Unsaturated Shear Strength of a Sri Lankan Residual Soil from a Landslide-Prone Slope and its Relationship with Soil–Water Retention Curve

Unsaturated soil mechanics has become a topic of interest among the Sri Lankan geotechnical community owing to its importance in different applications, especially slope stability. In this study, soil samples collected from a landslide area in Sri Lanka were subjected to soil–water retention curve (SWRC) tests and suction-monitored direct shear tests. SWRC tests were conducted on both undisturbed and statically compacted samples and the results were modelled using Van Genutchen equation. The compacted samples having the same dry density as the undisturbed block samples exhibited a similar SWRC. Consolidated drained direct shear tests and suction-monitored direct shear tests were conducted on compacted samples to determine saturated and unsaturated shear strength respectively. The unsaturated shear strength followed a non-linear envelope with respect to matric suction, as shown by a good correlation for the cases of second-order polynomial correlation (R² = 0.970) and the prediction using Vanapalli et al. model and SWRC with K = 1.2 (R² = 0.941). These test results confirm the validity of using SWRC for predicting unsaturated shear strength of a compacted Sri Lankan silty sand.