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Section 3: Case studies in land surface evaluation |
Department of Geological Sciences, University of Plymouth, Devon, UK
Purpose of survey
Engineering geomorphological mapping is primarily concerned with identifying and mapping the features on the ground surface. However, detailed mapping linked to limited subsurface data can provide the first approximation of a ground model in some situations. In this example mapping was carried out at the location of the proposal Channel Tunnel portal at Castle Hill near Folkestone, England, prior to any detailed ground investigations with the aim of establishing the nature and extent of the landslide complex that existed at the site. Full details of the study can be found in Griffiths et al. 1995.
The site
The Channel Tunnel terminal and portal on the UK side is located immediately below the Etchinghill escarpment, a scarp slope developed in the Lower Chalk of the North Downs to the west of Folkestone. Whilst the tunnel itself was positioned within the lower permeability zone of the Chalk Marl, the works for the terminal and portal required excavations in the Lower Chalk, the underlying Gault Clay and through the solifluction cover of Coombe Rock.
The 1: 10 560 scale BGS map identified six landslide complexes within the terminal and portal works area (Aarons et al. 1977), although there was no information available on the form, depth and current stability of these features. The design for the main tunnel portal, however, required it to enter the hillside through the centre of one of the landslides at Castle Hill.
Techniques used
Field mapping of the geomorphology was undertaken at a scale of 1: 500 using standard procedures for geomorphological survey work (Brunsden et al. 1975).
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This 250-word extract was created in the absence of an abstract.
