Lateral earth pressures in expansive clay soils

Abstract

Lateral earth pressures produced by saturated clays with negative pore-water pressures and unsaturated expansive clays with positive matric suctions are considered from a theoretical standpoint. Simple earth pressure equations are formulated in terms of total stresses using the Mohr–Coulomb failure criteria and the assumptions consistent with the Rankine earth pressure theory.Conventional practice is to separate the pressure that the soil exerts on the wall into the pressure produced by the soil structure (effective stress) and that produced by the water (neutral stress). Since two stress state variables are required to describe the behavior of unsaturated soils, the expressions can no longer be separated into two distinct components.The change in lateral pressures resulting from decreases in pore-water pressure or increases in matric suction are quantified by considering a 6 m high wall for the active and passive cases. Tension cracks are shown to have little effect on the conditions shown.The magnitude of the lateral pressure generated due to changes in matric suction under conditions where walls are restrained from moving depends upon the ratio (KT) of horizontal to vertical stress and the matric suction of the backfill at the time that it is placed behind the wall. When structural members are cast directly against undisturbed clays, similar criteria govern the magnitude of the lateral pressures that may be generated due to changes in matric suction. The maximum pressure that can be developed in some cases will be equal to the passive pressure of the soil when it is saturated.An expression for the critical height of a vertical bank for the saturated and unsaturated cases is developed. The role of tension cracks is shown by computing the critical height of a vertical slope for a typical soil both in the absence and in the presence of tension cracks.In many instances it is apparent that methods devised to sustain the matric suction at or near its original value may result in a substantial savings in design or that they will ensure adequate performance standards for existing facilities. Keywords: Lateral earth pressure, active pressure, passive pressure, unsaturated soils, expansive soils, tension cracks, retaining walls, unsupported excavations.