Failure Mechanism, Residual Shear Strength Back-Analysis, and Remediation Design of a Landslide in Weathered Gypsum Deposits

Landslides in weathered gypsum deposits present significant geotechnical challenges because progressive weathering, groundwater fluctuations, and residual strength degradation strongly influence slope stability. This study investigates the failure mechanism and remediation of a large translational landslide in weathered gypsum deposits in Çankırı, Türkiye. An integrated engineering geological assessment was conducted using data from 16 boreholes, laboratory tests, and groundwater/inclinometer monitoring records, followed by residual shear strength back-analysis and slope stability evaluation. A three-dimensional geological model was developed, and cross-sections were analyzed using the Morgenstern–Price limit-equilibrium method. Back-analysis identified residual shear strength parameters of c′ = 7.5 kPa and ϕ′ = 10° for the weathered gypsum, while laboratory direct shear tests yielded c′ = 4.0 kPa and ϕ′ = 9.9°. The friction angles obtained from the two approaches are nearly identical, whereas the back-calculated cohesion is slightly higher than the laboratory-derived value. Back-analysis parameters were used to design remediation measures, including slope unloading, rock buttress construction, toe fill improvement, and surface/subsurface drainage. Stability analyses increased the factor of safety to 1.76 under static loading, while pseudo-static analyses satisfied the recommended seismic design criterion (FS ≥ 1.10). Equivalent-linear Newmark analyses predicted a permanent displacement of 15 cm, within acceptable limits. The methodology provides a practical framework for assessing and stabilizing landslides developed in weathered gypsum deposits in seismically active regions.