Editing Channel Tunnel (section)

=== Geology ===
[[File:Channel Tunnel geological profile 1.svg|thumb|Geological profile along the tunnel as constructed. For most of its length the tunnel bores through a chalk [[marl]] stratum (layer).]]

Successful tunnelling required a sound understanding of topography and geology and the selection of the best rock strata through which to dig. The geology of this site generally consists of northeasterly dipping [[Cretaceous]] strata, part of the northern limb of the Wealden-Boulonnais dome. It has:
* Continuous chalk in the cliffs on either side of the Channel, with no major faulting, as observed by [[Verstegan]] in 1605.
* Four geological [[stratum|strata]], marine sediments laid down 90–100&nbsp;million years ago; [[pervious]] Upper and Middle Chalk above slightly pervious Lower Chalk and finally impermeable [[Gault Clay]]. There is a sandy stratum of [[Glauconitic marl]] (tortia), between the chalk marl and the gault clay.
* A {{cvt|25–30|m|ft||adj=on}} layer of chalk marl (French: ''craie bleue'') in the lower third of the lower chalk appeared to present the best tunnelling medium. The chalk has a clay content of 30–40% providing impermeability to groundwater yet relatively easy excavation with strength allowing minimal support. Ideally, the tunnel would be bored in the bottom {{cvt|15|m|ft|0}} of the chalk marl, allowing water inflow from fractures and joints to be minimised, but above the gault clay that would increase stress on the tunnel lining and swell and soften when wet.<ref name="Eng Geol">{{cite book | title = Engineering Geology of the Channel Tunnel | editor = Harris, C.S. | year = 1996 | publisher=Thomas Telford | location = London | isbn = 0-7277-2045-7 | page = 57 |display-editors=etal}}</ref>

On the English side, the stratum [[Strike and dip|dip]] is less than 5°; on the French side, this increases to 20°. Jointing and faulting are present on both sides. On the English side, only minor faults of displacement less than {{cvt|2|m|ftin|}} exist; on the French side, displacements of up to {{cvt|15|m|ft|}} are present owing to the Quenocs [[anticline|anticlinal]] [[fold (geology)|fold]]. The faults are of limited width, filled with calcite, pyrite and remolded clay. The increased dip and faulting restricted the selection of routes on the French side. To avoid confusion, microfossil assemblages were used to classify the chalk marl. On the French side, particularly near the coast, the chalk was harder, more brittle and more fractured than on the English side. This led to the adoption of different tunnelling techniques on the two sides.<ref name="Kirkland geol pp.21–50"/>

The Quaternary undersea valley Fosse Dangeard, and [[Castle Hill, Folkestone|Castle Hill]] landslip at the English portal, caused concerns. Identified by the 1964–1965 geophysical survey, the Fosse Dangeard is an infilled valley system extending {{cvt|80|m|ft|0}} below the seabed, {{cvt|500|m|ft|0}} south of the tunnel route in mid-channel. A 1986 survey showed that a tributary crossed the path of the tunnel, and so the tunnel route was made as far north and deep as possible. The English terminal had to be located in the Castle Hill landslip, which consists of displaced and tipping blocks of lower chalk, glauconitic marl and gault debris. Thus the area was stabilised by buttressing and inserting drainage [[adit]]s.<ref name="Kirkland geol pp.21–50"/> The service tunnel acted as a pilot preceding the main ones, so that the geology, areas of crushed rock, and zones of high water inflow could be predicted. Exploratory probing was done in the service tunnel, in the form of extensive forward probing, vertical downward probes and sideways probing.<ref name="Kirkland geol pp.21–50">Kirkland pp. 21–50</ref>