Overview

Location:	Between Pas de Calais, France and Kent, U.K.
Fire Event:	18 November 1996 A truck fire on a freight train towards England developed into an intense tunnel fire at 19 km into the tunnel from the French end, severely damaging the concrete lining and tunnel facilities. Nobody died. Fire duration = over 7 hours
Fire Damage:	Extensive tunnel lining spalling and damage to tunnel facilities over a length of 480 m.
Construction Type:	High strength reinforced concrete or cast iron linings
Fire Resistance:	No fire protection. No sprinklers.
Function:	Rail tunnel
Dimensions:	Length = 50 km Internal diameter of Running Tunnels = 7.6 m Internal diameter of Service Tunnel = 4.8 m

The Tunnel

The Channel tunnel, also known as Chunnel tunnel or Eurotunnel, is a railroad tunnel beneath the English Channel connecting Coquelles, Pas de Calais region in France and Cheriton, Kent in England. The tunnel has a length of approximately 50 km, of which 37 km are under the English Channel.

The tunnels were mainly constructed in a Chalk Marl layer about 40 m under the sea bed. Chalk Marl has high clay content and is relatively impermeable to water, which provides the ideal condition for a underwater tunnel.

The Channel Tunnel comprises 3 separate tunnels. The outer North and South Running Tunnels of 7.6 m diameter are 30 m apart, with each containing a single lane railroad line. The middle Service Tunnel of 4.8 m diameter is for maintenance and emergency. The Running Tunnels are connected to the Service Tunnel by cross passages at every 375 m. The two Running Tunnels are also connected by Piston Relief Ducts of 2 m diameter at every 250 m to balance the air pressure due to the "piston effect" caused by the passage of trains.

Most part of the tunnels is lined with precast high strength reinforced concrete lining rings of 1.5 m wide, with a thickness varying from 400 to 800 mm depending on the loading conditions. Where concrete lining was inappropriate, cast iron lining rings were used.

It is interesting to note that due to a poorer impermeable soil condition, the French undersea tunnels were lined with a water-tight bolted and gasketed segmental concrete lining. The English tunnel lining was a concrete expanded segmental lining with grouted voids to control water ingress.

Fire Protection System

The fire protection systems of the Channel Tunnel at the time of the 1996 fire are listed as follows:

Fire Protection System	At the Time of Fire in 1996
Fire protection to concrete lining	No
Compartmentation	Airlocks at the entrances to the Service Tunnel Exit of each cross-passage into the Running Tunnel has a fire-resistant door which is normally closed Piston Relief Ducts have dampers which are kept open during normal operations The undersea crossovers have fire-resistant doors which separate the tracks in the two Running Tunnels
Fire detection systems	The system comprises 33 detection stations in each Running Tunnel. Each detection station comprises: Ultraviolet and Infrared flame detectors Optical and ionization smoke detectors Carbon monoxide (CO) detectors Aspiration tubes around the circumference of the tunnels directing gases to the analysis units An unconfirmed alarm is triggered by the activation of a single ionisation or optical detector A confirmed alarm results from the activation of either a flame detector or from both an ionic and an optical detector Such two alarm levels is used to reduce false alarms
Water pipe system	250 mm diameter wet main along the Service Tunnel 100 mm diameter wet main along both Running Tunnels Wet mains inter-connected at cross passages, supplying hydrants at every 125 m in the Running Tunnels Total capacity of 4000 m3/hr
Ventilation system	The air pressure in the Service Tunnel is maintained at a higher level than that of the Running Tunnels to prevent smoke from entering the Service Tunnel and allow a "bubble effect" to be created at the opening of a cross-passage door The Supplementary Ventilation System functions to clear smoke away from any area in the tunnel to enable any emergency service
Emergency response teams	First Line of Response (FLOR) teams are stationed at the Emergency Centres near to the Service Tunnel portals Second Line of Response (SLOR) teams are the Kent Fire Brigade and the Fire and Medical teams in the Pas de Calais region.

The Fire

On 18 November 1996, a truck fire on Heavy Goods Vehicles (HGV) shuttle No 7539, travelling from France to England, forced the train to stop in the South Running Tunnel at about 19 km from the French entrance. The fire emitted intense smoke which rapidly engulfed the Amenity Coach and the front locomotive, preventing immediate evacuation of the 31 passengers, 2 crew members and the driver onboard. The evacuation could only commence about 23 minutes later.

The original drive-through strategy allowed the fire developed substantially while the train was still moving in the tunnel. After the train had stopped, fire development rapidly accelerated, first towards the front of the train due to the "piston effect". The fire spread towards the rear after the Supplementary Ventilation System had been activated.

