this report is one of a series of case studies of tunnel projects made from the point of view of site investigation. the site investigation allowed the route of the 1.7 km twin subaqueous power cable tunnels to be chosen, and their depths to be selected to allow construction in the london clay. boreholes and marine seismic reflection surveys determined the depths of the river bed, gravel and london clay surfaces, and dutch surroundings gave further information for shaft sinking on shore. construction in free air commenced from the north, employing expanding wedge-block lining, tunnelling machines and shields, although hand working and bolted linings were used where poor ground conditions dictated. exploratory probing ahead from the face was employed. after satisfactory progress for 1 km a major joint ahead of the west tunnel face caused an inflow of water, sand and silt, and the end section of the tunnel was sealed off. a further marine seismic reflection survey gave more detailed information on conditions ahead of the tunnel faces, and a more detailed programme of probing ahead was initiated. construction of the east tunnel continued from the north, but the west tunnel was made to deviate around the sealed-off section, and construction was commenced from the south; the final section in the region of the joint was completed using ground freezing. the general implications of the site investigation experience are reviewed.(a)

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