Landslide Mitigation in Volumetric-Sensitive Soils: Integrated Stabilization Design for the Kalibeng Formation

Abstract

Landslides and ground movements significantly threaten Semarang City, Indonesia, particularly in the Deliksari area. This region’s vulnerability stems from its location within the Kalibeng Formation, characterized by brownish-gray marly clay identified as montmorillonite. This expansive clay, combined with complex geological structures like joints and faults, creates high soil porosity and erosivity, leading to discontinuous layers prone to failure. This study conducts a geotechnical assessment and stability analysis. Field investigations via cone penetration tests (qc) yielded a cone resistance of 82 kg/cm² (8.04 MPa) at 20 m depth. Laboratory results showed a Liquid Limit (LL) of 148.12% and free swell of 425%. Using the Stark and Eid correlation, we determined the average friction angle (ϕAverage) to be 12.65°. Back-calculation via GeoStudio on the existing slope revealed a critical Safety Factor (SF) of 0.343. To mitigate instability, we modelled two reinforcement simulations. The first, utilizing a counterweight, gabions, bamboo piles, and one 600 mm diameter bore pile, yielded an SF of 0.684. The second simulation, which added a second bore pile, significantly increased stability to an SF of 1.611. Consequently, the recommended design features a 1V:5H slope counterweight supported by two bore piles, gabions, and 4 m deep bamboo piles. To manage pore water pressure, a 60 cm sand drain wrapped in non-woven geotextile is integrated between the embankment and original soil.