Designing More Resilient, Secure Tunnels with Passive Fire Protection Methods

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Passive fire protection is intended to contain fires, slow the spread of fires, and prevent structural collapse in tunnels.

Today’s complex underground structures and tunnels must be made ready for potential incidents affecting tunnel structures and services, especially tunnel fires. Fire protection comes in two specific forms, active and passive systems. Active systems are in the form of suppression, extinguishers, sprinklers, and extract ventilation. Passive fire protection (PFP) is an integral component of structural fire protection and fire life safety and is intended to contain fires, slow the spread of fire, and prevent structural collapse due to prolonged fire exposure through use of fire-resistant walls, floors, doors, and compartmentation (amongst other examples).

Passive fire protection is vitally important for tunnels and other underground structures because the large heat release rate potential (e.g., 10–20MW), limited escape facilities, and difficulties encountered by emergency services in gaining access for firefighting and rescue, can mean substantially increased risk factors. Providing passive fire protection will allow safe evacuation and emergency team access for firefighting. This article discusses explosive spalling during a fire incident in a tunnel and the different types of passive fire protection available for tunnels.

Explosive Spalling

During a fire incident, protection of the tunnel walls from explosive spalling (which refers to a sudden and violent breaking away of a surface concrete layer) is very important. In general, explosive spalling is likely to be damaging to the concrete as the heating period increases and will lead to structural collapse if the section is reduced substantially. Explosive spalling occurs due to the release of water content within the concrete when subject to temperatures above 250˚C and varies with many factors such as moisture in the concrete, aggregate size, applied load, concrete grade, and reinforcement concrete cover. The occurrence of explosive spalling depends on several factors:

  • the strength of the concrete (density);
  • the heat build-up rate;
  • the compressive stress in the concrete member; and
  • the thermal stability of the aggregate.

Methods of Passive Fire Protection

Passive fire safety provisions recommended in Eurocode 2: BS EN 1992-1-2 Design of concrete structures, indicate that the following methods may be adopted:

  • Providing a thermal barrier (either a protection board or sprayed fireproofing lining);
  • Adding polypropylene fiber (PPF) to concrete; and
  • Adding a sacrificial concrete cover (SCC) and providing a reinforcing mesh within the concrete cover.

The first two methods of passive fire protection provisions are recommended in the Guidelines for Structural Fire Resistance for Road Tunnels, published by the International Tunneling Association with cooperative effort of the World Road Association (PIARC); and the Handbook of Tunnel Fire Safety, published by the Institution of Civil Engineers and adopted in many tunnel projects worldwide. In general, with suitable surface roughness (e.g., 3mm) stated in the specifications, the provision of a thermal barrier will not affect the friction factors used for the tunnel ventilation.

One caution in using a thermal barrier in the tunnel lining, the tunnel will converge in the first few months to two years after construction to suit the ground conditions. As thermal barriers are rigid systems, frequent inspection and rectification may be required in the first two years when the tunnel converges. Whilst it is possible to carry out a visual inspection of the protection boards installed, inspection of the sprayed fireproofing lining can only be carried out by a hammering check or infra-red detection to locate any void behind the lining.

The use of a sacrificial concrete cover may not meet the requirement for avoiding explosive spalling and limiting temperature increases in the concrete, and reinforcement can only be used with careful verification and testing.

The installation of passive protection for a tunnel may reduce the space for tunnel systems (e.g., space for environmental sensors in a road tunnel) and may have an impact on the installation requirements and sequences for various tunnel services.

WSP | Parsons Brinckerhoff’s Recommendations

Passive fire protection (PFP) was used in the tunnels of the Hong Kong Express Rail Link, a high speed railway, and the Hong Kong Mass Transit Railway (MTR) South Island Line. PFP has now become a standard design for tunnels in Hong Kong MTR projects. In one of the recent passive fire protection projects done by WSP | Parsons Brinckerhoff, the following recommendations were made:

Polyprolyene Fibre Tunnels Passive Fire ProtectionFigure 1 - Microscopic view of Polyprolyene Fibre
  • A thermal barrier (sprayed or board) is recommended for a tunnel boring machine (TBM) excavation method using segmental concrete linings, including seismic joints made up of steel rings and rubber gaskets. There is a need to protect the gasket steel ring precast elements from excessive temperature during a fire. Sprayed non-vermiculate refractory cement (NVRC) fireproofing lining is preferred, as the concrete surface is curved. Intumescent paint and boards will be used at the seismic joints.
  • For the cross passage lining, special fire protection is not required as the area inside the cross passages is small and separated from the main tunnel by fire-rated doors.
  • For mass concrete lining in NATM1 tunnels, adding polypropylene fibre (see Figure 1) as passive fire protection to prevent explosive spalling of the concrete may be considered. The polypropylene fibre will be evaporated when subject to heat above 180˚C and form micro-cracks to release the water vapour, preventing the occurrence of explosive concrete spalling.
  • For cut-and-cover (C&C) tunnels, the concrete members behave as frame structures and maintaining the temperature of the rebar is critical. Thermal barrier (sprayed NVRC fireproofing lining or composite cement/galvanized steel board) is recommended.
  • Based on computational fluid dynamics (CFD) test results, the ventilation shafts will not be directly exposed to temperatures higher than 250°C. Hence, additional fire protection for concrete is not required.
  • It is recommended that the anchors for heavy equipment on the soffit of the tunnel be protected by intumescent paint and that mechanical expansion anchors be used.
  • For plant rooms in the ventilation buildings, concrete cover requirements for minimum fire resistance as per the local code is recommended as they will not be subject to the tunnel design fire and the associated fire curve.

Table 1 provides a comparison of the different means of passive fire protections.

designing resilient tunnels passive fire protection chartTable 1– Comparison of different passive fire protection measures (source: WSP | Parsons Brinckerhoff)


Designing more resilient and secure tunnels is a top priority for the engineering community, and this includes minimizing the impact of fire on tunnel structures and systems, especially those structures and systems involving life safety. The passive fire protection measures discussed provide fire protection and hardening of tunnel structural components. This, along with other fire life safety systems, helps provide passengers with safe egress in the case of a fire event, as well as protecting first responders. The measures also protect the tunnel structure from explosive spalling of concrete, ensuring that during and after a fire the structure withstands service loads and that damage to the tunnel structure is repairable, thereby minimizing economic impact.


  • PIARC Road Safety in Tunnels (1995)
  • Russell, Henry A (2004), ITA Guidelines for Structural Fire Resistance of Road Tunnels, WG6 International Tunnelling Association,
  • BSEN 1992-1-2(2010) Eurocode 2 Design of Concrete Structures Part 1-2 General rules - structural fire design
  • Yoshikazu OTA - Universal design concepts of mitigation measures to tunnel structures in case of fire
  • Clement, Frank: Zamecnik, Michal (2007) Fire Protection Options for Concrete Tunnel Linings, Tunnel Issue 1/2007
  • Khoury G.A. (2000) Effect of fire on concrete and concrete structure; Progress in Structural Engineering and Materials, Issue 2
  • Khoury G.A. (2008) Passive fire protection of concrete structure, ICE proceeding of structure and building, June 2008
  • Promat (2008) Tunnel fire protection for tunnel services and structures
  • Smith, Ken and Atkinson, Trevor (2010) PP fibers to resist fire induced concrete spalling, tunnel talk

1New Austrian Tunnelling method (NATM), also known as Sequential Excavation Method (SEM) describes a popular method of modern tunnel design and construction.

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