The shearing effect of suction

In light of a fundamental analysis of the microstructural behaviour of an unsaturated granular assembly (Li, 2003a), it was discovered that the impact of matric suction on the soil skeleton of an unsaturated soil was not simply determined by suction alone but with the fabric of the pore fluid. Owing to the tensorial nature of this fluid fabric, as opposed to the scalar nature of suction, and the observation of the fabric tensor being generally anisotropic, suction can bear a shearing effect and induce not only normal stress but also shear stress on the soil skeleton, in contrast to the traditional belief of the purely stabilizing effect of matric suction. Since shear is the major factor responsible for soil deformation and failure, investigation into this shearing effect is of fundamental importance. Therefore, an experimental program was carefully designed to investigate this shearing effect on a simple soil called Kaolin Clay or China Clay. The shearing effect was studied by observing the change of the behaviour, in terms of the change in volumetric response and strength characteristic, of soil specimens from fully saturated condition to unsaturated condition. Volumetrically, it was found that suction initiated and enhanced anisotropic deformation in an initially essentially isotropic specimen. On strength characteristic, the shearing effect caused an apparent shift of the ultimate stress ratios of the unsaturated specimens, with reference to the corresponding ratios of fully saturated specimens, in both the compression and the extension directions. The suction-induced shear rendered the unsaturated specimens apparently stronger in the compression direction and weaker in the extension direction. These findings not only provided encouraging support to the long-neglected phenomenon but also suggested the importance of taking this shearing effect into consideration for realistic description of the mechanical behaviour of unsaturated soil