To permit interchange between running tunnels, primarily to facilitate tunnel maintenance, the Channel Tunnel project incorporates two crossover caverns positioned under the Channel about a third of the way out from each portal.

At the location of the UK crossover the service tunnel, normally positioned between the two running tunnels, deviates below and to the side to enable the running tunnels to come into juxtaposition over a distance of some 500 m. The central portion of this run is opened out as a single cavern measuring 165 m long, 22 m wide and 15 m high—large enough to accommodate a five-storey building and probably the largest undersea cavern built to date.

Geological setting

The cavern is located within clayey carbonate mudstone sequences which straddle the boundary between the Chalk Marl (Lower Cenomanian) and the Gault Clay (Albian). The cavern crown is some 35 m below the seabed, the intervening strata comprising Chalk Marl overlain by the more fractured and therefore more permeable Grey Chalk which is weathered to within 20 m of the cavern crown.

The unconfined compressive strengths of the materials fall within the range 2 MPa to 16 MPa with the highest values being associated with limestone sponge beds in the Chalk Marl and the significantly stronger calcareous sandstone of the Tourtia or Glauconitic Marl.

Formation mass permeabilities are within the region of 1 × 10-7m s-1 and 6 × 10-9m s-1 at the cavern horizon, rising to in excess of 3 × 10-6m s-1 in the

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