using a representative station configuration. The model incorporated realistic fire heat release rates and examined multiple wind speeds and directions applied at the station perimeter. Key tenability indicators, including visibility, thermal exposure, and concentrations of hazardous gases, were evaluated at typical occupant height along the platform to understand how conditions evolve under varying environmental influences.
The findings emphasize the need to incorporate wind driven smoke behavior into fire safety assessments for above ground stations, where external conditions cannot be controlled or easily predicted. Considering these effects enables engineers to better evaluate potential worst case scenarios beyond standard code requirements, refine emergency response strategies, and identify whether architectural adjustments may be warranted to support expected passenger behavior. The study demonstrates that CFD serves as a powerful diagnostic tool for capturing complex wind–fire interactions that are otherwise difficult to predict, ultimately supporting more sustainable, resilient and informed design decisions for passenger safety in an above ground transit environment.

Background: The 2.8 km long Rokko Mountain Road Tunnel in Japan opened in 1967. It carries two-way traffic and was operated for many years using a semi-transverse ventilation system with an inlet air duct above a false ceiling. However, the collapse of the ceiling in Sasago Tunnel in 2012 triggered a review of the safety of all such ceiling structures, and potential improvements to ventilations were sought.

Renovation overview: In 2019, the ceiling panels in the Rokko Mountain Tunnel were removed, and the semi-transverse ventilation system was replaced by a longitudinal system using variable-speed jet fans in the tunnel together with air-supply through an existing ventilation shaft. This brought several advantages in addition to removing the potential structural weakness, but it also introduced a new problem.

In routine operation, the use of several jet fans at low rotational speed instead of fewer fans at higher speed provides the desired thrust in an energy-efficient manner. This is important because, in Japan, it is necessary to prevent the emission of polluted air through tunnel portals, so forced ventilation is required even in routine operation.

The central inlet shaft has been repurposed, and is now reserved exclusively for evacuation in the event of fire, in which case it can be maintained smoke-free.

In the main tunnel space, the variable-speed fans are used to reduce the airflow velocity to not more than 2 m/s withing 90 s after fire detection, as is required in Japan. In this tunnel, the longitudinal velocity is further reduced to approximately 0.5 m/s within three minutes of fire detection.

Operational outcomes and challenges: Monitoring of the actual energy usage in the tunnel before and after the renovation has shown significant benefits during routine operation. Furthermore, the rapid achievement of negligible axial flow after the outbreak of a small fire resulting from a vehicle malfunction demonstrated the effectiveness of the zero-flow control strategy. However, new challenges have also emerged, notably the formation of condensation in the end regions of the tunnel, i.e. close to the portals. This problem did not occur with the pre-existing semi-transverse system.

In addition to detailing the above items, the proposed paper will describe the transition from the semi-transverse system to a longitudinal system, and will explain how this was achieved with minimal disruption to the availability of the tunnel.

Once jet fan ventilation systems are installed and commissioned, little attention is afforded to their on-going performance as the tunnel ages and other systems are installed to replace older infrastructure. Reduction in effective jet fan thrust as a result of installation effects can be significant, and can worsen with time as new equipment is installed in the regions of fan outlets. Major impacts can be seen from secondary equipment causing form drag. Deflection of the jet away from obstructions by means of slanted silencers, vanes or flow deflectors has been shown to provide a means of improving the efficiency of such installations but not without loss associated with the deflection itself.

There are limited real-world data available on tunnel ventilation projects post-installation to inform design and refurbishment, particularly for ventilation systems in ageing tunnels where new equipment is often installed in limited available spaces, having impacts on the installed jet fan aerodynamic efficiency. This paper reports on the design, testing and performance evaluation of jet fan installations in a number of rail and road tunnel projects across the world, including presentation of data derived from deflector installation projects, cases of form drag performance losses and improvement techniques implemented.

Once jet fan ventilation systems are installed and commissioned, little attention is afforded to their on-going performance as the tunnel ages and other systems are installed to replace older infrastructure. Reduction in effective jet fan thrust as a result of installation effects can be significant, and can worsen with time as new equipment is installed in the regions of fan outlets. Major impacts can be seen from secondary equipment causing form drag. Deflection of the jet away from obstructions by means of slanted silencers, vanes or flow deflectors has been shown to provide a means of improving the efficiency of such installations but not without loss associated with the deflection itself.

There are limited real-world data available on tunnel ventilation projects post-installation to inform design and refurbishment, particularly for ventilation systems in ageing tunnels where new equipment is often installed in limited available spaces, having impacts on the installed jet fan aerodynamic efficiency. This paper reports on the design, testing and performance evaluation of jet fan installations in a number of rail and road tunnel projects across the world, including presentation of data derived from deflector installation projects, cases of form drag performance losses and improvement techniques implemented.

Road tunnel ventilation systems face the dual challenge of managing pollution during standard operations and smoke during fire emergencies. Traditionally, longitudinal ventilation systems, often combined with smoke extraction, are employed. However, the requirements differ significantly: normal ventilation needs to move large volumes of air at low pressure, while emergency systems require smaller air volumes at much higher pressures.
At West Gate Tunnel, two distinct ventilation systems tailored to their specific operational requirements were designed. Normal and congested traffic conditions are managed with large, low-speed, low-pressure fans, while emergency scenarios utilise smaller, high-speed, high-pressure axial fans in a single distributed extraction system. The project also includes a service tunnel under the road deck which is ventilated by its own ventilation system and also supports evacuation from the main road tunnel.
Additionally, the project integrates a feedback loop control system to enhance energy efficiency, adjusting air flow rates based on real-time traffic demands, and the design ensure no reverse jet fans operate during normal conditions, minimising further the energy consumption.
The innovative design is significantly more efficient than a single fan system. Overall, this approach not only enhances the effectiveness of the ventilation systems but also significantly reduces energy usage, setting a precedent for future tunnel ventilation design in urban environments.

Effective ventilation is critical for maintaining air quality and safety in road tunnels. This study investigates the optimization of ventilation systems in a road tunnel presenting unique architectural and operational challenges.
In this existing tunnel, induction fans—commonly used for smoke management in underground car parks—were installed at the entrance portal due to restrictions on vertical clearance within the tunnel. To accommodate these fans, the tunnel height was raised at the entrance. Computational Fluid Dynamics (CFD) simulations were employed to evaluate and optimize the configuration of the induction fans, ensuring their performance meets operational requirements. This diagnosis analysis concluded that additional jet fans were required to meet NFPA 502 critical velocity requirements. This necessitated the adjustment of the vertical clearance as well as accommodating the additional power supply and control system requirements.
This study offers practical insights into optimizing ventilation systems within architectural constraints, ensuring enhanced air quality, safety, and compliance with design standards by the means of CFD analysis. The findings contribute to the integration of engineering solutions with architectural considerations in tunnel environments.

