Tunnel Failure

Unique identifier / Notation

TL0213

Synonyms

Tunnel collapse (including tunnels, caverns, tunnelling incident, shafts and associated underground structures howsoever constructed and including the renovation of existing underground structures) underground failure subsidence events underpass failure passage failure

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Tunnels are artificial confined underground structures, which are used for different purposes; where, for example, collapses, fires, explosions and water ingress may damage tunnel facilities and cause injuries and human casualties, resulting in severe social harm (Adapted from Zafirovski et al., 2018 and Chien and Chao, 2021).

Primary reference(s)

Zafirovski, Z., Gacevski, V., Lazarevska, M. and Ognjenovic, S., 2019. Procedures for risk analysis and management in tunnelling projects. In E3S Web of Conferences (Vol. 135, p. 01001). EDP Sciences. Accessed 19 March 2025.

Chien L. L. and Chao F. C., 2021. Lessons learned from critical accidental fires in tunnels, Tunnelling and Underground Space Technology, Volume 113, 2021, 103944, ISSN 0886-7798. DOI: 10.1016/j.tust.2021.103944. Accessed 19 March 2025.

Annotations

Additional scientific description

Tunnels are civil engineering structures that are constructed for the purpose of securing space in the ground. They are categorized based on their intended use, such as road tunnels, railway tunnels, storage and waterway tunnels. Additionally, tunnels can be further classified by location conditions into mountain tunnels and city tunnels (JICA, 2018). Tunnels generally refer to online structures in which a predetermined cross-section is continuous in the longitudinal direction. According to the OECD definition, a tunnel is a cavity which is located below the ground surface and is used in some way and made into a prescribed geometry and has a cross-section of two square meters or more (OECD, 2001).

Tunnels are civil engineering structures designed to secure underground space. Their uses include transportation (road, rail, metro), hydraulic purposes (water, stormwater and wastewater), utility corridors (electrical, communications, gas), underground facilities (warehouses, hangars), renewable energy including hydropower, energy storage and nuclear waste disposal and scientific research applications.

Tunnels can be categorized by their primary function:

Transportation tunnels (railway, roadway, pedestrian, metro)
Hydraulic conveyance tunnels (water and wastewater, flood mitigation,)
Utility distribution tunnels (electrical, telecommunications, gas, heating)
Industrial and military tunnels (aircraft hangars, submarine shelters, underground storage facilities, industrial plants)
Tunnels used as part of hydropower installations and energy storage
Scientific and experimental underground spaces (Zafirovski et al., 2019)

Tunnels differ in structural form primarily influenced by functional requirements, but also in relation to the ground conditions (geotechnical factors) and the method of construction to provide ground support (lining). Lining materials include rock anchors, sprayed concrete, pre-cast segmental linings, cast-in-place concrete, and prefabricated steel or cast-iron segments.

Tunnel failure refers to the inability of a tunnel or its components to perform its function as specified by its design and construction requirements. This definition includes total collapses, partial collapses, excessive deformations, water ingress, fire, explosion, gas incursion, ground instability due to mining activities, tunnelling near weak surface structures, and sometimes excessive ground settlement leading to damage to overlying structures. Tunnel failures may occur during construction, during operational life, or due to external environmental impacts such as flooding, seismic activity, or fire.

Despite the severe consequences when failures do occur, the number of tunnel incidents worldwide remains exceptionally low in proportion to the number of tunnels constructed and operated daily. The underground environment, when properly engineered and managed, offers resilience and safety benefitting from the natural confinement provided by surrounding geology and the protection it offers against surface-level risks such as wind and extreme temperatures. Tunnel structures generally exhibit superior seismic performance compared to above-ground structures (Huan S et al 2024); however, they can still sustain catastrophic damage during powerful earthquakes particularly when crossing active fault zones (Zhang et al., 2023)

Operational risks include ageing infrastructure, changed use, operational error, corrosion of reinforcements, overloading, or inadequate maintenance. The most severe, but relatively rare incidents involve structural collapses, sometimes trapping workers or passengers inside. In rescue scenarios, further collapses or damaged ventilation systems can pose life-threatening conditions. Similarly, the 1999 Mont Blanc Tunnel fire, which killed 39 people, highlighted the fire risk from vehicle engines, emergency evacuation routes, advanced smoke extraction systems and emergency ventilation. A comparative analysis of the Mont Blanc, Tauern, and Gotthard tunnel fires (Voeltzel & Dix, 2004) emphasized the importance of risks from the users of the tunnel, rapid fire detection, inadequate emergency regulation, effective operator response, and timely self-evacuation by users.

Metrics and numeric limits

No standard reporting currently exists on tunnel failures, accidents or collapses. Various reports exist including information on previous large tunnel fires (OECD, 2001 and Chien & Chao, 2021) and studies on tunnel collapse failures such as Spyridis & Proske (2021).

