Mapping Quick-clay Sites for Geohazard Assessment – The Finneidfjord Case Study, Norway

On June 20th, 1996, a submarine land failure in a fjord near the village of Finneidfjord (Northern Norway) developed into a retrogressive quick-clay slide encroaching 100-150 m inland, resulting in the loss of life and significant material damage. Reports in the aftermath of the slide concluded that build up of excess pore pressure may have been the main reason for the slide, though other causes are invoked too. Several field campaigns in the fjord since 1997 consisted of sediment coring, VHR seismic surveys, and long term in-situ instrumentation. To complete offshore data with land data, several geophysical techniques were tested onshore in July 2007. Seismic, GPR and resistivity (method of choice in quick clay) were tried at two sites (one intact and one close to the 1996 slide scar), and all three techniques proved to be useful for a better definition of the underground. Despite clay-prone sites, a 50 MHz GPR antenna was used with success on one site because of the unknown presence of a thick layer of morainic material. Though standard refraction seismic profiles were acquired for P-wave tomography, surface-wave processing tests on the same data gave promising results.