Role of apparent cohesion in the stability of Dominican allophane soil slopes

This study examines the effect of loss of apparent cohesion from rainwater infiltration upon the stability of partly saturated, allophanic soil slopes of Dominica (West Indies). The parent material of the Dominican allophanic soils are the andesitic and dacitic volcanic rocks from ten volcanic centres of mainly Pleistocene age. Although simplifying assumptions are made to assess the depth of wetting front and magnitudes of true and apparent cohesion values of the partly saturated allophanic soils, certain inferences of practical significance emerge from the study.\ud Matric suction contributes to the effective stress of unsaturated soils and increases the shear strength of these soils by imparting them an apparent cohesion strength component. This apparent cohesion strength component of the partly saturated Dominican allophanic soils is considered to be related to the matric suction term (u(a)-u(w)) by a parabolic relationship.\ud The partly saturated allophanic soil slopes of Dominica would invariably fail from loss of apparent cohesion upon saturation of the soil mantle by the infiltrating water front only if (a) the average slope angle (beta) is greater than or equal to the drained shear strength parameter of the unsaturated soil (phi(d)) (slopes with beta greater than or equal to phi(d) are referred to as category 1 slopes in this study) and (b) the drained cohesion parameter (c(d)) belonging to the partly saturated soil of the category 1 slope is mainly contributed by matric suction induced apparent cohesion (c(app)) and contribution from true cohesion (c') is absent. However, the possibility of the category 1 slopes (for the case 1 situation) failing in the saturated condition from effective stress reduction due to rise in ground water table is shown to be an unfeasible proposition. In contrast, for the case 1 situation, the partly saturated category 2 slopes (beta