Large chunks of the fire damaged concrete lining in the Channel Tunnel have fallen off, the lead French investigator into the blaze on 11 September said this week.
"I'm no expert but there was significant damage to the concrete along the length of the tunnel where the train was standing.
"Quite a lot of concrete has fallen on the tunnel floor," said Bureau d’Enquêtes sur les Accidents de Transport Terrestre (BEA-TT) lead investigator Étienne Rambach.
He is leading the joint investigation between France’s BEA-TT and the UK’s Railway Accident Investigation Branch (RAIB).
But fire engineers said the extent of the damage was still unclear.
"It doesn’t take a genius to realise that there would be significant spalling after a fire of that magnitude burning for 16 hours but the real question at this stage is how much concrete has spalled underneath the steel reinforcement," said Halcrow director of fire safety Fathi Tarada.
Ordinarily, there would be a layer of reinforcement perhaps 50mm to 70mm under the surface of the concrete tunnel lining.
Tarada said spalled concrete behind the reinforcement would be difficult to replace.
"Rebar would have to be removed to replace the spalled concrete behind it and this steel would then have to be replaced and re-anchored," he said.
Imperial College research fellow Professor Gabriel Khoury – a leading expert on concrete and tunnel safety said reinstatement would be tricky if the reinforcement had to be removed first.
"When you reinstate, you've got to make sure the tunnel is structurally safe," he said.
Rambach said the BEA-TT's technical investigation was almost complete but that a judicial investigation being carried out by investigators from the town of Boulogne-sur-Mer was ongoing.
Rambach said that it was still difficult to assess the extent of damage to rolling stock in the tunnel.
BEA-TT gave the tunnel operators the green light to start moving sections of the 800m long fire damaged train from the tunnel last Wednesday. Since then Eurotunnel has moved around half of train from the tunnel.
The locomotive, the club car in which the HGV drivers travel and some of the wagons at the front of the train were functioning well enough to allow them to the towed out of the tunnel.
But Rambach and Eurotunnel said decisions on how to remove the remaining wagons would be taken on a case-by-case basis.
A Eurotunnel spokesman told NCE that despite having increased access to the fire damaged tunnel section since the removal some of the train, the company had still been unable to carry out its own formal investigation of the accident site.