Abstract:
In geotechnical practice, soils are generally assumed to have negligible tensile strength. However, in the context of unsaturated soils, this strength can be significant and important to consider. In contrast to the two-phase soil matrix considered in classical saturated soil mechanics, the presence of a third air phase in unsaturated soils brings about the manifestation of surface tension and matric suction. These phenomena provide unsaturated soils with a certain degree of tensile strength, which in turn affects the overall shear strength behaviour of the soil mass. This dissertation investigates the behaviour of three different unsaturated soils tested using the Brazilian Disc Test (BDT). The soils considered were two types of tailings from South African mines, i.e. gold and iron tailings, and one natural silty, clayey soil from Centurion, south of Pretoria. The tensile strength of 120 samples was determined using displacement controlled Brazilian Disc Tests with sample deformation recorded using Digital Image Correlation (DIC). Through the use of 30° curved loading strips, the BDT was found to consistently result in centrally located crack initiation by reducing compressive stress concentrations close to the points of load application. This modification of the loading conditions prevented the premature ‘crushing’ failures observed by previous authors. It was found that while the testing procedure was suitable for all soils considered, the increased ductility at high moisture contents resulted in a more complex material response upon loading. For these ductile samples, it was found that simply adopting the maximum achieved load as being indicative of the mobilisation of tensile strength is inadequate and can result in a gross overestimation of the tensile strength of these soils. The tensile strengths measured for each soil type, across a range of moisture contents, were subsequently compared to the Soil Water Retention Curves (SWRC) of each material. This comparison allowed for correlations between matric suction and tensile strength to be determined. For both gold and iron tailings, an increase in tensile strength was observed with an increase in suction. The contribution of matric suction to tensile strength was however found to diminish in the pendular regime. In contrast, the tensile strength of the Centurion soil increased with a reduction in saturation, achieved a peak value and decreased in the pendular regime. This trend was attributed to the dependency of both surface tension and matric suction on the contact area between particles, which in turn is affected by particle shape.
