Behaviour of a test excavation in soft Champlain Sea clay

Abstract

The field observations made during a sloped excavation in a soft sensitive clay are presented. The test site is located in Saint-Hilaire, in a typical Champlain Sea clay deposit, of 30 m depth. The trench was 8 m deep and 60 × 60 m square and its slopes were at 45, 34, 27, and 18° to the horizontal. The induced pore pressures and total lateral stresses have been recorded with vibrating-wire instruments since the beginning of the excavation. The steepest, 45° slope completely failed 1 day after excavating and the 34° slope, 2 weeks after. The piezometer readings have shown that the groundwater regime was modified proportionally to the changes in total stresses; these modifications took place within a time period of 5 months, after which the heads became more or less constant. The total stress cells indicated significant rotation of the principal axes at the toe of the slopes. Stability analyses using the lowest vane strength profile gave minimum factors of safety of 1.15 for the 45° slope and 1.23 for the 34° slope, thereby underestimating the risks of failure. Back analyses using regional effective shear strength parameters were made and the parameters mobilized at failure were found to be [Formula: see text] and [Formula: see text]. Key words: case record, excavation, soft clay, slope stability, back analysis.