This paper presents an experimental study aimed at quantifying the efficiency of a smoke curtain in controlling fire-induced smoke flowing upstream in a longitudinally ventilated tunnel. This work extends and complements a previous numerical study (Narcisse et al., Efficiency of a smoke curtain in a ventilated tunnel, ISAVFT 2024) which had already demonstrated the potential benefits of such a system.
Experiments were carried out in a small-scale (1:20) plexiglass tunnel allowing flow visualization and measurement to be performed with standard cameras and a LDV (Laser Doppler Velocimetry) system. The air-helium modelling technique was used to reproduce the fire source buoyant flux.
The relevance and validity of the experimental model were first assessed by comparing small-scale measurements with full-scale data and correlations commonly used in tunnel fire engineering to estimate critical and confinement velocities for a given fire heat release rate.
The influence of the smoke curtain on the longitudinal flow was then investigated under non-smoke conditions in order to characterise the aerodynamic disturbances it induces and to measure the size of the vortex developing downstream of this curtain.
Subsequently, a series of tests was performed for several fire heat release rates and curtain heights to determine the blockage velocity (the required ventilation velocity required to contain smoke backlayering behind the curtain) at steady state. The transient phase was also examined with and without curtain to highlight the influence of the downstream vortex on the development of backlayering.
The results show that, for a given backlayering length, the use of a smoke curtain allows the required longitudinal velocity to be reduced (by up to 40%). This reduction has a direct impact on the design of longitudinal ventilation system in tunnels for fire safety issues.

Backlayering refers to the formation of a stratified smoke layer that propagates upstream against the main flow in a longitudinally ventilated tunnel during a fire. The concept of critical velocity was introduced to define the minimum longitudinal airflow velocity required to prevent the backlayering occurrence. When the ventilation velocity is lower than the critical velocity, backlayering can develop and extend upstream until reaching a stagnation point at a distance L from the fire source. Numerous experimental studies have investigated the dependence of this upstream propagation length L on the longitudinal airflow velocity (whose value is lower than that of the critical velocity). These studies generally show good agreement with the well-known empirical correlation proposed by Li, Lei, and Ingason (Fire Safety Journal, 2010). From a purely theoretical perspective, however, this correlation has never been rigorously justified. Most existing modelling approaches treat backlayering as a simple entraining density current, an assumption that is not strictly valid. Indeed, under steady-state conditions, backlayering consists of two distinct layers: one flowing against ventilation along the ceiling, and another one flowing with the ventilation from the stagnation point toward the fire source. To address this issue, the present paper eports a set of experimental results obtained using a reduced-scale tunnel model. Vertical profiles of velocity and density were measured at several longitudinal locations within a stationary backlayering. These measurements provide insight into the flow structure and allow the longitudinal evolution of velocity and density within the backlayering to be characterized. Based on these experimental results, the relevance of more appropriate theoretical models is discussed, including formulations involving either two interacting density currents or a single detraining density current.

Tunnel ventilation and station HVAC systems can occupy up to 40% of the usable volume in an underground metro station. Stations developed in dense urban locations benefit significantly from compact Tunnel- and Station- Ventilation System (TVS and SVS) solutions, which minimises spatial footprint, construction cost, and, importantly, civil design complexity. However, such space-efficient configurations introduce complex airflow interactions between tunnels, trackways, and station volumes, necessitating advanced numerical modelling to verify performance across all operational modes and ensure compliance with stringent fire–life safety requirements.

Brisbane has a large dedicated busway network that has progressively expanded over 25 years and includes a significant number of tunnels. The fire safety systems and operational requirements have evolved progressively adapting to changes in vehicle types, safety strategies and available system provisions. This means that the tunnels can have varying provisions and operations.

The tunnels that were initially developed are at an age where system upgrades are currently being considered. These upgrades need to not just consider equipment replacement/refurbishment but also reconsideration of strategies, operations and the potential incorporation of new systems. Ideally upgrades would bring these tunnels to a level of safety equivalent to new modern standards. Various constraints however mean that this is not necessarily possible.

Work has been undertaken to develop a framework in which to assess the existing infrastructure in order to determine the requirements to arrive at a position where the tunnels can be considered to meet fire safety requirements So Far As Is Reasonably Practicable (SFAIRP). This is considered to be a health and safety requirement that needs to be achieved. This paper describes the considerations that were undertaken in order to arrive at a process that would work within the constraints of a large operating busway system. These considerations include items such as legal requirements, physical limitations, operational constraints and new vehicle technology. The work undertaken can offer guidance into how older existing infrastructure can be assessed in order to meet fire safety requirements.

Computational fluid dynamics (CFD) simulations are widely used in ventilation and fire engineering but they remain computationally demanding, particularly for parametric analyses and scenario-based assessments. Physics-Informed Neural Networks (PINNs) have emerged as a promising surrogate modeling approach capable of approximating CFD solutions while embedding governing conservation equations directly into the training process. By enforcing physical laws as constraints, PINNs can achieve physically consistent predictions with fewer labeled data than purely data-driven models. However, practical implementation still relies on CFD-generated datasets for supervised training, which limits overall efficiency gains.

The performance of Saccardo nozzle systems is driven by a complex interaction between geometric configuration and jet development, making their designs inherently challenging. Australian metro projects have shifted toward shorter, more compact nozzle designs to satisfy spatial and cost constraints. While these configurations enhance constructability and reduce capital expenditure, designers face a critical trade-off with the risk of reduced nozzle efficiency and compromised system performance. Conventional theoretical design methods provide good estimates of induced tunnel pressure rise for long nozzles with uniform jet profiles, but they cannot capture complicated three-dimensional (3D) flow characteristics of compact nozzles. This makes it difficult to assess how compact nozzle design affect system performance, particularly at the fan, without detailed 3D CFD analysis. This study presents a systematic investigation of the Saccardo nozzle performance through a parametric study, quantifying the influence of key nozzle characteristics on effective nozzle thrust. Results are compared with theoretical predictions, and a momentum exchange coefficient (as established in the literature) metric is used to measure deviation from idealised behaviour. An overall nozzle efficiency is introduced to compare the effective nozzle thrust against the work done by the fan. These metrics aim to provide practical benchmarking for designers. Additionally, the sensitivity of CFD predictions to boundary conditions is assessed to identify modelling approaches that best reflect real project conditions. The findings in this paper demonstrate that compact nozzles generate complex 3D flow behaviour, which can increase momentum exchange coefficient and can raise predicted effective thrust compared with theoretical methods. However,
this also reduces nozzle efficiency, which must be accounted for in the fan performance. Despite these limitations, compact nozzles can achieve effective performance when critical geometric parameters are appropriately configured.
The paper also provides practical design guidance and establishes a consistent framework to support nozzle application in tunnel ventilation projects.

Lighting and walkway configuration have a clear impact on movement in a smoke-filled rail tunnel. During design, analysis and evacuation modelling, walking speed should be adjusted according to the visibility and the configuration of the tunnel. The walking speed used in evacuation modelling in tunnels is often based on a tunnel environment which does not fully correspond with environments as seen in today’s newer rail tunnels. Newer rail tunnels are often equipped with adequate lighting and handrails providing an enhanced egress environment. If the walking speed used during design and analysis can be increased due to a more beneficial environment in new tunnels, it could lead to distances between exits being extended. Hence, a refined analysis can provide large cost-savings.

A refined egress analysis was commenced through a case study in the paper “A modified approach to walking speed within smoke-filled rail tunnels“ presented by the authors at the ISTSS conference in 2025. The paper focused on the lighting configuration and walking speed reduction due to visibility. The paper presented at ISTSS had a narrow perspective in terms of tunnel geometry and evacuation conditions being studied. However, it highlighted that the constant K, used for calculating visibility distance, as well as the minimum walking speed, can have a considerable impact on evacuation. This signifies the need of further sensitivity analysis. The paper being proposed for ISAVFT 2026 will be an advancement of the previous paper presented at ISTSS. The paper being proposed aim to broaden the tunnel geometries and evacuation conditions being considered. This will mainly be conducted through variation of fire heat release rate and walkway width.

