Under fire: a review of safety protocol during tunnel construction12 April 2013
The risk of fire in tunnels under construction can be reduced in several ways. Barry O’Donoghue, Mott MacDonald site contract manager; Francois Pogu, Vinci, Lee Tunnel project director; Dave Bulbrook, London Fire Brigade group manager; and Donald Lamont of Hyperbaric & Tunnel Safety review the Storbelt and A86 Socatop tunnel fires, protocol for working with fire brigades and current legislation required for contractors. John Corcoran reports.
The risk of a fire in a tunnel under construction is minimised by good planning, coordination with the emergency services, procurement of equipment with fire safety in mind, and awareness of procedures and documentation.
Case study: Storbelt tunnel fire
The contract involved two bores that were 8km long, 7.7m internal diameter (ID) and 75m below sea level, using four Howden earth pressure balance (EPB) machines. The tunnel-boring machines (TBMs) incorporated twin track within the gantries, and were considered to have advanced firefighting provisions, including two water curtains, AFFF fire suppression systems controlled by a central control panel housed in a non-fireproof container, and ten fire reels. As the control panel was not in an explosion-proof container, it was not connected to the emergency power supply.
On the morning of Saturday 11 June 1994, around 7.30am, just after handover from the nightshift to the dayshift, there was a large flash around the shove rams. Within seconds, the TBM methane alarm cut the mains power to the TBM. Emergency power systems were activated and thick black smoke engulfed the TBM.
The well-drilled crew donned MSA sets and then gathered at the permanently available man-rider at the back of the TBM. The TBM engineer and foreman went to the fire control panel, but this was inactive as it
was connected to the mains supply. Consequently, they were unable to activate the sprinkler system.
Four trained members of the crew donned breathing apparatus sets and operating as two teams carried out a search of the upper and lower levels of the TBM. Once they were satisfied that everyone was accounted for, the crew evacuated the TBM. Those working on adjoining cross passages and the second tunnel drive also evacuated to the surface.
The fire crews tried several times on the day of the fire to enter the tunnel. They also attempted to introduce CO2 into the tunnel air mains to contain the fire, but this failed due to freezing. During the day of the fire the ventilation system was shut down.
Entry was only possible towards the end of the second day, when it was apparent that extensive damage had occurred to the tunnel lining, with up to 200mm of the 400mm-thick concrete tunnel lining spalling.
The initial fuel source of the fire was attributed to hydraulic oil atomising from a hydraulic ram. The source of ignition was never established. There were no injuries and the crews were well drilled in emergency procedures.
There had been a fire on one of the adjacent machines the previous year, which had resulted in an extensive review of procedures. Emergency drills were also emphasised due to the risk of flooding - particularly from cross-passage excavation - being carried out at the same time as TBM excavation.
Extensive repair work was required following the fire. The site took the precaution of constructing two water-tight bulkheads at the portals prior to repair work, and a third bulkhead to enable compressed air to carry out these repairs. The fire extended the contract by nine months at a cost of $33 million.
Case study: A86 Socatop tunnel fire
The 10km, 11.5m internal diameter A86 Socatop tunnel was being excavated in 2002 using a TBM capable of working either in EPB or slurry mode (at the time of the fire, it was in EPB mode). This unique tunnel to the west of Paris was part of the circular road being constructed around the French capital.
The tunnel was split into four horizontal levels, which were created during construction. Two horizontal slabs were concreted to split the tunnel into separate chambers: the upper part for the conveyor and ventilation, the lower part for TBM traffic.
The tunnel train was approximately 70m long and was powered by diesel engine. The tunnel concrete separation slabs were being constructed using timber and plywood, and were being installed as the tunnel was being excavated.
During the planning stages, extensive reviews of fire scenarios were prepared, including fires occurring at any of the three levels within the compartmentalised tunnel. The TBM was equipped with a man lock to gain access to the cutterhead, and there were fire detection and fire suppression systems, water screens and sprinklers. In addition, the Paris fire brigade was involved with the project, and monthly inspections and training drills were carried out.
On 5 March 2002 at 10.30pm, one of the diesel locomotives caught fire when situated under the tunnel timber formwork. Four trained men tried to fight the fire and the TBM crews were informed. The TBM crews activated the rear fire-curtain and went to the front of the TBM. As the fire blazed, the tunnel ventilation collapsed. Communications were lost and the ventilation was switched off. The Paris fire brigade tried, but was unable to gain access to the TBM.
A total of 19 operatives on the TBM were trapped for nine hours. They did not panic and remained in the TBM man lock as required by their emergency procedures. Within the man lock was water and fresh air. It had subsequently been calculated that there was 48 hours of breathable air supply available to them.
