The tunnel in the area where the fire broke out is likely to be embedded in the most fault-ridden part of the chalk marl strata, which was chosen to house the twin bore structure.
During investigations for an earlier crossing in the 1880s, engineers discovered the 15m to 30m thick band of chalk marl stretching between the English and French coasts at the base of the English Channel’s seabed chalk stratum.
A century later, construction of the Channel Tunnel took advantage of the same chalk marl band – a decision based on the logic that it was stable and would cause relatively few tunnelling problems. Click here for image of tunnel geology.
But, geophysical investigation boreholes uncovered a number of faults on the French side of the Channel – predominantly along the first few kilometres of the tunnel alignment from Sangatte. Few were found on the UK side.
University of Plymouth school of Earth, Ocean and Environmental Sciences associate dean Professor Malcolm Hart said tunnellers feared water ingress through these faults, creating adverse tunnelling conditions.
However, the conditions confounded tunnellers: “The French used earth-pressure balanced machines because they were expecting wet ground due to faults in the rock above the alignment. In fact the ground was largely dry,” a senior geologist directly involved in the tunnel’s construction told NCE.
In addition, during tunnelling, workers had to take care to locate boreholes drilled for earlier investigation work. If the tunnel boring machine had met one of them it could have resulted in seawater ingress into the tunnel workings.
As advances were made, workers took cores from the ground surrounding the service tunnel, which helped the team keep check on the tunnels exact position within the chalk marl by determining the horizons of the strata above and below the tunnel.