Critical state shear strength of an unsaturated artificially cemented sand

This paper presents the results of a set of 22 triaxial tests on an unsaturated artificially cemented sand. The results are used to explore the applicability of a number of unsaturated soil frameworks for interpreting the shear strength. Constant water content triaxial tests were carried out on unsaturated specimens, using the axis translation technique to measure suctions during shearing. The test results on the unsaturated material were referenced against a series of drained and undrained triaxial tests that were carried out on saturated specimens. The results of the unsaturated tests were analysed to investigate the effect of the suction and degree of saturation on the shear strength at the critical state. The results show that the critical state stress ratio in terms of net stress (M_a) was found to be larger than the saturated critical state stress ratio (M_s). It was also found that the stress ratio in terms of suction (M_b) reduced as suction increased (when the degree of saturation reduced below 30%). Interestingly, during the initial desaturation phase when the degree of saturation reduces considerably, the stress ratio M_b was largely unaffected by desaturation. It was only when the suction increased sufficiently that the micro-voids within the cementing material could start to desaturate that a reduction in M_b was seen. This occurred at suctions in excess of the residual suction when the global degree of saturation was changing very little. This implies that the suction is contributing to the strength of the cementing material itself.