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.