Integrated Geotechnical and Seismic Stability Assessment of Tropical Residual Slopes in Sitinjau Lauik, West Sumatra, Indonesia

Sitinjau Lauik, a national road corridor in West Sumatra, Indonesia, is one of the most landslide-prone segments of the Bukit Barisan volcanic belt. This study assesses the geotechnical and seismic stability of three tropical residual soil slopes at this location by combining cone penetration testing (CPT), laboratory index testing, consolidated-drained direct shear testing (DST), and Morgenstern–Price limit-equilibrium analysis under static and pseudo-static seismic loading. CPT profiles revealed a weak layer (qc = 0.4–0.6 MPa) at 1.2–2.8 m depth in Slope 1. This slope also has a high plasticity index (PI = 39.7%), a high void ratio (e = 1.50), and a low dry density (γd = 10.06 kN/m³), resulting in lower shear-strength parameters (c' = 17.8 kPa, φ' = 19.2°) than Slopes 2 and 3 (PI < 10%). Static factors of safety (FS) were 1.08, 1.54, and 1.49 for Slopes 1, 2, and 3, respectively. A pseudo-static coefficient of kh = 0.15, consistent with regional peak ground acceleration zoning, reduced the FS values to 0.77, 1.23, and 1.17, making Slope 1 unstable during earthquakes. At kh = 0.20, none of the slopes met the minimum FS of 1.1 required by the Indonesian National Standard (SNI) 8460:2017. High plasticity, a high void ratio, and low CPT resistance distinguished the critical Slope 1 from the other slopes. Because only three slopes were examined, these findings are site-specific; nevertheless, they provide a basis for prioritizing slope remediation and early warning along this corridor.