Behaviour of sands under generalized drainage boundary conditions

Abstract

An experimental study of the behaviour of sands under generalized drainage boundary conditions is presented. The influence of partially drained conditions, which generally is a reflection of the loading rate and the permeability of the soil, has been studied by limiting the volumetric deformation between drained and undrained states. The effect of potential pore-pressure variations in situ has been assessed by simulating various levels of volumetric deformation during shear. Conventional drained and undrained tests were also carried out on the same sands to enable a direct evaluation of the effect of drainage. A triaxial device with the ability to control strain-paths was used to carry out the tests, and all tests were performed under compression loading with no change in total lateral stress. A systematic change in the stress–strain response was noted as the drainage conditions gradually change from drained to undrained. The maximum excess pore pressure generated owing to inhibition of drainage is almost linearly related to the amount of drainage blocked. These results support the contention that the undrained state may not represent the most damaging scenario under field loading conditions. Much smaller minimum shear strength values compared with the undrained strength were measured when pore-pressure boundary conditions caused expansive volume changes. The domain of strain-softening response, and hence liquefaction susceptibility, increased owing to such loading.Key words: partial drainage, liquefaction, strain softening, laboratory testing, soil mechanics.