ENGINEERING GEOLOGY OF THE CHANNEL TUNNEL

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ENGINEERING GEOLOGY OF THE CHANNEL TUNNEL

• Richard Rae Robert Hazelhurst
• March 2006

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Contents

• Geology beneath The English Channel
• Selecting the best location for a tunnel
• Successful tunnel construction (1987-1991)

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ENGLAND
SECTION LINE
FRANCE
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The Weald

• Sediment laid down when area was underwater
• Dome arose in Miocene period (60m years ago)
• Dome weathered away over time

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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
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Route

• Tunnel construction began in 1883 abandoned for
  military reasons
• 1980s 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

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Construction

• 8.2 m diameter TBMs, met in middle
• English machines - 'open face' (faster)
• French - 'earth pressure balance'

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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

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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

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• Thank you