Building site investigation by joint shear wave reflection seismic and geotechnical drilling at Tønsberg hospital area, eastern Norway

Abstract

In northern hemisphere countries such as Norway, Sweden, Finland, Russia, Canada and Alaska (USA), so called quick-clays seriously affect the safe building of settlements, and depth to bedrock is a prerequisite for safe building foundation. Such clays show a mineralogical structure where the stability is dependent on the ionic content in pore water. The composition is sensitive to leaching by low mineralized water. Originally deposited in a marine or brackish environment, clay formations composed of silt and clay are exposed to freshwater owing to the isostatic uplift of nearly 200 m (Bjerrum et al. 1967, ca. 180 m in our study area Tønsberg, www.ngu.no) above sea level after deglaciation. This may have caused leaching to low salinity depending on the time and volume of fresh water inflow, which may destabilize the formation up to a sudden liquefaction collapse. The detection of safe building ground e.g. bedrock and the knowledge of the internal soil structure above it is therefore essential in areas prone to quick-clay. Typically, quick-clays are not exposed to the surface and covered by other lithological units, which makes it difficult to map their area in the subsurface. The administration of the central hospital of Tønsberg (Sykehuset I Vestfold, SIV), Norway, planned to expand the hospital with new buildings in an area prone to quick-clay (Figure 1). Past borehole investigations indicated an undulating bedrock topography below soil, with clay, silt, and anthropogenic infills estimated up to 25 m thick and that a dense borehole grid would be needed for accurate depth-to-bedrock knowledge. Ground Penetrating Radar (GPR) was tested, but failed owing to high electric conductivity in marine sediments. Geoelectric and electromagnetic methods previously applied in other locations by Long et al. (2012) and Solberg et al. (2016) were considered, but were discarded owing to lots of buried hospital infrastructure e.g. pipes, cables and underground transportation tunnels, and the disturbing urban environment. Seismic refraction could not provide the resolution required and was also limited in application owing to the restricted space, the nearby buildings and the asphalt pavement at the surface. Therefore NGU, as geophysical project leader, advised SIV to provide shear wave reflection seismic surveying prior to a focused drilling campaign. Because of the lack of competence of this research in Norway, NGU established a joint research expertise enabling the full range from shallow reflection seismic acquisition and geotechnical analysis towards geological model building for construction site planning.