Unsaturated Soil Characteristics of Residual Soil in Indonesia

Unsaturated soil mechanics offers a rigorous framework for analyzing geotechnical problems in which the near-surface strata remain partially saturated for many years. Many slope failures are initiated within these layers, particularly during rainfall stability assessments, and must account for hydraulic mechanical coupling beyond saturated assumptions. This study characterizes the hydraulic behavior of residual soils through the soil–water characteristic curve (SWCC), relating matric suction to water content, as the principal descriptor for incorporating suction effects into strength and deformation analyses. Field observations and laboratory testing were undertaken; matric suction was measured using the filter paper method, and SWCCs were derived to parameterize suction-dependent shear strength for limit-equilibrium evaluation of slopes subjected to infiltration. This work demonstrates how adopting SWCC-based parameters refines the safety factor during transient wetting and clarifies the contribution of suction to apparent cohesion. Although the routine use of unsaturated parameters is not yet a standard practice in Indonesia, the results provide a practical pathway for integrating matric suction into conventional stability calculations and developing context-appropriate guidance. More broadly, these findings reinforce the need to embed unsaturated soil mechanics in slope design and risk assessment, where rainfall-induced failures are of concern. Keywords: unsaturated soil mechanics; residual soil; matric suction; soil–water characteristic curve (SWCC); rainfall infiltration; slope stability; soil shear strength.