Test fill on soft plastic marine clay at Onsoy, Norway

The test fill at Onsoy, close to the town of Fredrikstad in Norway, was built on a very soft marine clay with in situ vane strength as low as 11 kPa and a plasticity index varying from 34 to 50. The dimensions at the bottom of the long fill were 20 m x 60 m and the height 2.3 m. The fill, as placed, had a computed factor of safety against foundation failure of 1.35 based on in situ vane strength. The fill was allowed to sit for about 3 years (from 1972 to 1975) before it, in a second stage, was brought to failure by raising the height. The main purpose of the first stage was to study the stress-strain-time relationships for the soft clay and compare the values of geotechnical parameters determined by careful laboratory tests with those back-calculated from the observed field behaviour. The test fill foundation was heavily instrumented to measure clay deformations and pore pressures with time. Field observations showed that the Onsoy test fill turned out to be more of a drained loading case than an undrained case in spite of the fairly rapid construction of the fill. Actually, subsequent undrained triaxial and direct simple shear tests strongly indicate that the fill might have failed if less drainage had taken place, i.e., if the drainage paths had been longer and (or) if the coefficient of consolidation had been lower.