A brief account of the fire development is given as follows:

Time	Fire Development
21:48 (French time)	The train entered the South Running Tunnel. A 1~2 m fire flame was seen beneath a lorry abroad the train by some security guards and reported to the Terminal Control Centre in the French terminal.
21:49	The Terminal Control Centre informed the Rail Control Centre. Tunnel fire detection system gave first "unconfirmed" alarm.
21:50~21:52	Four further "unconfirmed" alarms. The Rail Control Centre informed the train driver of the possible onboard fire and the train would be diverted to the emergency siding in the UK terminal. The onboard fire alarm system warned the driver of a fire in the rear locomotive.
21:53	A fire on the rear locomotive was confirmed by both onboard and tunnel fire detection systems. The train had travelled 10 km into the tunnel.
21:56	The French First Line of Response (FLOR) team comprising 8 firefighters left the French Emergency Centre.
21:58	The train stopped adjacent to the cross-passage at PK 4131.
22:01	The train driver was trapped in his cab and the passengers could not be evacuated due to dense smoke.
22:02	The French FLOR team entered the Service Tunnel. One minute later, the UK FLOR team also entered the Service Tunnel.
22:22	Supplementary Ventilation System had been reconfigured to move smoke along the South Running Tunnel towards France. The train passengers were evacuated.
22:28	The French FLOR team arrived at cross-passage 4131 and saw the evacuated passengers. The train driver was later rescued from his cab.
22:53	The UK FLOR team entered the South Running Tunnel to inspect the exact location and extent of the fire. It was found that the fixed tunnel equipment had been damaged and five wagons were involved in the fire at the rear rake of the train.
23:39	Fire was confirmed between cross-passage doors 4163 and 4201. In the following 5 hours, the fire was attacked by the combined force of the French and UK firefighters.
05:00	The centre of the fire was extinguished. Minor fires were extinguished during the early morning. Smouldering debris continued to be dealt with until 03:00 on 20 Nov.

The Damage

The fire caused considerable damage over 480 m long of the tunnel structure including:

Damage Zone	Length	Extend of Damage to Concrete Lining
Extreme damaged zone	50 m between PK 4186 and PK 4191	In many places, the lining thickness was reduced to an average of 17 cm In a few places, as much as 40 cm thick concrete spalled, leaving only 51 mm of concrete remaining and exploding all steel reinforcement No damage to concrete grouting and rock The whole section was reinforced and rebuilt
Severely damaged zone	290 m between PK 4180 and PK 4209	In many places, the depth of concrete spalling was between 5 and 20 cm, exploding the first layer of steel reinforcement The lining was repaired without replacing the steel reinforcements
Substantially damaged zone	480 m between PK 4172 and PK 4220	Superficial damage to concrete in some places, but the steel reinforcement was not exposed The lining was repaired without replacing the steel reinforcements

Besides the concrete lining, the cross-passages and Piston Relief Ducts near to the fire, the walkways and the concrete track-bed were largely undamaged.

In addition, the tunnel equipment over a considerable distance was seriously damage by the high temperatures and smoke, including:

• Over 500 m of railway track and supporting track blocks
• Over 800 m of traction power catenary
• Several kilometres long of electrical supply and fibre optic communications cables, together with some lighting systems, fire detection stations, signalling systems, and electromechanical equipment for cross-passages and Piston Relief Ducts

The fire had little effect on the front rake of the freight train but the rear rake was very severely damaged. Ten HGV wagons and their contents and the rear loading wagon were completely destroyed. Three wagons and the rear locomotive were seriously damaged.

Analysis

The 1996 fire highlighted the potential disaster of explosive spalling behaviour of high strength concrete (HSC) in high temperatures. During the fire, large quantities of concrete exposed to the fire had spalled off from the tunnel lining. This resulted in very fine concrete rubble collecting on the access walkway and the roof of the HGV wagons. The consistently falling off of the hot concrete debris endangered the life safety of the emergency personnel who were carrying out the rescue and fire fighting missions.

The extend of the concrete spalling was shown by the ultimate collapse of the roof of some HGV wagons in a "V" shape due to the weight of the collecting concrete debris. Thankfully, the extensive spalling of the concrete lining did not endanger the stability of the tunnel.

The experimental studies in history have shown that moisture content, strength and stress levels are the main factors governing spalling of concrete at elevated temperatures. Compared to normal-strength concrete, HSC subjected to high temperature heating has a higher susceptibility to explosive spalling. This may be due to the following reasons:

• low permeability of HSC retains the moisture inside the concrete, resulting in a high moisture content
• dense cement paste prevents heated moisture from escaping at elevated temperatures, resulting in a high pore pressure
• HSC is normally subjected to higher compressive stresses than lower strength concrete

Generally, a combination of pore pressure, compression in the exposed surface region of concrete as well as internal cracking are all required to cause explosive spalling.

Two possible ways of minimising the risk of extensive spalling of concrete lining for tunnels are (for more information, please see the Section on Concrete Materials):

• Providing fire-proof coatings to the exposed surface of concrete which is unable to resist fire
• Adding polypropylene fibres into concrete which melt during a fire thus creating paths in the matrix for water vapour to escape

It is important to take into consideration the actual or realistic material behaviour in the design of the fire protection systems of a tunnel to protect the integrity of the tunnel in a fire, particularly for tunnels built through poor ground condition.

Sources of Information

• BBC News Online / World / Europe - UK Edition
• Department of Transport - Channel Tunnel Safety Authority (1997). Inquiry into the Fire on Heavy Goods Vehicle Shuttle 7539 on 18 November 1996, The Stationery Office, London.
• Comeau, E. and Wolf, A. (1997). "Fire in the Chunnel!" NFPA Journal March/April 1997, pp58-64.
• Kirkland, C.J. (2002). "The fire in the Channel Tunnel." Tunnelling and Underground Space Technology, 17, pp 129-132.