Geotechnical Data Analysis Contributing Rainfall Triggering Landslide Risk Assessment at Kawkhali Upazila of Rangamati District, Bangladesh

Abstract

On 13 June 2017, a catastrophic landslide struck the south-eastern Tertiary hilly region of Bangladesh, marking one of the deadliest disasters in the country’s history. At least 165 people, including children and four army personnel, lost their lives in multiple slope failures across Rangamati, Bandarban, and Chittagong districts. Detailed field investigations were conducted in Kawkhali Upazila of Rangamati district, where original slope angles were measured at affected sites. Soil samples were collected for laboratory analyses at the Engineering Geological Laboratory of the Geological Survey of Bangladesh, following relevant ASTM standards. Particle size distribution was determined using sieve and hydrometer analyses, while shear strength parameters were obtained through direct shear tests under consolidated drained conditions, ASTM D 3080. Sieve analysis revealed that sand content ranged from 95.84 to 99.51 percent, with silt and clay comprising 0.49 to 4.13 percent. The uniformity coefficient varied between 2.79 and 6.5, and the coefficient of curvature ranged from 0.55 to 1.96. Hydrometer results indicated variable proportions of sand, silt, and clay, leading to classification as silty sand, sandy silt, and clayey silt. The average liquid limit was 34.94 percent, indicating moderate plasticity and potential deformation. Cohesion values ranged from 2.41 to 15.56 kPa, and internal friction angles from 26.75° to 35.40°. High sand content, low cohesion, and steep slope angles collectively contributed to slope instability and failure.