Shear Strength of Unsaturated Organic Clay across Natural Dry-Density Gradients from Landslide Sites in Bogor, Indonesia

Rainfall-induced landslides in tropical regions are closely linked to the loss of apparent cohesion as unsaturated soils approach saturation. This study examines how the apparent cohesion (c′) and internal friction angle (φ′) of unsaturated organic clay respond to variations in natural dry density (γd) across a range of degrees of saturation (Sr), using natural dry density values obtained from three actual landslide sites in Bogor, Indonesia, rather than from laboratory-compacted specimens. A total of 231 direct shear tests (3 sites × 7 specimens × 3 normal-stress levels × 3–4 repetitions) and 42 filter-paper tests (3 sites × 7 specimens × 2 repetitions) were performed on remolded specimens reconstituted to in-situ γd values at seven Sr levels. The results reveal four main findings. The three soils have very low and uniform air-entry values (1.52–2.09 kPa), so the suction contribution is lost soon after infiltration. Apparent cohesion does not decrease monotonically but peaks at intermediate saturation (44–56%), with peak values tiered according to density (10.60; 6.30; 4.69 kPa). c′ and φ′ respond to Sr through different mechanisms: c′ peaks while φ′ decreases monotonically. The cohesion advantage of the densest soil, prominent under moist conditions, vanishes near saturation, where the cohesion of the three densities converges to a nearly identical low value. A multiple linear regression model, c′ = 16.162·γd − 0.089·Sr − 5.076, is proposed as a practical estimator. These findings imply that initial density alone does not guarantee slope stability under heavy rainfall.