Using CFD for fire modelling coupled with evacuation simulations, the study aims to highlight the shortcomings in current knowledge and provide a greater understanding for important input parameters when conducting egress analysis.

Snowy 2.0 is one of the largest pumped‑hydro infrastructure projects undertaken in Australia, comprising approximately 27 km of underground tunnels, multiple deep shafts, and a major underground Power Station Complex within Kosciuszko National Park. The scale, depth and phased excavation of these interconnected underground assets present significant construction fire risks, particularly during drill‑and‑blast and tunnel boring machine operations. Our role encompassed establishing the regulatory framework, defining fire safety objectives and developing a performance‑based design methodology across multiple construction packages, with a focus on maintaining risks SFAIRP supported by a live fire risk register and stage‑dependent evacuation analysis.

The project presented several unique challenges. The underground network comprised complex and continuously evolving geometries, including blind and upward‑oriented tunnels, creating uncertainty in smoke movement and emergency access. Common and converging evacuation routes increased the risk of congestion and prolonged exposure to smoke. Deep shafts excavated from the surface to depths of up to 250 m further constrained evacuation due to long ascent times. Increasing travel distances as excavation progressed made walking evacuation under smoke‑affected conditions a dominant life‑safety constraint.

Accordingly, refuge chamber spacing, capacity and travel distances were engineered as primary risk controls. Evacuation timelines were assessed against tenability criteria for visibility, temperature and toxic species, accounting for workforce density and shift patterns. A combination of engineered and procedural measures was implemented, including enhanced personnel training, robust lighting and wayfinding, compartmentation to limit smoke spread, and resilient communication systems to maintain coordination with emergency response teams in a rapidly changing environment.

Innovative mitigation measures included fixed water curtain systems at strategic locations to limit smoke propagation and protect critical junctions and evacuation routes. These systems were integrated with construction ventilation and operational procedures to create defensible tenable zones during credible high‑consequence scenarios such as heavy plant and diesel fires. Computational Fluid Dynamics modelling was applied to evaluate design fires and predict smoke spread, stratification and tenability along escape paths. The modelling informed ventilation strategies, water curtain deployment and refuge chamber performance, demonstrating that Available Safe Egress Time exceeded Required Safe Egress Time during defined construction phases.

The Snowy 2.0 case study demonstrates how integrated fire engineering, ventilation design, advanced modelling and coordinated emergency planning can maintain life safety in deep and dynamically evolving underground construction environments.

Keywords: Tunnels, pumped hydro, construction fires, CFD, evacuation, refuge chambers, risk management

Retrofitting smoke exhaust systems within operational underground transportation infrastructure presents unique engineering challenges: delivering life-safety upgrades while the facility remains occupied and connected with active tunnel and building systems. This paper presents a case study involving the upgrade of an existing smoke extraction system serving a subsurface public concourse linked to railway tunnels, ancillary underground spaces, and adjacent high-rise developments, where maintaining safe operation during construction became a primary design driver.
A performance-based and risk-informed design approach was adopted to reconcile legacy conditions with current code requirements while maintaining operational continuity, using scenario-based smoke modelling and tenability assessments to demonstrate equivalent life-safety performance where prescriptive compliance was impractical due to existing spatial and operational constraints. The project required selective replacement of ductwork, dampers, and mechanical equipment within severe spatial and access constraints while preserving fire-rated compartmentation and maintaining indoor air quality during phased system outages.
A key technical challenge was implementing the upgrade in a live environment with limited redundancy, requiring temporary risk mitigation measures, staged system isolation, and careful coordination with active building and tunnel interfaces. Constructability planning focused on minimizing downtime while ensuring the modified system could be safely tested, accessed, and adapted for future operational needs.
A structured testing and commissioning plan has been developed to verify performance targets and ensure integration with existing life-safety systems. The project also evaluates lifecycle and energy implications associated with retrofit versus full system replacement and incorporates future-proofing strategies to accommodate evolving safety standards and operational demands.
The project demonstrates how performance-based ventilation retrofit strategies can safely modernise critical underground infrastructure without operational shutdown, offering transferable lessons for resilient, future-ready upgrades in complex urban transport environments.

Understanding smoke movement in large, interconnected underground environments remains a significant challenge, particularly where aerodynamic connections with varying elevations create strong buoyancy-driven stack flows which can overwhelm mechanical ventilation. This case study presents a high-level overview of an integrated assessment framework developed to support the fire and life safety design of a complex subsurface concourse connected to railway tunnels, ancillary underground spaces, and high-rise buildings.
The methodology combines targeted empirical on-site measurements with multi-scale simulation tools to enhance confidence in smoke control performance and the achievement of tenability conditions. Field observations were used to characterize baseline resistance and flow behavior and identify the influence of operational and environmental seasonal conditions. These measurements informed both 1D network airflow modelling and higher-resolution computational fluid dynamics analysis, enabling cross-validation of predictions and improving the robustness of the design process. The integrated approach allowed for steady-state assessment of smoke containment within defined ventilation zones and transient analysis of fan response to verify dynamic system performance, ensuring no smoke migration into protected areas, while maintaining tenable conditions within designated egress paths.
To verify the modelling outputs, dedicated cold smoke tests are to be conducted on site, allowing direct comparison between predicted and observed smoke behavior across critical paths within the network.
The study highlights the importance of combining empirical data with simulation-based tools to address the complexities of ventilation performance in multi-node underground systems. The resulting methodology provides a resilient and evidence-based pathway for developing smoke control strategies in large aerodynamically-coupled environments, explicitly accounting for seasonal variations in natural stack-driven airflow. While ambient conditions significantly altered baseline pressure differentials and flow patterns, containment objectives were maintained across seasonal scenarios without changes to the fire emergency response strategy.

The Gotthard Road Tunnel is a key transalpine corridor and one of Europe’s busiest road tunnels. Originally opened in 1980, it requires now comprehensive refurbishment. To maintain continuous connectivity during the renovation, the Swiss Federal Roads Office (FEDRO) has commissioned the construction of a second tunnel tube, that is currently being excavated. The construction of the second tube of the Gotthard Road Tunnel constitutes one of Europe’s most complex underground infrastructure projects. A key challenge is the design of a modern, robust, and fully compliant ventilation system for the new tube, which must operate both independently and in coordination with the existing tube.
This paper presents the design principles, performance assessment, and safety considerations underpinning the new system. The original 1980 tunnel ventilation relied on full transverse ventilation while the new system adopts a hybrid concept combining longitudinal ventilation and smoke extraction, with ventilation shafts connected to underground buildings, a false ceiling with smoke dampers and 60 jet fans. During normal operation, the ventilation is primarily longitudinal and exhausted air can be replaced with fresh air at the ventilation stations, if required. In a fire scenario, the smoke is extracted through dampers and the jet fans provide longitudinal flow control. The design complies with FEDRO standards and includes additional project-specific requirements, notably the need to operate in both unidirectional (final phase) and bidirectional (during the refurbishment of the first tube) traffic regimes.
The paper details aerodynamic modelling assumptions, components design, system performance and operational strategy in both normal operation and fire scenarios. With enhancements in safety, redundancy, and maintainability, the new system substantially enhances smoke control, operational flexibility and maintainability, ensuring long-term safety and availability of one of Europe’s most critical transalpine transport corridors.

