Constitutive testing of soil on the dry side of critical state

Abstract

Heavily overconsolidated clays tend to fall on the dry side of critical state and exhibit brittle behaviour in terms of localized strains. Testing of such brittle soils is conducted mainly under triaxial conditions, whereas actual field problems often occur under plane strain (PS) conditions. Experimental stress-strain data on stiff clays from PS tests is very limited due to difficulties with existing biaxial apparatus.A novel biaxial compression device has thus been developed that allows simplicity and ease in sample preparation. Direct measurement of intermediate principal stress by using total stress cells and lateral strain using micro laser sensors has also been incorporated in the set-up. A series of drained and undrained PS compression tests have been conducted on heavily overconsolidated clay to investigate its constitutive behaviour and shear band characteristics. Triaxial compression, extension and direct shear tests were also conducted to understand the role of shear modes on stress-strain behaviour. The new biaxial device was found to allow accurate investigation of the onset and development of localized deformation in compression testing of stiff soils. It was found that shear banding initiated before peak in PS tests and after peak in triaxial tests. It was also found that the residual friction angle is the same in all the shear modes.The modified Cam clay (MCC) model has been generalized to 3D stress space using Mohr-Coulomb failure criterion and incorporating the Hvorslev surface in the supercritical region. The model has also been regularized to give objective results in order to evaluate its performance when applied to problems involving discontinuities. The performance of the model in predicting global stress-strain behaviour and shear band formation is evaluated.