Failure of soil under water infiltration condition

•Advanced laboratory triaxial tests were conducted on unsaturated soil samples.•Soil failed by shearing–infiltration has higher strength than that under CD test.•Infiltration alone is possible to generate soil failure without any addition loading.

To reduce and mitigate rainfall-induced landslide problems, there is a need to improve our understanding of the failure mechanism of soil under water-infiltration condition. This study aimed at understanding the effect of water-infiltration on the transformation of shear stress and its eventual failure in unsaturated soil. The method of study was via a series of advanced laboratory triaxial test of two specific types: (1) constant-suction shearing test, and (2) shearing-infiltration test. The shearing-infiltration test results indicated that infiltration of water reduced the matric suction of the soil by the generation of excess pore-water pressure; it however was not accompanied by a reduction in shear strength, instead, the unsaturated soil failed under a constant shear stress applied prior to the infiltration. Excessive deformation and the eventual softening of the soil were found to be the main causes of water-infiltration induced failure.