Editing Channel Tunnel (section)

=== Site investigation ===
Marine soundings and samplings were made by Thomé de Gamond in 1833–67, establishing the seabed depth at a maximum of {{cvt|55|m}} and the continuity of geological strata (layers). Surveying continued for many years, with 166&nbsp;marine and 70&nbsp;land-deep boreholes being drilled and more than 4,000{{nbsp}}line{{nbsp}}kilometres of the marine geophysical survey completed.<ref name="Kirkland geol pp.22–26"/> Surveys were undertaken in 1958–1959, 1964–1965, 1972–1974 and 1986–1988.

The surveying in 1958–1959 catered for [[immersed tube]] and bridge designs, as well as a bored tunnel, and thus a wide area was investigated. At that time, marine geophysics surveying for engineering projects was in its infancy, with poor positioning and resolution from seismic profiling. The 1964–1965 surveys concentrated on a northerly route that left the English coast at Dover harbour; using 70&nbsp;boreholes, an area of deeply weathered rock with high [[permeability (earth sciences)|permeability]] was located just south of Dover harbour.<ref name="Kirkland geol pp.22–26"/>

Given the previous survey results and access constraints, a more southerly route was investigated in the 1972–1973 survey, and the route was confirmed to be feasible. Information for the tunnelling project also came from work before the 1975 cancellation. On the French side at Sangatte, a deep shaft with [[adit]]s was made. On the English side at Shakespeare Cliff, the government allowed {{cvt|250|m|ft|0}} of {{cvt|4.5|m|ft|0|adj=on}} diameter tunnel to be driven. The actual tunnel alignment, method of excavation and support were essentially the same as the 1975 attempt. In the 1986–1987 survey, previous findings were reinforced, and the characteristics of the gault clay and the tunnelling medium (chalk marl that made up 85% of the route) were investigated. Geophysical techniques from the oil industry were employed.<ref name="Kirkland geol pp.22–26">Kirkland pp. 22–26</ref>