This study reports on results from a series of geohydraulic, geomechanical, and mineralogical tests conducted on 12 different landslide samples consisting of broadly graded and potential internally unstable soils. The test samples contained almost 50% coarser or rock (cobbles-gravels) fraction and 50% finer or soil (sand-silt) fraction. The current test results characterized the landslide samples as highly erodible, while their regraded rock and soil fractions showed non-erodibility with good strength and drainage characteristics. For instance, the landslide samples suffer from a classical suffusion failure at critical hydraulic gradients below 0.38 with up to 20% erosion, while their regraded sand-silt fraction remains internally stable even at \({i}_{cr}\) up to 0.80 with less than 4% erosion. Subsequently, the results of pinhole erosion testing revealed that the silt component exhibited a high dispersion index D1. Similarly, the coarser fraction showed more than 95% slake durability, while the point load testing both parallel and perpendicular to the planes of weaknesses yielded a shale rating of nearly 8 reflecting high durability against abrasion. Results of both XRD and SEM analyses revealed that the intact landslide samples contained up to 50% Quartz and 42% Calcites with traces of Halite, Bornite, Silicon, and Fluorite minerals. However, the Calcite content of the displaced samples reduced to as low as 11% with subtle variations in other minerals that would result in lower inter-particle cementation and enhanced erodibility, thus making the entire soil matrix a highly erodible material. In essence, the Calcite provides sufficient cementation to the finer fraction that enables the particles to stick together and stay intact inside the constriction network of coarser fraction to ensure stability. However, the loss of Calcite and hence the cementation would trigger the internal erosion of soil fraction to eventually develop a landslide on steeper slopes.
