Shear strength characteristics of a compacted soil under infiltration conditions

A significantly thick zone of steep slopes is commonly encountered above groundwater table and the soils within this zone are unsaturated with negative pore-water pressures (i.e., matric suction). Matric suction contributes significantly to the shear strength of soil and to the factor of safety of unsaturated slopes. However, infiltration during rainfall increases the pore-water pressure in soil resulting in a decrease in the matric suction and the shear strength of the soil. As a result, rainfall infiltration may eventually trigger a slope failure. Therefore, understanding of shear strength characteristics of saturated and unsaturated soils under shearing-infiltration (SI) conditions have direct implications in assessment of slope stability under rainfall conditions. This paper presents results from a series of consolidated drained (CD) and shearing-infiltration (SI) tests. Results show that the failure envelope obtained from the shearing-infiltration tests is independent of the infiltration rate. Failure envelopes obtained from CD and SI tests appear to be similar. For practical purposes the shear strength parameters from the CD tests can be used in stability analyses of slopes under rainfall conditions. The SI tests might be performed to obtain more conservative shear strength parameters and to study the pore-water pressure changes during infiltration.