This is an update on the open-source development of the subway environment simulation (SES) computer program (OpenSES) since the 2022 ISAVFT paper, “Open-source SES Past, Present, and Future”. The goal is to encourage others to participate and use this updated version of SES.
Digital Output and Verification
OpenSES’s results are verified against the older SES 4.1 version. SES 4.1 outputs are limited to text files that require parsing, whereas OpenSES introduces a new digital format. This new format enables easier verification of OpenSES results.

Conversion to SI
Previously, SES 4.1 required a cumbersome double conversion process—inputs and outputs had to be translated between SI and IP units, complicating simulations for global users. Source code updates also pose challenges, as they require knowledge of unfamiliar IP conversion factors.
The initial code updates to OpenSES enable fire simulations in the SI system via an input switch (IP or SI). With native SI calculations, more engineers can understand and update the source code. Projects outside the United States can use OpenSES for fire simulations without a double conversion.
This update streamlines workflows for engineers, reduces conversion errors, and enables more accurate simulations for international projects.
Future SI conversion focuses on enabling normal, non-emergency simulations.
First New Feature
The paper introduces a new simulation feature to derate impulse (jet) fans during fires. This example sets a precedent for the documentation and format of updates.
Documentation
Existing documentation for SES 4.1 is made available. The paper explains the update and maintenance of OpenSES’s user manual, programmer manual, and new documentation.
We invite engineers and researchers to contribute to OpenSES’s ongoing development and documentation

Computational Fluid Dynamics (CFD) is a well-established modelling tool for 3D detailed analysis of fire emergencies in tunnel environments. Control of smoke back-layering for safe evacuation in emergencies and the prediction of hot gas temperatures for impacts on rolling stock and eventually tunnel structure, are important for assessing risks to people and infrastructure. These factors are largely impacted by Tunnel Ventilation Design.
This work presents a novel fire modelling methodology for practical industrial applications, with tie-in between smoke and radiation modelling in a fast chemistry model setting – aspects that increase the rigour and accuracy of detailed analyses for large fire simulations in tunnel emergency scenarios. Model parameters controlling the link between smoke concentration and radiation from the plume, influence plume energy distribution and plume shape, directly impacting back-layering control and temperature distribution in the tunnel environment. This more explicit interaction between radiation and soot brings further rigour and realism to detailed analyses for fire emergency scenarios, impacting the accuracy of predictions.
The proposed methodology is verified/validated for different fire sizes, against public large-scale fire tests, including those from the Memorial Fire Ventilation Test Program (MFVTP) and the EUREKA Project for parameters of interest. These benchmarks provide a valuable basis for evaluating model performance under tunnel ventilation conditions and time dependent fire heat release rates. By identifying current modelling strengths and limitations including uncertainties, this work contributes to improve the reliability and confidence in detailed 3D design analyses for tunnel fire emergency scenarios, promoting safer design and emergency response planning.

 In underground rail stations, smoke exhaust systems are traditionally treated as either fully operational or failed. While such systems generally incorporate a degree of redundancy, such as an N+1 fan arrangement or tolerance for the failure of a single damper module, most designs do not account for multiple or combined failures.
This binary approach can result in overly conservative operational decisions, including unnecessary station closures, even when sufficient system capacity remains to manage smoke safely with additional controls, where necessary.
The paper will introduce the concept of an “impairment mode”, developed and implemented as part of the retrofitment of platform smoke exhaust systems to the 40+ year old MURL stations. Impairment mode defines a controlled intermediate state in which the smoke exhaust system continues to operate with reduced performance following partial equipment failures, allowing safety to be maintained while avoiding immediate service shutdown.
Also, the paper will discuss the smoke exhaust system design, explains how impairment and pre-impairment conditions were defined, and describes the process used to identify impairment thresholds. Additionally, it will highlight several challenges associated with working in a brownfield environment.
Station-specific impairment thresholds were established for each station and translated into control system logic that generates impairment alarms for operators. These alarms differ from the typical degradation alarms commonly adopted for similar systems. The paper will explain how Rail Operators can benefit from such impairment alarms.
The MURL experience shows that an impairment-based approach can better balance life safety and service continuity, giving rail operators the confidence to make informed, risk-based decisions during system degradation. The approach provides a practical and transferable framework for other underground rail networks facing similar operational and safety challenges.

The rapid adoption of battery electric buses (BEBs) is changing the fire hazard profile of enclosed transit garages, where vehicles are commonly parked in dense arrays and may be simultaneously connected to high-power charging infrastructure. This paper presents a performance-based evaluation of fire hazard conditions in a BEB storage garage. The evaluation focuses on a credible severe-case ignition scenario, in which thermal runaway occurs within a roof-mounted battery energy storage system enclosure (ESSE) and acts as the initiating fire source. A simplified fire growth model is developed to capture the coupled effects of (1) energy release from thermal runaway and vented combustion products, (2) formation of a hot upper smoke layer beneath the ceiling, and (3) thermal radiation from flames and hot gases to adjacent vehicles and interior materials. The model links roof-level battery pack ignition to multi-vehicle exposures by predicting heat flux to neighboring bus roof surfaces and sidewalls, accounting for convective heating from the smoke layer and radiative view factors between vehicles.
The study emphasizes the influence of bus parking separation distance on fire spread pathways. Reduced spacing between parked buses increases radiative feedback and accelerates heating of adjacent bus rooftop ESSE components. The study includes scenarios where neighboring ESSEs are pre-warmed from a recent charging cycle, lowering thermal margin and potentially shortening the time to secondary vehicle involvement. The analysis also examines the potential for roof burn-through, enabling downward flame impingement and elevated radiative transfer into the bus cabin. Particular attention is given to ignition susceptibility of interior seating and flooring materials subjected to sustained heat flux and hot smoke infiltration. Results are used to identify conditions under which the fire is likely to remain confined to the bus of origin versus transitioning to multi-vehicle involvement.
Finally, the paper discusses a layered mitigation strategy, combining hot smoke venting (natural or mechanical), overhead sprinkler protection (including deluge configurations for rapid, area-wide application), early fire detection (e.g., aspirating smoke detection), and fixed ceiling-mounted curtain boards intended to limit smoke-layer spread and reduce radiative exchange between parking bays. Operational measures are also evaluated, including emergency response procedures to relocate non-incident buses, and standard operating procedures that integrate fleet telemetry to monitor ESSE temperatures and charging status for abnormal thermal conditions. Collectively, these measures provide a framework for reducing detection time, limiting exposure severity, and improving incident manageability in enclosed BEB storage environments.
Suggested keywords: battery electric bus; thermal runaway; fire hazard analysis; smoke layer; thermal radiation; vehicle-to-vehicle fire spread; sprinklers; deluge; aspirating smoke detection; curtain boards; telemetry monitoring.

Water-mist fire suppression systems are increasingly used in environments where rapid heat reduction, smoke control, and minimal collateral damage are critical. Despite their growing adoption, many projects lack a structured approach to evaluate mist system performance for both fire mitigation and structural protection. This paper introduces a scenario-driven qualitative evaluation method that integrates Activity Hazard Analysis (AHA) and reliability/availability assessments to guide system design and configuration.

The framework addresses task-level hazards throughout the system lifecycle—commissioning, maintenance, and emergency operations—emphasizing human-factor controls and operational dependencies. Reliability considerations include component failure modes, redundancy, maintainability, and readiness metrics, with design heuristics aimed at eliminating single-point failures and ensuring robust system response.

