Test fill brought to failure on soft plastic marine clay at Onsøy, Norway

Abstract

A test fill at Onsøy, 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 test fill were 20 m × 60 m with a height of 2.3 m. The fill, as placed in the first stage, 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 was, in a second stage, brought to failure in about 4.3 days by raising the height. This second stage is the subject of this paper. Back-calculation of the failure by undrained stability analysis based on in situ vane tests performed shortly before the start of the second stage, based on undrained triaxial and constant volume direct simple shear tests on specimens from tube samples taken shortly before the start of the second stage and reconsolidated to the in situ effective stresses, gave factors of safety varying from 0.88 to 1.45 depending on type of analysis. Settlement values computed at the centreline of the fill for the second stage based on the results from undrained compression triaxial tests agreed reasonably well with the field measurements.