ROLE OF APPARENT COHESION IN THE STABILITY OF DOMINICAN ALLOPHANE SOIL SLOPES

This study examines the effect of loss of apparent cohesion from rainw ater infiltration upon the stability of partly saturated, allophanic s oil slopes of Dominica (West Indies). The parent material of the Domin ican allophanic soils are the andesitic and dacitic volcanic rocks fro m ten volcanic centres of mainly Pleistocene age. Although simplifying assumptions are made to assess the depth of wetting front and magnitu des of true and apparent cohesion values of the partly saturated allop hanic soils, certain inferences of practical significance emerge from the study. Matric suction contributes to the effective stress of unsat urated soils and increases the shear strength of these soils by impart ing them an apparent cohesion strength component. This apparent cohesi on strength component of the partly saturated Dominican allophanic soi ls is considered to be related to the matric suction term (u(a)-u(w)) by a parabolic relationship. The partly saturated allophanic soil slop es of Dominica would invariably fail from loss of apparent cohesion up on 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)) (slo pes with beta greater than or equal to phi(d) are referred to as categ ory 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 pos sibility of the category 1 slopes (for the case 1 situation) failing i n the saturated condition from effective stress reduction due to rise in ground water table is shown to be an unfeasible proposition. In con trast, for the case 1 situation, the partly saturated category 2 slope s (beta<phi(d)) are insusceptible to failure from reduction in effecti ve stress due to loss of matric suction but fail in the saturated cond ition from effective stress reduction due to rise in ground water tabl e. If the allophanic soils were to possess a sufficient magnitude of t rue cohesion (c'), even the partly saturated category 1 slopes possess ing slope angles (beta) much in excess of their phi(d) values (beta-ph i(d) = 15.1 degrees) would remain stable upon total elimination of the matric suction induced cohesion. However, partly saturated category 1 slopes possessing a true cohesion component and insusceptible to fail ure from reduction in effective stress upon loss of matric suction, in variably fail in the saturated condition from reduction in effective s tress due to rise in ground water table. Using the methodology develop ed in this study, two previously reported case histories pertaining to rain induced translational landslides in the residual soil areas of B razil are re-examined and the results obtained in this study are found to be in agreement with the findings of the previous researchers.