A key focus is on optimizing mist spray density and distribution for tunnels with large cross-sectional areas and high aspect ratios, where conventional test protocols may underestimate required mist delivery. The framework provides guidance on scaling droplet flux, nozzle spacing, and spray overlap to maintain effective suppression and thermal attenuation in complex geometries. Evaluation also highlights proper system sizing-encompassing pump selection, pressure control, and distribution-line design is essential for ensuring the system can maintain required performance across all operating modes.

Operationally, the evaluation supports coordinated fire department response, highlighting features such as zoned discharge, manual overrides, and compatibility with ventilation systems to enhance responder safety and tactical control. The result is a repeatable, evidence-based approach that enables engineers to compare mist system options, justify configurations, and implement operational controls that improve fire mitigation, responder safety, and structural resilience.
The initial development of this framework was to compare mist-based fire protection systems by their ability to absorb the heat energy of a fire through the evaporation of the mist droplets. This initial product comparison utilized a basic heat transfer analysis to determine the amount of heat absorbed as the mist droplets experience a temperature increase from ambient to boiling before evaporating completely. This heat transfer model will require future refinement to account for the variables that exist in a real-life tunnel fire scenario, but it can be used currently to roughly evaluate the performance of different product offerings for water-mist fire protection systems.

The integration of an existing tunnel ventilation system with newly constructed underground rail infrastructure presents significant aerodynamic, operational, and safety challenges. With the opening of the City Rail Link, the Britomart Station tunnel ventilation system required assessment and validation to confirm it could operate effectively as part of an expanded rail network while continuing to meet congestion management and fire life safety requirements.
To support this integration, a structured program of baseline airflow testing and All Systems Integration Testing (ASIT) was undertaken. The scope included verification of installed fan performance, assessment of airflow distribution within tunnels and station environments, evaluation of aerodynamic interaction between Britomart and the adjoining network, and validation of modelling assumptions used in one-dimensional and computational fluid dynamics analyses. Airflow measurements were complemented by pressure and acoustic assessments to better understand system resistance characteristics and overall ventilation behaviour under representative operating scenarios.
Testing considered multiple operating scenarios, including normal operations, multi-section congestion, and emergency fire modes. Attention was given to cross-system interactions, environmental influences such as portal wind effects, and the influence of existing asset condition on operational performance. Where necessary, operating modes were reviewed and amended to ensure coordinated performance across the expanded system.
This paper outlines the testing methodology, integration challenges, and the structured validation process adopted to confirm ventilation performance within an operational underground rail network.

Road tunnel ventilation systems represent one of the most significant operational energy demands within Australian tunnel infrastructure, driven in large part by stringent portal emissions control requirements. Historically, ventilation design has been guided by conservative emissions assumptions, whereby the World Road Association (PIARC) 2012 report on Vehicle Emissions and Air Demand for Ventilation recommended minimum longitudinal air velocities in the order of 1.0–1.5 m/s to manage pollutant concentrations associated with high-emitting heavy goods vehicles (HGVs).

In 2019, PIARC updated this guidance to incorporate contemporary fleet emissions data, reflecting the progressive decarbonisation and technological improvement of vehicle fleets. The revised report identifies reduced pollutant emission rates and correspondingly lower air demand requirements for both normal and congested operating conditions, challenging legacy design assumptions still embedded in many tunnel projects.

This paper presents a road tunnel case study to quantify the implications of applying updated fleet emissions data on ventilation design outcomes. Comparative assessment was undertaken to quantify the differences in required airflow rates, fan duty, and whole-of-life energy consumption between designs based on superseded versus current emissions datasets.

The findings demonstrate that reliance on outdated emissions assumptions can lead to ventilation system oversizing and overrunning, resulting in unnecessary capital expenditure and operational energy penalties. Conversely, adoption of contemporary fleet data enables right-sizing of ventilation capacity and/or operation and reduction of operational carbon footprint while maintaining compliance with air quality and safety criteria.

The Brazilian national and state highway network is expanding very rapidly, and road tunnel expand both in number and length. As an essential step towards enhanced safety levels, a new national regulation was recently issued, and local regulations are being developed in most states. The implementation of the new regulations is impacting in a significant manner the safety of the extensive national infrastructure.The proposed paper shall focus on the new national regulation and on their practical implementation in new road tunnels. The proposed paper shall touch on the following topics:
•Specific national conditions and issues and their impact on tunnel ventilation
•General objectives of the new national regulation and selected normative approach
•Specific highlights and comparison with leading national regulations
•Illustrative practical applications, with particular focus on the São Paulo state, the Nation’s economic engine

In case of fires in tunnels with bi-directional traffic, the ventilation strategy is a particular challenge. One strategy is to maintain the direction of flow upon fire detection and to  ensure a flow of 1.0 to 1.5 m/s. Focusing on the speed of egress, another strategy is to aim at having about zero velocity at the position of the fire. A third strategy is not to try to intervene and let the smoke spread according to the natural forces.

Norway has numerous road tunnels operated with bi-directional traffic and therefore decided to launch a research project in order to investigate full-scale fires at low flow velocities. Consequently, 11 full scale tests were conducted in the about 1520 m long Runehamar tunnel.

In addition to measurements of the flow velocity, extensive measurements were taken of temperatures. Moreover, videos were taken to establish the smoke behaviour including its stratification. Gas analysis was conducted at one length position.

This paper focuses on the reporting of the tests results in the light of the resulting egress conditions.

Longitudinal ventilation is commonly used in Japan’s expressway tunnels. Some 1,200 jet fans are installed in tunnels managed by East, Central and West Nippon Expressway companies (NEXCO). Jet fans must be installed outside the clearance envelope. The required height for this envelope is mandated in the Japanese Road Structure Ordinance. Recently, the mandated vertical clearance has been increased, from 4.5m to 4.8m, to allow taller trucks to pass and further promote logistics efficiency in Japan. This new clearance height also applies to existing tunnel sections. Signs and lights can be raised where necessary to meet the clearance, but many jet fans cannot be lifted far enough, and must be replaced with smaller diameter jet fans to secure the newly required clearance.
NEXCO’s research institute, NEXCO RI, establishes technical standards for NEXCO-run expressways, including specifications for all mechanical, electrical, communication equipment and architectural facilities. Following prior practice, installed jet fans are mostly 1,030 mm or 1,250 mm diameter. In response to the clearance height change, NEXCO RI, working with manufacturers, developed a new 800 mm jet fan.
Prototype tests were carried out in a full-scale tunnel to quantitatively evaluate the pressure boost by measuring the pressure distribution along the tunnel before and after the fans. In the test, each pressure boost was measured while varying tunnel air speed 0, 2 and 4 m/s under assumed operational condition. As a result, the new 800 mm jet fan demonstrated its performance, proving jet fan could be effectively harnessed even within the restricted spatial constraints. More specifically, the measurement data show not only the pressure rise performance characteristic of the new 800 mm fans, but also the effects of tunnel air velocity. Furthermore, this test provided findings how the pressure rise effect depends on fan installation configurations, such as tandem or parallel two jet fans, and the use of deflectors.
This paper describes the process by which the required performance of the new 800 mm jet fan was achieved, presents the pressure-rise characteristics obtained from various test cases, and reports the adoption of the new jet fan as standard equipment in NEXCO tunnels in July 2025.