Key relevant UN convention / multilateral treaty

The United Nations Committee of Experts for the Transport of Dangerous Goods enhances global transport efficiency through a harmonized regulatory framework (ECE Inland Transport Committee ADR 2024, Volume I and Volume II)

Amendments to and Implementation of the 1968 Conventions on Road Traffic and on Road Signs and Signals and the 1971 European Agreements Supplementing them: Safety in Tunnel (UNESC 2003)

Drivers

Tunnel failures are much more likely to occur during construction than during operational phases. When they occur, it may be from a combination of factors including workmanship, design, construction sequencing, intrinsic geotechnical (ground) conditions due to unanticipated rock behaviour, high ground stresses, karstic voids, and unanticipated water ingress. Failure to manage the design and construction process in view of the ground conditions whether from insufficient ground support, or inadequate ground treatment or drainage or operation of specialised plant and equipment, during the operation of a tunnel, there are potential fire and explosion risks with vehicle fires, hazardous material spills, gas explosions, electrical system failures (Chien & Chao, 2021). Driver error or communication breakdown are also among human hazard drivers.

Some of the most common tunnelling hazards include tunnel stability issues, such as rock falls and rock bursts, and changing ground conditions, including variation in strata and stress fluctuations. Furthermore, since it has limited space and access challenges, it is vulnerable to air contamination, oxygen depletion and risks of fire or explosion (Safe work Australia, no date). While the tunnel is in service, the open ends of a tunnel are susceptible to natural and meteorological disasters such as rock falls, avalanches, debris flows and earthquakes (JICA, 2018).

Impacts

The primary threat in the event of a tunnel collapse is the direct crushing impact caused by falling concrete, rocks, structural elements and flood water. If individuals become trapped under debris, or behind debris and rescue operations are delayed, their chances of survival decrease. Additionally, damage to the ventilation system may lead to oxygen shortages and toxic gases asphyxiation. The collapsed structure can also obstruct rescue teams' access, and the unstable conditions may result in further collapses during rescue efforts.

Furthermore, vehicle collisions or damage to electrical systems within the tunnel can significantly increase the risk of fire, and if hazardous materials are involved, there is also a potential explosion risk (Li et al, 2022; Tyagi, 2024; Pireddu, 2022).

Given these complex risk factors, tunnel failure and/or collapse pose severe dangers, necessitating a swift and well-coordinated emergency response. Comprehensive safety management to prevent accidents, along with the establishment of an effective rescue system, is essential. Additionally, failure of a tunnel can result in substantial economic, social, and cultural impacts.

Multi-hazard context

The figure below summarises common interactions between tunnel failure and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

Risk management in tunnelling requires a multi-disciplinary approach involving owners and operators, engineers and geologists contractors, and policymakers. Proprenter (2018) outlines key mitigation strategies:

Advanced monitoring and early warning systems: AI-based predictive modelling (Zhang et al., 2023), geotechnical instrumentation, and real-time ground movement tracking
Robust design and planning: Incorporating observational methods, stress modelling, and thorough site investigations
Resilient infrastructure: High-performance linings, fire-resistant materials, and redundancy in ventilation and drainage systems
Emergency response preparedness

The IMIA/ITA Code of Practice for Risk Management of Tunnel Works (2023) recommends fundamental good practices for avoiding losses and mitigating consequences including:

Competency and resource adequacy among all stakeholders
Risk-aware culture and open information sharing
Defined risk allocation in contracts
Timely resolution mechanisms for engineering disputes
Thorough site investigations prior to awarding contracts
Independent supervision and real-time monitoring during construction
Integrated health and safety planning before excavation begins

The geological composition of tunnels is inherently intricate, and potential disasters can be severe, with many underlying risks (Cao et al., 2018; Zhu et al., 2020). Since tunnel collapses can result in significant casualties and property damage, risk prediction and early warning are essential. For example, soft rock tunnels have a high risk of collapse during construction. An early warning system helps detect and predict large deformation risks in advance, allowing for appropriate countermeasures. In assessing these risks, hydrogeological conditions, design factors, and construction factors serve as the primary indicators, while rock quality index, groundwater influence, and tunnel depth are considered secondary indicators. AI models can be utilized to monitor data, enabling the early detection of potential risks (Zhang et al., 2023).

Monitoring

Structural health monitoring over the lifecycle of the structure is used in some countries (Stepanovic et al., 2020). Structural Health Monitoring comprises the use of permanently installed sensors or instrumentation, combined with data analysis, to generate data that permits the continuous monitoring of structures such as bridges, dams, tunnels and buildings. This allows engineers to make timely, informed decisions regarding the monitored structure’s integrity, detect structural defects, predict or estimate a structure’s remaining life span and maximise the opportunity to operate the structure safely.