The present research study aims to create accurate and comprehensive inventory mapping while investigating the geomorphological and geotechnical characteristics of the large, deep-seated, and damaging El Kherba landslide triggered by the August 7, 2020 (Mw 4.9) Mila earthquake. The methodology relies on the analysis of results obtained through detailed field investigations, satellite image interpretation, deep boreholes equipped with piezometers, laboratory tests, in situ tests, and numerical simulations. The resulting landslide inventory map reveals a significant earth slide with an active zone covering a surface area of 1.565 km2, extending approximately 2,166 km in length, with a width ranging from 40 m to 1.80 km, and a volume of 25,784,909 m3. Geomorphological field mapping results revealed a large and deep-seated morphological deformation related to: (i) the weak mechanical resistance and low stability slopes that the seismic strengths caused a reduction in the shear strength of the soil; (ii) Miocene clays, highly altered and potentially subject to shrinkage and swelling; (iii) a partial reactivation of a previously existing large landslide; (iv) human activity such as slope excavation and unplanned urbanization; and (v) topographical and lithological site effects. The results of geological and hydrogeological investigations indicated the presence of: (i) thin and thick weak-resistance interlayers of altered and plastic clays with weak resistance, which may constitute shear surfaces; (ii) a shallow aquifer that impacted the mechanical resistance characteristics. Laboratory tests revealed that the fine clay in the soil was highly weathered, with a low dry density and a high moisture content, along with a high saturation and plasticity, making it very sensitive to the presence of water. Undrained triaxial cyclic loading tests indicated a high potential for the generation of excess pore-water pressures in the material during seismic loading. The direct shear test showed that the disturbed soils had an average cohesion of 33.4 kPa/m2 and an internal friction angle of 18.21°, indicating poor structural and shearing strength. The results of the oedometer test indicated that the soils are compressible to highly compressible, overconsolidated, and have the potential for swelling. According to the Manard pressuremeter test (MPT) and available empirical relationships, the landslide exhibited a deep-seated nature, with sliding surfaces located along weak geotechnical characteristics interlayers at a depth ranging between 10 and 40 m. The depths of failure obtained from the MPT were consistent with those determined by the empirical relationships available in the literature and numerical simulations. This comprehensive research provides valuable data on earthquake-induced landslide and can serve as a guide for the prevention and mitigation of landslide risks.