Client requirements for ventilation of an access tunnel in the embankment of a proposed 1 km long dam wall included:
– Forced (N+1) ventilation for occupant safety, comfort and extended material and equipment life
– Design for potential inundation
– Maximised use of natural ventilation
Maximised use of natural ventilation led to a hybrid ventilation design whereby natural ventilation driving forces (wind and thermal/buoyancy) were assessed as being adequate with high reliability, with forced ventilation available for times of inadequate natural ventilation. The forced ventilation was then designed for high reliability of control (its ability to overcome natural ventilation driving forces) without redundancy, and high reliability of ventilation, with redundancy, when natural ventilation driving forces are inadequate. The tunnel air flow monitoring and control system was designed to boost natural ventilation when inadequate and cease operation when not required. Jet fan diameter was maximised to minimise power requirements whilst achieving both noise and thrust requirements with resulting jet fan power requirements at each installation of less than 1.5 kW.

Emergency ventilation systems in underground tunnels are critical life-safety systems. Their performance depends not only on fan capacity but equally on the actuator/damper assemblies responsible for smoke extraction and high-temperature exhaust control during fire scenarios. Actuators must operate reliably under highly variable and extreme conditions, including elevated temperatures, dynamic pressure changes, and emergency control demands. The selected actuator technology directly influences safety, reliability, controllability, energy consumption, maintenance effort, and overall life-cycle cost. Inadequate selection can introduce system-level vulnerabilities, whereas an optimized design reduces operational risk, increases availability, and stabilizes long-term operating expenses.

Reliability, Availability, Maintainability, and Safety (RAMS) frameworks define how complex systems are specified, validated, and accepted to ensure dependable performance over their full lifecycle. Safety Integrity Level (SIL) classification quantifies the probability of a safety function performing correctly when demanded. In tunnel ventilation systems, the final element—actuator, damper, power supply, and control interface—often represents the limiting factor in achieving higher SIL levels (SIL 0–4).
Ventilation specifications frequently reference functional safety standards such as IEC 61508 and railway-oriented standards including EN 50126 and EN 50129. While all address SIL designation, they differ significantly in lifecycle models, system boundaries, assumptions, and acceptance criteria. Product-specific standards such as NFPA 130, UL 50, UL 61010-1, UL 429, IEC 60529, and CSA 22.2 No. 139-13 are also commonly cited for dampers, enclosures, and electric actuators.

This paper compares these standards and evaluates electromechanical, pneumatic, electrohydraulic, and magnetic actuator technologies—with and without fire-resistant protection—highlighting their respective advantages, limitations, and risks. Attention is given to factors affecting standard compliance, operational continuity, and total cost of ownership in long-life tunnel infrastructure.

In modern underground metro systems equipped with full-height platform screen doors (PSDs), pressure relief shafts (PRSs) are often provided either at both ends or at a single end of stations, typically in combination with a ventilation shaft. Their primary purpose is to mitigate pressure effects on the PSDs.
For single-bore metro tunnels where the tunnel cross-section can be significantly larger than in twin-bore configurations the pressure effect on PSDs may be acceptable without PRSs. This raises the following questions: Are PRSs still necessary? Are they required for other purposes, such as air temperature control or air renewal along the tunnel?
This paper aims to provide a holistic answer to these questions.
First, the ventilation principles during normal operation are reviewed:
•
Mechanical ventilation using fans.
•
Natural ventilation using the train piston effect, facilitated by a bypass within the ventilation shaft.
•
Mixed solutions combining the above, for example by removing PRSs and utilizing the piston effect through fan shafts, allowing flow directly through the fan blades.
Next, design criteria are defined in terms of air temperature behavior and air renewal requirements. One-dimensional analyses are then performed for different PRS configurations (at both ends of each station, at one end of each station, or at either end) with various ventilation strategies. Characteristic curves for fans operating in freewheeling mode (airflow through stationary blades) are estimated and incorporated into the analysis.
On the one hand, maximizing the number of PRSs reduces fan operating time but increases infrastructure capital expenditure. On the other hand, minimizing the number of PRSs lowers infrastructure capital cost but increases fan operating time and thus operational expenditure. An optimal balance must be identified.
The paper proposes a comprehensive assessment framework that accounts for civil works costs, equipment costs, and operational costs.

 Heat generated by train traction, braking, auxiliary systems, and passengers accumulates in tunnels, leading to rising air temperatures that can affect passenger comfort and equipment reliability. Numerical tools such as SES (Subway Environment Simulation) are commonly used to predict airflow and temperature evolution in tunnel networks. However, the relative influence of key design and operational parameters on tunnel temperatures is not always well quantified. This information is essential for engineers to make informed decisions during project development and to advise on effective mitigation strategies.
This study presents a parametric sensitivity analysis using SES, applied to the future Line 9 of Santiago Metro (Chile), a 27-km underground line with 19 stations. Seven parameters were selected based on their potential impact on tunnel air temperature (train headway, tunnel wall thermal conductivity, deep ground temperature, regenerative braking efficiency, auxiliary heat dissipation per empty car, passenger load, and annual ambient temperature amplitude). Each parameter was varied independently around reference values derived from project data and from a review of previous SYSTRA projects, ensuring realistic ranges. Simulations were run for peak-hour conditions with natural ventilation only, representing the most thermally demanding scenario.
Results show that train frequency has the strongest impact on tunnel temperatures. Regenerative braking efficiency and auxiliary heat dissipation also play major roles. Passenger load, deep ground temperature, and annual temperature amplitude have moderate effects, while tunnel wall conductivity has a negligible influence.
These findings provide quantitative guidance for design choices and thermal regulation strategies. Optimizing regenerative braking, limiting on-board auxiliary power, and managing headway are the most effective levers for controlling tunnel temperatures. The study also helps prioritize which parameters require accurate data and which can be treated with default values, reducing uncertainty in future projects. The methodology can be extended to evaluate mechanical ventilation scenarios and to support the definition of activation thresholds for temperature control systems.

Underground railway stations often experience high concentrations of microparticles exceeding health safety thresholds, particularly on lines with older rolling stock and without platform screen doors (PSDs). These microparticles primarily originate from brake wear and the infiltration of outside air, along with miscellaneous sources. They are transported to the platforms by mechanical ventilation and the trains’ piston effect.
Although numerical modeling is a key tool for assessing this phenomenon, the literature contains very few studies detailing the modeling approach, emission assumptions, and interactions with surfaces, which limits the accuracy of the models. This study proposes an initial approach to simulate particles dispersion, combining a prior analysis of measurement campaigns to establish baseline levels and identify key parameters contributing to particles emission and dispersion. Subsequently, a CFD model [ANSYS] is developed to understand the spatial distribution of these microparticles, incorporating all assumptions and boundary conditions determined beforehand by a 1D model (SES).
The methodological framework was applied to the Nation station (RER A – France) by simulating the stopping and departure of two trains. Brake-related emissions were quantified using operational data and literature sources. Attention was paid to the behavior of particles on the surfaces of moving trains, and parametric studies were also conducted to assess the impact of particle/surface interactions.
The results show that that particle rebound is essential, as it determines the number of particles deposited, re-suspension, and redistributed within the volume over the course of train movements. Although the simulated trends evolve chaotically (as observed in the measurements), some discrepancies were noted due to the simplifying assumptions made and the very limited simulation duration.

