Joint Shear Wave Reflection Seismic and Geotechnical Drilling for Site Investigation at Tønsberg Hospital Area

Summary Quick-clays in Nordic countries seriously affect building settlements safely, especially essential infrastructure. The detection of safe building ground for top priority infrastructure buildings is therefore essential in areas prone to quick-clay liquefaction. In 2015 the administration of the central hospital of Tonsberg (Sykehuset I Vestfold, SIV), Norway, intended expansion towards an area prone to quick-clay. Former drilling investigation indicated a dubious bedrock topography below soil, with clay, silt, and anthropogenic infills estimated up to 25 m thickness. A dense drilling grid would be required to analyze the depth to bedrock for pile foundation, but could not be realized due to the building density and the installations at and below the surface. Since Ground Penetrating Radar (GPR) failed in the area due to high soil conductivity and seismic refraction did not provide the resolution required, high-resolution shear wave reflection seismic was suggested combined with a focused geotechnical drilling campaign, to expand the drilling results in the area. This unique experiment was realized in June 2016 and led to a comprehensive interpretation of the bedrock topography and the soil thickness above. It strongly improved the planning process regarding the foundation construction and reduce the financial risk compared to conventional foundation construction.