ENGINEERING ENGINEERING GEOLOGY OF GEOLOGY OF THE CHANNEL THE CHANNEL

TUNNELTUNNEL

Richard Rae & Robert Hazelhurst March 2006

Contents

• Geology beneath The English Channel

• Selecting the best location for a tunnel

• Successful tunnel construction (1987-1991)

ENGLAND

FRANCE

SECTION LINE

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 m3

• 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

Thank you