Multi-line rapid transit interchanges that share a single, aerodynamically connected station present distinctive challenges for the safe operation of tunnel ventilation systems (TVS) during maintenance and incident response. This paper examines a complex underground interchange where Line A and Line B operate on orthogonal alignments with separate tunnel ventilation systems, but are coupled through a multi-level station volume without fire doors or other barriers to airflow. In this configuration, maintenance activities on either line can generate hazardous atmospheres (e.g. combustion products, silica dust, fumes) that may migrate across the interface, creating shared risks for workers and passengers.

The paper describes a structured work-planning and coordination protocol between two maintenance organizations and a single operator, emphasizing joint hazard assessment, explicit consideration of cross-line impacts, and ventilation-led risk mitigation. It introduces operational “basic rules” and preferred simultaneous TVS scenarios that either move air away from the station on both lines or strictly limit push–pull arrangements drawing air toward the interface, thereby maintaining uncontaminated flow at the Line A–Line B boundary. The paper also presents emergency ventilation strategies involving use of the TVS on the non-incident line to assist the TVS on the incident line during a fire emergency.

The resulting framework links procedural controls (work windows, prioritization, and approval) with engineered controls (pre-programmed TVS modes and directional airflow management) to maintain safe and tenable conditions in an interconnected environment. The paper concludes by proposing these principles to aid in the development of Standard Operating Procedures for multi-line, aerodynamically integrated stations, with relevance to both routine maintenance and emergency response in complex rail tunnel systems.

The Eglinton Crosstown Light Rail Transit (ECLRT) project includes a 19-kilometre corridor with a 10-kilometre underground section and fifteen underground stations supported by a dedicated Tunnel Ventilation System (TVS). The TVS is designed primarily for smoke extraction and smoke control of tunnels and stations in the event of a fire emergency. Across the alignment, the system includes 29 jets fans located at tunnel portals and unique underground locations, and 52 station fans (42 horizontal and 10 vertical) that generate airflow within underground stations.
Modern TVS installations increasingly rely on electronically controlled motor-starting technologies such as Variable Frequency Drives (VFDs) and soft starters to achieve energy efficiency, reduced mechanical stress, and improved operational flexibility. However, these electronic devices introduce potential failure modes that may compromise ventilation performance if not properly mitigated. To enhance system resilience, the ECLRT design incorporates bypass contactors for both VFD-driven and soft-starter-driven motors, enabling automatic transfer to full-voltage across-the-line (ACL) operation in the event of drive faults or electronic control failures.
A Reliability, Availability, and Maintainability (RAM) analysis framework is applied to quantify the improvement is system availability achieved through bypass integration and architectural redundancy. The TVS architecture is intentionally designed to avoid single-point failures and ensure full-speed fan operation during degraded conditions and emergency scenarios.
This work demonstrates that integrating robust bypass paths with modern electronic motor starters provides a practical, fault tolerant, and high-reliability strategy for maintaining life-safety ventilation in large underground transit systems such as the ECLRT.

The body of work was conducted on a metro system during commissioning, which highlighted the need to understand the sensitivity of design input parameters and to perform post-design validation from commissioning tests. The assessment integrates design reviews and extensive field testing to confirm the system’s operational reliability in a complex tunnel environment.
The tunnel configuration includes two parallel rail tracks separated by a concrete centre wall, with periodic cross-passage doors for emergency egress. The tunnel geometry varies along its length, transitioning from rock-bored sections to large, open underground spaces. Station geometric constraints such as fully enclosed platform environments with platform screen doors (PSD), elevators only access between the surface and the below-grade concourse and limited pathways to the surface, connections to adjacent buildings for station entrances and expansive open volumes for a tunnel portal. The distinct geometries introduce dynamic airflow behaviour, including pressure differentials, leakage paths, and potential recirculation zones, all of which were evaluated through field measurements and model calibration.
The studies were a comprehensive evaluation of the as-built Tunnel Ventilation System (TVS), examining its ability to satisfy required critical-velocity and smoke-confinement performance criteria under fire-emergency conditions. The work employed a combination of 1D models (Subway Environment Simulator, IDA Tunnel, and CONTAM), calibrated against real-world airflow measurements, and 3D CFD simulations (Fire Dynamic Simulator and OpenFoam) for areas with complex geometries—such as curved tunnel segments, jet-fan regions, and portal interfaces. Field testing included airflow measurements, annular velocity around rail vehicles, station pressure logging, individual fan airflow measurements, and videography of smoke-movement observations. Instrumentation was placed throughout multiple adjacent tunnel segments to allow correlations of mass flow and understanding of air movement in the system.
The paper will present the following study parameters:
• Modelling and accounting for PSD’s in design assumptions, specifically operational considerations and implications on system performance,
• Design pressure loss and system leakage determinations through PSD’s for the unique system geometry,
• Implications of ventilation design assumptions for constrained boundary conditions, and
• The importance of holistic system testing using multiple air flow measurements during commissioning.

This study explores the potential advantages and trade-offs of using one and two-dimensional computational fluid dynamics (CFD) to estimate vehicle-induced pressure on road tunnels’ ceilings compared to more complex three-dimensional CFD approaches. It examines accuracy, computational cost, and flow representation to guide the selection of appropriate simulation methods.
In this study, two scenarios are considered. In the first scenario, a semi-trailer truck, deep inside the tunnel, travels at a constant speed of 80 km/h. This allows us to assume entrance effects are negligible. The second scenario is a semi-trailer truck entering the tunnel at a constant speed. This scenario has been assessed to identify the capability of different modelling approaches in capturing the transient entry pressures.
The 1D model provides a computationally efficient approach by considering pressure variations along the tunnel length but does not account for cross-sectional flow dynamics. The 2D approach introduces lateral flow effects, improving accuracy in pressure distribution across the tunnel height while assuming uniformity in the third dimension. The 3D model resolves spatial flow structures in full detail, capturing complex interactions between the truck and airflow, though at a significantly higher computational cost.
This comparison highlights the balance between modelling complexity and predictive capability, aiding in the selection of suitable simulation methods.

The Melbourne Underground Rail Project (MURL) was first opened to the public in the early 1980s and consists of 4 underground rail tunnels travelling between three underground stations in Melbourne’s central business district. By the early 2000s, multiple assessments identified that the Fire & Life Safety systems fell short of international standards. A study commissioned by the operator recommended installation of a mechanical platform smoke extraction system to support safe passenger evacuation during fire events.

This paper presents the detailed design development and challenges encountered in retrofitting smoke extraction fans into the existing station shafts. A risk and performance-based design approach was adopted to meet safety objectives within the constraints of the legacy infrastructure.

This paper also presents how digital engineering and CFD modelling were used to assess the performance of the fan airflow paths in the constrained brownfield environment. Point cloud surveys of existing conditions such as back-of-house areas, draught relief shafts, platforms, and tunnels were integrated into a comprehensive BIM model. These were used to create 3D computational fluid dynamics (CFD) simulations to support fan sizing and procurement of the equipment from suppliers.

A changing shift in perspective has been occurring and is one we, as a transit fire safety industry need to be at the forefront of. It is the shift from a single priority, i.e., keeping passengers safe during fire events, to also protecting those who dedicate their lives to protecting ours.

Firefighters acknowledge that they may need to risk their lives during any fire or rescue event they respond to, so it is vital to ensure that their needs during these emergency operations are adequately considered in the design of new transit systems. This shift has been slowly pervading into the mass transit world. Now, with the continued advancement in speed at which 3D simulations can occur we are better able to grasp exactly how bad an environment can quickly become in an emergency, especially at times when the fire service enters an underground area to begin lifesaving and firefighting activities. Some experiences from recent projects working with various fire services included the application or consideration of the following requirements, which are beyond typical code requirements. These applications are described in more detail in this paper.

