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Channel Tunnel concrete lining fire protection flawed, say experts

Tunnel safety and concrete experts said this week that Eurotunnel had failed to heed warnings about the fire resistance of the Channel Tunnel's concrete lining before the 1996 fire.

The 1996 fire burned for seven hours and damage caused cost more than £200M to reinstate. Last week’s fire burned for 16 hours, and could cost more to repair.

Imperial College research fellow Professor Gabriel Khoury – a leading expert on concrete and tunnel safety – said he had written to Eurotunnel before the 1996 fire to express his dissatisfaction at the lining's fire resistance.

"High strength concrete is usually of the order of C50/60 and is highly impermeable to corrosive agents. The concrete used in the Channel Tunnel was well in excess of C50/60. The concrete however was not designed to withstand intense heat loads and not be explosive," he said.

Khoury said high strength concrete is susceptible to spalling in the intense heat created by, for example, hydrocarbon fires in tunnels which can reach up 1,000° C within four minutes.

The spalling process begins when water in the concrete
turns into steam.

The low permeability of high strength concrete hinders the release of this steam resulting in explosive spalling, with layers of lining cracking and peeling away from the tunnel wall.

The concrete is also put under differential thermal stress as superheated areas exposed to fire expand more quickly than the concrete closest to the tunnel wall.

In the 1996 fire, temperatures in tunnel reached 1,100°C
and the fire burned for around seven hours causing structural damage over a 500m length of tunnel (NCE 28 November 1996).

Within that section, 50m of lining suffered severe spalling.

Shortcomings in the Channel Tunnel lining's concrete design could be mitigated by the installation of thermal barriers. These act as an extra layer of lining which would absorb heat, protecting the precast concrete lining segments.

Thermal barriers are a protective lining which can be applied to the tunnel lining to absorb heat. They can be panels, or
can be sprayed or manually applied and are usually made from substances with high insulation properties.

Khoury said that the installation of thermal barriers was rejected on cost grounds, although this had turned out to be a false economy.

"The installation of thermal barriers would cost something in the order of £48M to £70M which, when set against reinstatement costs of around £200M after the 1996 fire, seem not unreasonable," he said.

This sentiment was qualified by Stuart Matthews, chief engineer (construction) at research body BRE.

BRE has been working closely with the Highways Agency on the installation of thermal barrier systems.

"With hindsight, maybe Eurotunnel needs to reassess its cost-risk analysis.

"On the subject of installing thermal barriers, the company needs to examine spend versus disruption costs," he said.

Eurotunnel was unavailable to comment on the fire resistance of the tunnel lining.

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