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

Abstract

On 13 June 2017, a large-scale landslide hit the south-eastern Tertiary mountainous region of the country, surpassing all previous disaster records in Bangladesh. At least 165 people, including children and four army personnel, were killed in different landslides caused by deep landslides in different parts of the Rangamati, Bandarban, and Chittagong districts. In the fieldwork at Kawkhali Upazila of Rangamati district, the original slope angle is measured at different sites where landslides took place. Samples were gathered from various destinations of the field for grain size investigation and direct shear test individually. It was completed at the Engineering Geological Laboratory of the Geological Survey of Bangladesh (GSB), considering the ASTM Standard Test Strategy for particle size investigation of soils. The direct shear test is covered in ASTM D 3080 - standard test method for direct shear test of soils under consolidated drained conditions. Grain size, Hydrometer, Atterberg limit and shear strength parameter tests are executed to observe geo-information to determine the risk assessment. The test result obtained from sieve analysis shows that the amount of sand in the sample is 95.84 - 99.51% whereas the total amount of silt and clay is 0.49 - 4.13%. The uniformity coefficient Cu is 2.79-6.5 and the coefficient of curvature Cc is 0.55-1.96. From the hydrometer analysis result it is seen that sand particles vary from 0.44 to 58%, silt particles vary from 29.52- 75.10%, and clay particles vary from 4.5- 42.5%. Samples can be categorized as 5 samples are silty sand, 6 samples are sandy silt, and 6 samples are clayey silt, depending on the particle sizes present in soil. The average liquid limit of the review region is 34.94%, demonstrating the high expansion potential of the soil. In this condition, the deformation of the slope is possible ultimately resulting in the slope being unsteady. Results from the direct shear test showed that the cohesion of slope materials ranges from 2.41 to 15.56 kPa, while the internal friction angle varies from 26.75° to 35.40°. Hence, a higher percentage of sandy materials, low cohesion value and greater original slope angle than internal friction angle were the main attributes that severely affected slope stability, which, in turn, triggered landslides in the study area.