– Requirements for Emergency Rescue Zones.
– Standpipe requirement on platforms with challenging access for fire hose.
– More stringent Emergency Exit Building (EEB)/Cross Passage spacing.
– Consideration of Firefighter Air Replenishment Systems (FARS) underground.
– Requirement to provide a Safe Firefighter Staging Area in underground stations.

Mass transit systems are a benefit to society, though, a balance needs to be struck between budget and requirements, such that constructing transit stations is not cost prohibitive. Protecting our firefighters and ensuring successful firefighting operations should be a priority, because keeping the fire service safe ultimately keeps passengers safe as well. Knowing the motivations behind the requirements can help us engineer better solutions.

Deep Geological Repositories (DGRs) occupy a space between the fields of both nuclear and mining, both of which are well regulated and have well established knowledge bases. Nuclear stems from theory and academia, while mining originates from a long history of experience. Fire safety in underground mining is approached differently from many other industries. Due to the limited access underground and convoluted network of shafts, drifts and raises, water-based fire protection is not usually feasible. These challenges lead to the use of other methods, among which, most critically is fresh air ventilation. Several smoke ventilation concepts from various industries exist and can also be utilized, which include:

– Longitudinal ventilation and critical velocity
– Point extract/transverse ventilation
– Smoke extract [vents or shafts]
– Oxygen deprivation/gas suppression
– Containment/confinement
– Pressurization
– Compartmentation or partial barriers
– Dilution

This paper will present fire ventilation design considerations for a Deep Geological Repository, that are rooted in the traditions of mining ventilation concepts, though, augmented for the unique nuclear application. In some cases, due to the technically advanced nature of DGR facilities, some fundamental mine principles can be omitted. In addition, well established concepts from the tunnelling and mass transit industry can provide an additional lens through which to design the DGR smoke ventilation strategy. Ultimately, the goal is to design a flexible system that can be adapted to various scenarios, that is also simple, robust and safe.

Lithium-ion batteries (LiBs) continue to proliferate through our everyday life. In our ears, our pockets and in our bags, we routinely carry LiBs throughout the day, and increasingly, they carry us. Incidents involving light electric vehicles (LEVs) (e-scooters and e-bikes) on trains and in underground stations are occurring with increased frequency and so they need to be considered seriously for fire life safety design underground. With the release of toxic and flammable gas mixtures, rapid heat generation and fire growth, the limited egress from a train, and dense occupancy, the potential for harm is significant.

This paper reviews the literature on LEVs within the underground rail context and summarises the mechanisms that cause TR, the results within the unforgiving rail context, how harm can be mitigated, and how residual LEV risks may affect the safety case for the infrastructure.

Incidents involving LEVs in rail environments are discussed, assessing consequences to the infrastructure and vehicles, and identifying design considerations to protect occupants and responders. Hazards are quantified using available data.

Current operational control measures adopted globally are reviewed at high level, with commentary on the effectiveness and practicality of implementation, along with the societal value balances that must be considered. Recommendations are made for design and operation of rail infrastructure and rollingstock, and for government policy on LEV regulation and use.

Fires in tunnels pose a critical threat to structural integrity, operational continuity, and economic outcomes. When concrete is exposed to high temperatures, it undergoes reduced compressive strength, spalling, micro-cracking, and decreased modulus of elasticity. The bond between concrete and embedded steel reinforcement may degrade, porosity increases allowing chloride ingress, and the residual strength of steel and tension in prestressing tendons may be lost.

Exposure to onerous fire scenarios, such as the RWS (Rijkswaterstaat) and RABT-ZTV (German standard) fire curves, can produce extreme heat over short periods, accelerating structural degradation and highlighting the importance of proactive protection measures. It is important to note that the state of concrete after heat penetration is often unknown. Studies show that after cooling from fire temperatures of approximately 800 °C, the concrete’s compressive strength can be reduced to around 15% of its original value, making the material highly susceptible to further damage even under normal loads.

This study considers a fire-exposed tunnel project of 1KM. To mitigate fire-induced damage, passive fire protection systems, specifically calcium silicate boards, are applied as a sacrificial layer to walls and ceilings. These boards insulate the concrete and reinforcement beneath from extreme heat, limiting surface temperature and slowing structural degradation. By reducing the thermal exposure of concrete, they prevent spalling and protect embedded steel, while allowing tunnels to remain operational for longer. The effectiveness of these systems can be assessed using finite element analysis (FEA) to predict temperature distribution and ensure the boards provide sufficient protection under design fire scenarios.

For this fire-exposed tunnel project, the total cost will be estimated, which includes inspection, concrete breakout, materials, labor for shotcrete application, traffic management, and the repair of residual weaker concrete caused by the fire, alongside the cost to repair a tunnel which has been protected with the sacrificial calcium silicate boards, highlighting the large economical saving.

Passive fire protection also enhances insurability. Australian tunnel insurance policies often require fire safety measures for coverage and may limit payouts if passive systems are absent. Installing sacrificial calcium silicate boards or cement based vermiculite sprays demonstrates proactive risk management, potentially reducing premiums and avoiding coverage disputes.

Beyond cost and insurance considerations, passive fire protection supports structural resilience. By maintaining the integrity of concrete segments and embedded steel on walls, ceilings, and curved surfaces, passive fire protection systems extend tunnel service life, reduce the need for intensive post-fire repairs, and limit long-term degradation caused by micro-cracking and chloride ingress.

In conclusion, incorporating passive fire protection boards in Australian fire-exposed tunnels is a prudent, economically sound, and technically justified strategy. It addresses vulnerabilities of concrete and steel under fire, reduces total project life-cycle costs, supports insurance coverage, and safeguards users and stakeholders. Passive boards provide a cost-effective, long-term solution, balancing upfront expenditure against avoided structural damage, while limiting surface temperatures and enhancing resilience under extreme fire scenarios.

Digitalisation is increasingly influencing the design and operation of road tunnel systems. One practical implementation is the use of simulation-based digital twins that replicate the physical behaviour of tunnel ventilation systems using their real control logic. Such models allow engineers to analyse system responses and operational strategies in a virtual environment before the real installation is available.
This paper presents two decades of experience with the application of a tunnel ventilation simulator used as a digital twin in road tunnel projects. The simulator represents the aerodynamic behaviour of the tunnel and calculates parameters such as airflow, air quality and smoke propagation in response to control actions and boundary conditions. By connecting the actual control software to this virtual environment, the interaction between ventilation equipment, control algorithms and traffic conditions can be examined under a wide range of operating scenarios.
The approach enables systematic testing of ventilation control strategies and emergency scenarios that are difficult or impractical to reproduce in real tunnels, such as large fire events or complex traffic conditions. It also supports the verification of control logic during factory acceptance tests and prior to on-site commissioning, allowing potential issues to be identified before the physical installation becomes operational.
Experience from multiple road tunnel projects shows that the digital twin approach facilitates the preparation of commissioning activities and the validation of operational strategies when tunnels are constructed or opened in stages. In such situations, simulation allows extensive testing of the ventilation control system even when the full infrastructure is not yet available for real-world testing. The paper discusses the capabilities and limitations of this approach and reflects on lessons learned from its long-term practical application.