Abstract

The sub-seabed investigations for the Channel Tunnel in 1986–1988 included major marine geophysical surveys as well as 11 deep boreholes in which geophysical techniques were also employed. This was the first time that the significant advances in geophysical techniques which had occurred during the 1970s were applied to a major civil engineering project.

High-resolution multi-trace seismic data was obtained to a greater accuracy and resolution than had been possible during previous studies by means of multi-trace digital techniques. These provided a channel-wide assessment of the tunnelling environment, specifically within the proposed tunnelling medium of the Chalk Marl of the Lower Chalk to a depth of 150 metres below sea bed. Deeper seismic acquisition techniques were used to investigate the sub-tunnel structure to about 800 metres below sea bed to assist with seismic risk evaluation.

The field investigations were complemented by a number of in-depth reviews carried out by a joint Anglo-French team of specialists drawn from the Bureau de Recherches Geologique et Minieres (BRGM) and Mott, Hay and Anderson (MHA). J Arthur & Associates were appointed as specialist sub- consultants to MHA to specify and supervise the engineering geophysical studies and assist with the data interpretation.