Effects of Drying and Soil-Base Interface on the Behavior of an Expansive Soil Mixture

Expansive soils exhibit complex behavior associated with significant volume changes triggered by moisture variations induced by changes in environmental conditions. This type of process impacts on soil properties, such as shear strength, stiffness, and permeability. Also, cracks generally develop upon desiccation. There are still several gaps in the current knowledge in this area, particularly in relation to the factors that affect the shear strength of soils and control the formation of drying cracks. This paper focuses on the impact of some of these factors on crack formation and shear strength of an expansive soil mixture made up of 75% kaolin and 25% bentonite, % by mass. Three factors were considered in this research; soil structures (associated with different sample preparation methods), initial saturation, and textures at soil-base interface. Two laboratory series consisting of desiccation plate and soil-base interface shear tests were conducted. For each series, three types of soil specimens were prepared considering different preparation methods (with the associated different soil structures) and initial saturation conditions namely; slurries, fully-saturated compacted and unsaturated compacted specimens. Soil-base interface shear strengths were determined using four different interface textures; grooves oriented perpendicular to shear direction, spiral (circular) indentations, grooves aligned parallel to the shear direction, and smooth surface. For the desiccation plate tests, two types of textures were adopted for the plate-base, namely, circular indentations and smooth to study constrained and free shrinkage conditions, respectively. Results of desiccation tests revealed that cracks developed in the constrained plate (e.g., with circular indentations), whereas no cracks were observed under free displacements conditions (e.g., smooth surface). The direct shear tests revealed that the soil-base interface shear strength is strongly affected by the specimen moisture, soil structure and the direction of the shearing  respect to the grooves orientation. For the three set of experiments conducted (i.e., slurry, compacted-saturated, compacted-unsaturated), the highest strength was found for the surface with groove oriented perpendicular to shear direction and the lowest one for the smooth plate. The shear strength associated with the circular-grooves base is in-between those ones observed for the plate with grooves oriented perpendicular and parallel to the shear direction.