Abstract
Physico-chemical forces of repulsion and attraction exist in swelling or active clay-water systems. A modified shear strength formulation that extends the classical Terzaghi effective stress relationship to include physico-chemical forces is proposed in this paper. Residual shear strength comparisons were carried out for five carefully controlled, large-strain direct-shear tests on Na-montmorillonite before and after leaching with distilled water and under constant overall volume conditions. These tests indicated that the volume change and residual shear strength characteristics of Na-montmorillonite are governed by the true effective stress, σn*, defined in the modified effective stress relationship as the difference between the total stress, σn, and the summation of pore water pressure, uw, and the net physico-chemical forces of repulsion and attraction (R - A). Additional data available in the literature on residual strength of swelling clay minerals also appear to support the modified strength formulation. Actual measurements of overall (R - A) forces during leaching of the samples appeared to agree well with theoretical estimation of the net physico-chemical forces.
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