Development of swelling induced shear and slickensides in Vertisols

Abstract

Vertisols shrink and crack as they dry, and swell as they wet. Shear surfaces, sometimes called slickensides, are often observed within the profile of such clay soils. A hypothesis for the appearance of these surfaces is in reaction to lateral swelling forces that induce shear within the soil mass. No direct proof supporting this hypothesis is found in the literature.Test data presented verify the development of lateral soil swelling pressure in response to wetting of a compacted clay soil. The data illustrate that the swell induced pressure can cause the soil to reach failure conditions and in essence, shear itself.During wetting, laboratory compacted specimens of clay were subjected to constant vertical pressure and allowed to swell in the vertical direction. Strain in the lateral direction was restrained and the horizontal soil pressures monitored as they approached a residual magnitude. From the series of tests it was found that the ratio between the residual horizontal swell pressure to the applied vertical pressure decreased monotonously with increase of applied vertical pressure.Mohr's circles representing the state of stress within the specimen at the residual state align along the Mohr-Coulomb failure envelope for tests performed at vertical pressures of 4 kPa through 27 kPa.These results provide credence to the hypothesis that slickensides are formed due to internal shear of the clay, and as such are expected to form at limited depths.