Stability of long uniform slopes in cohesionless soils with seepage parallel to slope surface

A method is employed in this paper for the computation of earth pressures mobilized by landslides in cohesionless soil slopes against retention systems. For calculation purposes, the water table is taken to be parallel to the slope and at a depth zw. Steady seepage is assumed to be taking place in a direction parallel to the slope. End effects are taken into account. In the upper part of the slope, the soil is considered to be in a Rankine active state of equilibrium and a Rankine passive state of equilibrium is assumed to reign in the lower part of the slope. The infinite slope concept is applied to the central part of the slope. The theory shows that the slip surfaces below the water table are curved, not straight lines. The theoretical procedure developed in the present paper was applied to the analysis of an initially unstable long slope that failed in spite of the presence of a shear pile wall made up of a row of closely spaced 1m-diameter bored piles. The retaining system was designed based on the Rankine active state of equilibrium, using a factor of safety of about 1.7. It is shown that, as the shear pile wall was located in the lower part of the slope, it was subjected to a much greater force, compatible with a Rankine passive state of equilibrium, corresponding to a factor of safety of about 0.9. As a result, the shear pile wall was doomed to fail.