Understanding the failure mechanism of a gently inclined earth slide in highly weathered pyroclastic rocks (Java, Indonesia)

Most landslides in tropical regions initiate in steep terrain, but we investigate an earth slide on a gently inclined pyroclastic slope in West Java. On 18.02.2024, cracks were observed on a slope with a gradient of only 12°, composed of completely weathered pyroclastic rocks, indicating precursory signs of a large-scale earth slide. Eleven days later, on 29.02.2024, slope failure occurred, leading to the displacement of more than 90,000 m³ of material and causing damage to 39 houses, one elementary school, and an inter‑village road. We acquired UAV-based photogrammetry to build a digital elevation model and we reconstructed the pre‑failure topography. Fieldwork based on geomorphological-geological mapping and geotechnical drilling was performed to investigate the spatial distribution, thickness, geometry and kinematics, geological structure, and hydrogeological setting. Samples from selected outcrops and core drillings were collected for comprehensive laboratory testing to determine grain-size distribution, clay mineralogy composition, strength and hydrogeological properties. In addition, meteorological data were analyzed and geomechanical modeling were done to elucidate the failure mechanism. Our geological-geometrical model indicates a translational earth slide with a gently dipping basal shear zone of about 10° at a depth of 13–14 m, and a rotational secondary slide at the toe. Geologically, a three‑layer model from bottom to top is derived: Layer 1 consists of a sequence of sandstones and claystones with a gently dipping bedding that locally aligns with the sliding direction; Layer 2 is composed of highly weathered pyroclastic rocks, i.e., tuffaceous claystone and sandstone; and Layer 3 is a completely weathered tuff unit. Our investigations indicate that the clay-rich basal shear zone is located at the contact between Layer 1 and 2. Extensive clay mineralogy analyses of 37 samples show that smectite and vermiculite are the primary clay minerals of all layers. Considering a 25-year rainfall record, the 1.9.2023 to 29.02.2024 rainy-season accumulation of 1,666 mm is near the upper bound of 1,746 mm and clearly above the mean of 1,181 mm. A back analysis taking into account the observed groundwater conditions and assuming a safety factor of approximately 1 yields a cohesion of 4 kPa and a friction angle of 10°, which indicates a smectite-rich shear zone with exceptionally low strength behavior. Our results show that heavily weathered pyroclastic rocks in tropical regions promote the formation of landslides on slopes even with a low angle of inclination. In addition, the slope was exceptionally wet during the rainy season leading up to the failure in February 2024. This factor, combined with changes in land use over time, may have had a negative impact on slope stability. Therefore, hazard mitigation measures should be based on controlling surface runoff through a modern drainage system and on land use planning.