Shear behaviour of peat at different stress levels

Consolidated undrained triaxial compression tests were conducted from 1.65 m depth subjected to stress levels of 10.4–40.5 kPa. The results showed that most test data failed to reach the tension cut-off line (q/p′ = 3). For peat, the membrane correction effect on peat shear resistance was strain dependent – generally small within 10% shear strain, but significant above 10% shear strain. A critical state line for peat was determined based on the maximum curvature approach. Of the data recorded for peat, 78% fell within the range 30–60°, increasing to 90.4% when ignoring points lower than 10 kPa. Organic content (OC) also had an effect on the shear behaviour. For OC > 75%, the deviator stress behaved like an organic soil; otherwise, the peat behaved more like a mineral soil. In peat samples with OC > 75%, the direct shear box test gives higher estimates of shear strength than the triaxial shear test, but which are not necessarily accurate – the mechanism of direct shear acts only at the centre of a specimen, while triaxial shear can shear throughout the specimen.