The Weald Sediment laid down when area was underwater Dome arose in Miocene period (~60m years ago) Dome weathered away over time
Tunnel Location Lower chalk layer –soft but stable – easy to excavate, but will stand up even without supporting walls –impermeable – comprises 80+% chalk, with some clay mixed in –depth and thickness of layer acceptable for a tunnel –layer continuous across whole width of channel Section through chalk layer beneath English Channel
Route Tunnel construction began in 1883 – abandoned for military reasons 1980’s – advanced oil techniques used 4000km of geophysical surveys 116 marine and 68 land boreholes Buried valleys presented major hazard – route had to stick to rock
Construction 8.2 m diameter TBM’s, met in middle English machines - 'open face' (faster) French - 'earth pressure balance'
Construction Tunnels are 51 Km in length, of which 37.5 Km is undersea 2400 tonnes/hour spoil = 4 million m 3 360,000 m² of land reclaimed TBM placed segmental concrete linings Gasket or grout to prevent water ingress 150 year design life
Problems Surveys only provided samples – in between unknown. e.g. aquiclude at crossover Water ingress caused TBM breakdowns on English side Poor ground conditions on French side But, generally highly successful - up to 426m per week – completed ahead of schedule