In the confines of a structure with the geometry of a tunnel, a primary safety issue is eliminating the possibility of fire. Complete prevention of a fire event has an improbable chance of success due to the number of parameters involved in the starting of a fire, but measures must be in place to enable action to be taken swiftly to minimise damage to both life and structure.
Low probability, high consequence incidents like the Mont Blanc Tunnel fire, which killed 41 people, and the Tauern Tunnel fire, which claimed a further 12 lives, emphasise the fact that fire safety in tunnels cannot be seen as an afterthought but must be incorporated into the planning, design and construction of tunnels. In the case of the Mont Blanc Tunnel fire, several faults were reported: only one trained fireman was on duty; an out of date ventilation system was in use; and an ineffective warning system was present. These faults were compounded by an inadequate communication system between the French and Italian sides.
Now under consideration is the building of major transport tunnels in twin bore, which will ensure a separate escape route. A second tube for the Tauern Tunnel is currently under preparation for tendering, with completion estimated for 2005.
Computational Fluid Dynamics
Dr Fathi Tarada and Dr Peter Reinke of ventilation designers and consultants HBI Haerter of Switzerland report that the increasing complexity of client requirements has meant a move towards more technologically advanced tools such as Computational Fluid Dynamics for the calculations of 3D flows of smoke in tunnels. HBI is assessing the technical requirements to propose feasible solutions for the ventilation system during construction of the 33km Lötschberg Tunnel through the Swiss Alps.
Rock cover extending to a depth of 2000m poses particular demands on the ventilation and cooling of the work areas. The starting point was the inclusion of specification of air flow rates and air directions (Fig 1). Major design aspects of the system were possible releases of methane from rock and fire scenarios at different locations. Calculation of the continuously changing geometry of the tunnel system during construction was modelled, as were the effects of cooling installations simulated using the specially developed computer program. HBI is currently studying the emergency system in the Mont Blanc Tunnel with a brief to improve the system’s efficiency.
Fire detection
Under the extreme conditions that prevail in a tunnel fire, an early warning fire detection system is an essential means of protection. Such a system will allow automation of the required rescue and evacuation measures, which include: prevention of further traffic entering the tunnel; making emergency lanes available; activating a pre-programmed intervention plan; activating ventilation; and switching on emergency lighting. Training of response staff is crucial.
Because fire spreads so fast, constant monitoring of the tunnel is an overriding necessity. A device that can be used in both construction and operation of a tunnel for detecting possible fire situations is a temperature sensitive cable like the Fibro Laser II from Cerberus. The cable combines laser and fibre-optic technology and provides continuous linear monitoring of the entire area under surveillance and precise location of temperature changes, even with dynamic behaviour. Software allows assessment of the fire’s size and direction of spread.
A key element in the detection and prevention of fires is the use of gas detectors to monitor levels of flammable and toxic gases. These are readily available and come in a range of designs, from portable to wall mounted. The Gasmaster range from Crowcon can detect fires and changes in oxygen levels as well as the presence of flammable and toxic gases. The compact control system can be wall mounted and has as LED display to indicate gas levels, which flashes if a detector senses gas at a concentration higher than the designed range. In the alarm state, both visual and audible warnings are activated. Relays are provided to control other equipment, including ventilation systems, automatic doors and valves, depending on the alarm state of the control unit.
During operation, road tunnels, by their very nature, are prone to traffic bottlenecks, particularly in an accident. Major tunnels have sensors that monitor the slowing or stopping of traffic. This information is relayed to traffic management control centres, where the system or operators can warn traffic to slow down or stop.
However, traffic controllers need to see exactly what is going on to know which actions to take and which emergency services to call. This is where monitoring via CCTV can give an instant picture of the situation. Philips Communication & Security Systems (Philips CSS) produces the LDH 803 camera for such applications. Philips can supply everything from the conventional analogue transmission over copper wire to the most modern Asynchronous Transfer Mode (ATM) digital transmission, which allows new and existing CCTV applications to integrate with other systems.
Using Philips ATM interface systems, it is possible to combine traffic management and other data with video and audio data in one integrated package. Also available from Philips is a public address system that can be used to guide large crowds to emergency exits when there is the danger of fire or further accidents in the tunnel.
Fire fighting and rescue
With the amount of technical equipment available, specialised fire fighter or emergency manager training can be easily overlooked. Fires in tunnels pose many different hazards, and the combination of an accident and a fire is difficult to control because of the limited rescue facilities in a combined space. In the worst case, only the two tunnel portals are available for rescue operations, limiting possibilities for rescue teams and fire brigades to reach the scene of the fire.
RISC Ruhr has a training centre in Dortmund, Germany, specifically designed to build up experience for fighting tunnel fires. The company has two tunnels, a railway line connection and a flashover module for more realistic, safe and environmentally acceptable training. At the centre, trainees learn how to behave for their own safety and how to react in a tactical manner. In the firehouse and the tunnels they can practise various methods which include how to orientate under invisible conditions, how to work with long-term respiratory equipment, and how to find and rescue people under such difficult conditions.
