| Abstract: | A laboratory Investigation was conducted to assess the Influence of suction on the shear strength of unsaturated soil. Variables which were investigated included the effects of compacting specimens of an expansive clay at different water contents, shearing specimens at various densities and adding potassium chloride (KCl) to the pore fluid of selected specimens. To assess the influence of matrix suction on the shear strength of unsaturated soil, specimens were compacted at water contents wet or dry of optimum and consolidated by selected stresses to various densities prior to shear. The effects of density on the strengths of both saturated and unsaturated specimens were normalized by Hvorslev's "true friction-true cohesion" concept. Constant water content tests were conducted on all unsaturated specimens, although drainage of pore water may have occurred as some specimens were consolidated to high degrees of saturation by the applied stresses. It was determined that shear strengths of unsaturated soils were dependent on the applied stress, density and water content of specimens at failure. A modified Mohr-Coulomb strength relationship was proposed to predict the shear strengths of unsaturated soils. The effect of matrix suction was to increase the value of cohesion in this model, although the measurement of suction was not required to apply the model. A method was proposed to characterize the influence of matrix suction on the shear strengths of unsaturated soils. It was determined that the magnitude of suction was dependent upon the water content and the degree of saturation of the unsaturated specimens at failure while the effect of suction was a variable which was dependent upon the degree of saturation of the specimens. To assess the influence of solute suction on the shear strengths of unsaturated soil, selected specimens were treated with KC1 prior to compaction. The effect of solute suction was to increase the value of cohesion in the modified Mohr-Coulomb strength relationship. As compared to untreated specimens, the effects of matrix suction in treated specimens were reduced. |
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