Multi-method geophysical mapping of quick clay

Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. The purpose of this paper is to assess the use of a number of geophysical techniques for identifying quick clay. The investigated area, Smørgrav, located in southern Norway has a history of quick clay sliding, the most recent event occurring in 1984. Geophysical techniques that are used include electromagnetic conductivity mapping, electrical resistivity tomography, seismic refraction and multichannel analysis of surface waves. These results are compared to geotechnical data from bore samples, rotary pressure soundings and cone penetration testing. A number of these approaches have proved promising for identifying quick clay, in particular electrical resistivity tomography and electromagnetics, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and sampling. Seismic refraction was useful for determining the sediment distribution as well as for indicating the presence of shallow bedrock whereas the multichannel analysis of surface-waves approach suggested differences between the intact stiffness of quick and unleached clay. It is observed that quick clay investigations using discrete rotary pressure soundings can be significantly enhanced by using, in particular, electrical resistivity tomography profiles to link together the information between test locations, perhaps significantly reducing the need for large numbers of soundings.