Characterization and engineering properties of the NGTS Onsøy soft clay site

In the 1960s, NGI investigated numerous potential research sites and established the first Onsøy soft clay site in 1968. In 2016, the NGTS project established five research sites, one of which is the NGTS Onsøy soft clay site. The purpose of this paper is to provide a reference for further in-situ and laboratory testing at the site and easy access to high-quality soil investigation data. The paper interprets the soil conditions providing engineering soil properties, comments on data reliability and quality, and correlations between different soil parameters where appropriate. The lightly overconsolidated soft clay site has been characterized based on extensive in-situ and laboratory testing. In-situ methods include electrical resistivity tomography, multichannel analysis of surface waves, total soundings, rotary pressure soundings, cone penetration testing with and without seismic and electrical resistivity measurements, dilatometer with seismic measurements, self-boring pressuremeter, electric piezometers, thermistor string, hydraulic fracture testing and earth pressure cells. Soil samples were obtained using Sherbrook block sampler, Geonor φ 72 mm piston sampler, Geonor φ 54 mm composite piston sampler and hydraulic piston and push samplers with liners. Laboratory tests include multi-sensor core logging, index testing, cyclic and monotonic, consolidated and unconsolidated triaxial tests, bender element tests, oedometer tests with constant rate of strain and incremental loading and direct simple shear tests