Geo-engineering properties of shallow landslides occurred due to large scale anthropogenic interventions: a case study of the humanitarian crisis-affected Rohingya Refugee camps in Bangladesh

Landslides occur every year in the Chittagong Hill Districts (CHD) of Bangladesh and cause life and property damages, especially during the monsoon. Due to the ethnic and social cleansing in Myanmar, nearly a million Rohingya populations have been fled to Bangladesh since 2017. They took shelters on a tectonically active, steep slope, and fragile lithological region the Kutupalong Rohingya Camps (KRC). They raze forests and cut slopes for their shelters and food. Which immediately resulted in numerous landslides and killed many peoples in the monsoon. The KRC area is stratigraphically composed of upper cohesive soil and lower non-cohesive sandstone. They belong to the Pliocene to Pleistocene age DupiTila, Girujan clay, and Tipam Formations. The exposed slopes were relatively high (>15 m) and steep ranging from 40 - 70 that have numerous polygonal tension cracks and fissures. The landslides are mostly of slide and slump type and have multiple heads like stair-step morphology and have retrogressive failure modes. The upper residual soils have a bulk density of 1.49 - 1.97 g/cm3, liquid limit of 25 - 48%, plasticity index of 5 - 16%, and undrained shear strength of 23 46 kPa and shows strain-softening behavior under stress. Whereas the lower sandstone unit has a bulk density of 1.44 - 1.94 g/cm3, an internal friction angle of 34 - 40, and cohesion of 0.5 - 13 kPa. The mineralogical composition determined by the X-ray diffraction shows low clay mineral content, which does not affect landslides. The low shear strength of materials, strain-softening of upper soil facilitate landslides in the KRC area. The presence of a high permeable layer underlying the residual soil and tension cracks allow preferential water infiltration during rainfall which may also contribute to the landslide occurrence. The state of equilibrium determines by LEM and FEM simulations using the field shear strength parameters shows that under natural conditions i.e., without any overburden loads slopes are stable. But slopes get unstable or critically stable when extra overburden loads are applied. Before the Rohingya settlement, there was no relict of landslides in the KRC area which in addition to the above-mentioned factors demonstrate that the large-scale anthropogenic interventions by the Rohingyas are the major cause of landslide occurrence.