The most badly damaged section of the tunnel runs through blue chalk marl riddled with faults. Geological engineers had previously denied that the ground conditions were a concern after the fire (NCE 18 September 2008). But this week Eurotunnel said that it had to act to limit the risks of the damaged section of tunnel collapsing.
An open-sided truck wagon caught fire in the Channel Tunnel on 11 September 2008, triggering a 16 hour blaze 11km from the French portal, destroying the 400mm thick concrete lining. The tunnel was closed for a month after the fire.
Engineers inspected the damaged lining and decided to install over 1,000 rock bolts in the worst affected section to secure it before repair work began. "We weren’t worried [that the tunnel would collapse]. But the fracturing near to this section where the fire was meant we did have a question in our mind. Using rock bolts was our response to that question," said Eurotunnel logistics and infrastructure maintenance director Jean-Luc Pochet.
Site workers installed the 50mm diameter, 3m long hollow rock bolts into the tunnel walls. Once drilled to depth, grout pumped through the rock bolts filled the annulus between bolt and rock to secure them. Ground strength tests conducted after the bolts were in place proved that the concrete and tunnel bore was strong enough for work to continue.
The three and a half months of repair work that followed have involved relining the tunnel with 4,000t of shotcrete.
The Channel Tunnel runs for 50.5km from Folkestone to Sangatte in France and is on average 40m below the seabed.
The twin bore tunnel runs for most of its length through a 15m to 30m thick band of blue chalk marl. But towards the French end of the tunnel, strata are riddled with faults.
The fire broke out in the tunnel where it sits among these faults, leading to questions about whether the fire would weaken the lining and destabilise the tunnel.