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A laboratory investigation was conducted to assess the influence of density on the shear strength of unsaturated soil. To vary density, compacted specimens of an expansive clay soil were subjected to different stress histories prior to shear. Tests were performed at a constant water content. The Hvorslev “true friction-true cohesion” concept was used to normalize the effects of density differences on the shear strengths of both saturated and unsaturated specimens. It was determined that shear strengths of unsaturated soils were dependent on the stress state, density, and water content of specimens at failure. A modified Mohr-Coulomb failure model to predict the shear strength of unsaturated soils is proposed. The effect of suction is to increase the value of the cohesion intercept in this model. The actual measurement of soil suction is not required to apply the model.
unsaturated soils, expansive soils, clays, soil suction, laboratory tests, psychrometer, strength tests, consolidation tests, compaction tests
Research civil engineer, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS