Shear behaviour of peat at different stress levels

In this study, a series of consolidated, undrained triaxial compression tests were conducted to investigate peat shear behaviour on samples from 1.65 m depth when subjected to different stress levels from 10.4 kPa to 40.5 kPa. At the consolidation stage, the triaxial test specifically investigated the peat isotropic compressibility at low stress levels, showing an agreement with oedometer test data available in literature. The subsequent triaxial shearing stage results show most of the test data failed to reach the tension cut-off line (q/p’ = 3), which indicated that the deviator stress may represent more of an interparticle connection than the tension of fibres and woods in peaty soils. For peat, the membrane correction effect on peat shear resistance is strain dependent; generally, small within 10% shear strain, but becomes significant above 10% shear strain. A critical state line for peat was determined based on the maximum curvature approach, where the Mohr-Coulomb model has difficulty in determining the friction angle for peat. Of the data recorded for the peat, 78% fell within the range of 30 to 60 degrees, increasing to 90.4% when ignoring points lower than 10 kPa; the previous test data for very low stress level (less than 10kPa) might not be sufficiently reliable due to limitations of conventional triaxial testing apparatus, specimen preparation and etc. In addition, organic content also plays an important role on the peat shear behaviour. In general, when the organic content exceeds 75%, the deviator stress behaves like organic soils, otherwise, the peat behaves more like a mineral soil. In peat samples with organic content higher than 75%, the direct shear box test gives higher estimates of shear strength than the triaxial shear test, but not necessarily accurate — the mechanism of direct shear acts only at the centre of a specimen, while triaxial shear can shear throughout the specimens.