Two of the main dangers in a fire are the associated smoke and fumes. Not only do they suffocate, they make visibility in some instances virtually impossible. TBM Cutters & Wearparts (TBM C&W) is the licensed manufacturer of a unique device patented by Thames Water. It is called the Tunnel Stopper and is an inflatable airbag that plugs the tunnel profile, preventing smoke from filling the whole tunnel. Martin May, Thames Water’s senior safety advisor and inventor of the tunnel stopper, says: “One of the biggest risks to our construction staff and contractors is from fire in one of our underground tunnels . . . the Tunnel Stopper blocks off the smoke and gives valuable extra time to both workers and rescuers.” Workers and rescuers can reach safety through a sealable escape hatch in the centre of the stopper.
The original smoke stopping device is made from frequently welded PVC which, when deflated, has a diameter of 2.8m. The airbag is constructed from a 600mm x 600mm section of tube bent to form a doughnut. The unit is inflated through two non-return inlet valves using compressed air as a power source through a life raft-derived valve. It is housed in a welded steel rectangular box with a roller shutter door on the front, which protects the airbag, and, on activation, rolls back into the housing.
Inside the housing are the roller shutter spring section, two steel compressed air bottles and the inflation valve. A steel gate acts as the operating mechanism. The device is intended for use in tube station corridors, tunnels and underground passageways. The device earned TBM C&W the Beaumont Safety Trophy award last year (1999) from the Construction Health & Safety Group in the UK. Recent research by TBM C&W has resulted in further designs which allow the unit to be used in different situations.
The first smoke stopping devices were designed for use in TBM tunnels, where the main hazard is smoke rather than fire. Now it is intended to manufacture the airbags out of a rubber polymer which has been approved for use in London’s Underground system. Other materials under consideration are PTFE and kevlar derived woven fabric. The use of fire resistant foams and gels, along with water curtains and sprays, is also under review. As each application has its own requirements unique to a particular situation, TBM C&W offers tailored solutions to engineering problems.
Removal of smoke and fumes
The creation of smoke and fumes in a tunnel needs a ventilation system that can cope with the added load of extraction, an important part of this being the damper system. Richard Peers of Howden Buffalo Calidair Division, designer and manufacturer of tailored damper systems, told T&T International, “Fire and smoke dampers form a vital part in tunnel ventilation, whose primary function is to prevent the spread of fire and smoke in the event of a fire. This is achieved by opening the dampers local to the source of the fire and closing the other dampers in the tunnel to cause smoke to be exhausted through ventilation shafts. This enables the performance of the smoke extractor fans in transverse ventilation systems to be more effective as air is extracted local to the fire.”
Howden conducts heat resistance testing as specifications call for operation of dampers at 250°C for up to 2h and, in some cases, temperatures of up to 400°C. Temperature testing depends on the design of the installation and the location of the dampers in the ventilation system. In metro applications, trains are now travelling at high speeds and greater regularity, therefore dampers must be designed to accommodate pressure differentials up to 6kPa.
Calidair damper blades have been subjected to analysis and fatigue testing for 7.8m pressure cycles at 6kPa. Due to the corrosive nature of tunnel environments, the correct choice of material for damper construction is imperative. Howden stresses that stainless steel or galvanised mild steel should be used, with moving parts such as damper shafts and linkage always made of stainless steel.
Woods Air Movement designs high performance ventilation systems that meet exacting smoke extraction and airflow specifications. Woods is currently meeting the air ventilation requirements of Hayden Australia, a contractor for the M5 East transport link project in Sydney, Australia. The $3.2m contract is to supply 140 jet fans and 15 large Aerofoil fans, the largest of which has a 2.8m diameter, for the 560m and 4km long tunnels on the Sydney Orbital road network.
AMCO Plastics manufactures flexible ventilation ducting for tunnelling and mining companies throughout the world. The company produces forcing and extraction flexible plastic ventilation ducting from 200mm to 3000mm diameter. The company can also provide ventilation ducting cassettes that can be configured to mount on any TBM in either steel or fibreglass, depending on the operating characteristics required.
SMJ in the US is a designer and manufacturer of fans; scrubbers; cassette-ducting systems; and accessories for tunnelling and mining. The fan designs include axial flow single and 2-stage; contra-rotating with 2-4 stage operation; and turbo-axial and inline centrifugal systems. Fan diameters range from 300mm to 3600mm. Impeller assemblies are offered in fixed and adjustable at standstill.
The ideas and equipment types in this article are most effective when used in conjunction with each other. A systems approach is key to the design and implementation of an efficient, integrated tunnel safety system.
Related Files
Figure 1: ventilation plan for the Lötschberg base
