Saeid kontratchi - Department of invironmental engineering, atehran University, Tehran, Iran
Mohammad Reza Mohaddes Nia - Department of geotechnical engineering, Mohaghegh Ardebili University, Ardebil, Iran

In this study, the filter paper method was used to obtain the soil water characteristic curve. Samples with different initial MCs undergone the direct shear test under unsaturated conditions. Using Khalil and Khabaz (1998) equation, the effective stress in different moisture contents was determined, given the equivalent suction values. Results show that the obtained strength values for different MCs have no significant differences with each other. Negligible variations in cohesion and soil internal friction angle can be attributed to the small shear-induced pore pressure changes. Results also support the validity and usefulness of effective stress principle in unsaturated soils; i.e. if valid estimate for effective stress is available, there will be no need to measure strength parameters in the unsaturated state, and parameters from the saturated state will be still reliable and applicable.

shear strength, unsaturation soil, saturation soil