Prolonged exposure of a stretch of the 40km (25-mile) south tunnel to high temperatures during the fire has caused such significant damage to some of the concrete segments that they will have to be removed and new ones installed.
Replacement of the segments is such an intricate engineering operation that the tunnel operator, Eurotunnel, has admitted that Eurostar passenger trains and Le Shuttle car freight trains will be unable to make full, normal use of both tunnels until the spring.
"It will be at least three months," John Noulton, Eurotunnel's director of public affairs, said yesterday. "That estimate includes clearing away the remains of the shuttle train, replacing fixed equipment and replacing and repairing part of the tunnel."
Structural engineers have been examining the concrete fabric to assess the long-term damage caused by the intense heat. The fire, which raged for more than eight hours, during which temperatures reached 1,000C, appears to have "cooked" some of the concrete segments.
Concrete starts losing its structural strength when it is exposed to temperatures above 400C. During Monday night's fire, chunks of the concrete lining fell away because the heat caused some of it to "spall" or disintegrate, with the moisture in the material turning to steam. Tony Morris, of the Building Research Establishment, said: "Concrete is like an onion: it peels off layer by layer. In the heat as it falls it makes a sound rather like popcorn popping."
Eurotunnel has until now said that it had yet to decide how the damage to the concrete lining of the tunnel will be repaired, but yesterday admitted that the preferred option was to replace the segments. This, together with replacing the cabling and electrical equipment in the tunnel, will cost at least pounds 200m."The engineers prefer to replace like with like," said John Noulton.
The 7.6m (25ft)-diameter tunnel is built from rings each 1.5m long, formed from interlocking concrete segments. Some time will be saved during the repair work by using segments from a stockpile left over during the original concrete process. Specialised gear will have to be brought in to carry out the job.
Yesterday French judicial officials continued their inspection of the burnt-out freight wagons left inside the tunnel, but the inter-governmental safety commission suspended its discussions. Talks between the commission and Eurotunnel over the resumption of passenger and car services will continue tomorrow. Yesterday shadow transport spokesman Glenda Jackson said she would be writing to the Secretary of State for Transport, Sir George Young, asking why a single safety regime has not been implemented for the Channel Tunnel. Earlier this year the Government said it would try to ensure the co-ordination and enforcement of such a system after the Transport Select Committee criticised the lack of a unified system of inspection.