Once the fire brigade managed to get through to the TBM, all 19 operatives were safely removed over a three-hour period. There was one injury (an ankle sprain) to a fireman due to falling through an opening between the first slab.
The 400mm-thick concrete tunnel lining was damaged due to the fire. It had behaved as the designers had predicted during a fire, with up to 100mm of spalling.
The cause of the fire in the diesel engine was attributed to an oil leak in the turbo. Following the fire, all diesel locomotives were changed, the fire-suppression systems were improved, and the durability of the tunnel communication systems was upgraded. The fire plan was revised, training was reinforced and goggles were supplied with the tunnel MSA re-breather sets. Fire and smoke suppression systems on the TBM were improved.
The fire brigade's expectations upon arrival on site
The purpose of the fire and rescue service in law is a responsibility to plan for operational incidents, to carry out a preventative function and, fundamentally, to respond to emergencies. In addition, following a fire, the service is charged with carrying out investigations into the causes.
The brigade acknowledges that the circumstances when dealing with a fire in a tunnel under construction can often involve additional challenges to its more normal mode of operation when dealing with a fire in the conventionally built environment.
If a fire occurs in a tunnel environment, communications are vital, as is the water supply. The lack of ventilation and long travel distances associated with a tunnel fire place additional stress on a firefighter, while the manual handling consideration can also cause significant physiological effects.
For example, a single 20m length of water hose weighs 100kg. This is cumbersome, and if a number of lengths are required, will significantly impact on the commencement of a firefighting attack in terms of personnel and breathing apparatus. Construction sites present challenges, which can be overcome with proper planning, preparation and communications, particularly on arrival.
A firefighter in full gear relies on breathing apparatus and on integral communications, which are intrinsically safe for hazardous environments. In the past, contractors have wanted to issue hand-radios to the firefighters, but this presents problems.
For a start, it takes away the use of one of their hands. Heat stress can have a detrimental effect and thought processes become more difficult. Tunnels are enclosed and can be a very stressful environment during an emergency situation. A dedicated radio channel for the emergency services does involve additional cost, but is fundamental if those services are to operate efficiently.
Water supply is also crucial, and history has shown that if enough water is available, the fire will eventually be put out. It is also important to minimise the amount of combustible material permitted within a tunnel environment. The heat that can develop in a confined space is immense, and can make conditions intolerable for human activity if combustibles are not properly managed.
It is vital that upon arrival the fire service is given current, accurate information. This includes the number of people involved, the best access to the incident - including any new construction points of access - newly constructed shafts and cross passages, smoke curtains, fire-suppression systems, the location of any refuges and bridgeheads.
This information can make a crucial difference to decisions that need to be made by the incident commander. A key requirement is a responsible person who meets the fire crew on arrival, and can assist the incident commanders in making decisions based on a dynamic risk assessment. The better the information available, the better the decisions that are made. If there are lives to be saved, a higher level of risk will be taken by the fire and rescue service, but without lives at stake, less risk will be taken.
During the planning stage it is important for the contractor to work with the fire and rescue service to ensure that information is given to local fire stations so that site-specific plans can be prepared and initial plans are in place. This gives the fire crews a vital headstart when they arrive at an incident. This should be viewed as an ongoing process, with any significant change to access or response facilities communicated to the service.
The overall responsibility for the rescue ultimately resides with the brigade's incident commander, but incident management is a team response and the responsible person has an important role to play.
Also vital is how many people are within the tunnel. Increasingly, more complicated systems are being used to record the number of people underground. How readily available is this information? Is it kept at a central location? Can it be made immediately available to the incident commander? If lives are at risk, it makes a huge difference to the decisions the incident commander has to make.
On large contracts it is a good idea to have one point of contact in the brigade, who will develop fire and rescue policy. On larger projects, a standardised policy for levels of fire protection, as well as controls and emergency procedures that need to be put in place when dealing with an incident, are recommended.
Flammables and cylinders present particular issues. For example, the brigade no longer closes down large areas of London when a cylinder is alight, but acetylene in particular will lead to an area being closed down due to the risk of explosion. It is recommended that a tally system is put in place when cylinders are in use, and in the event of an evacuation, to get the cylinders out.
One other observation involves the human element of barriers between contractors. The brigade will try to overcome these personality issues, but if it cannot, then it will escalate the issue to the client. The brigade is always keen to develop good relations.
The fire and rescue services have a national incident command system. The contractor's responsible person will be an equal partner in this system. On arrival, the brigade requires quality information on which to base its decisions. This contractor's responsible person should be easily indentified so the brigade can trust the level of information being supplied. This person then forms part of the brigade's response team.
This article was first published in Urban Transport Agenda's sister publication, Tunnels & Tunnelling International.