Shear strength and dilatancy behavior of unsaturated diabase and granite residual soils

Residual soils are commonly found in slope areas in Brazil. Considering variations in stress and confinement, geotechnical studies often identify issues related to landslides, slope instability, and soil collapse, particularly under unsaturated conditions. This research aims to understand the unsaturated shear behavior of two residual soils in their intact state at different weathering levels, correlating soil structure and suction with shear strength. First, sampling locations were selected, and two types of soils were defined: granite residual and diabase residual. Undisturbed samples were saturated and then dried using the filter paper method, achieving suctions between approximately 0 and 1000 kPa. Subsequently, the samples were sheared under normal stresses of 50, 100, 200, and 300 kPa. A comprehensive investigation was carried out based on dilatancy analysis and three-dimensional nonlinear envelopes. Results showed that the soil structure, formed by different weathering processes, plays a significant role in strength gain. The granite residual soil, with a bimodal structure, exhibited a 50% increase in dilatancy strength compared to the saturated condition. Conversely, the diabase residual soil, characterized with a unimodal structure, showed insignificant influence of suction on stress increase.