Mineralogical Influence on the Geotechnical Properties of Shallow Landslide Hazard Site Soils of Rangamati Sadar Area, Bangladesh

Abstract

This study has evaluated the mineralogical influence on geoengineering properties of some soil samples of the landslide site in the Rangamati Sadar area, Bangladesh. This research has assessed field data interpretation, i.e. borehole lithology, SPT data analysis, the mineralogical composition, and basic engineering properties such as grain size analysis, moisture content, specific gravity, Atterberg limit i.e., liquid limit, plastic limit, plasticity index, engineering soil classification of Rangamati Sadar, Bangladesh. The studied soil is broadly divided into cohesive and non-cohesive soils and the SPT values increase with increasing depth. The field SPT values suggest that the ground condition of the studied cohesive soil is mainly stiff to hard silty clay (CL-ML) and non-cohesive soils are medium-dense to densely compacted sandy (SM) soil. The uppermost part of cohesive soil up to 3m is mainly composed of silty clay with low SPT values which is highly vulnerable to landslides. The non-cohesive soil sand up to 7.5 m is also highly vulnerable to landslides. The mineralogical information on soil clay and non-clay minerals has been identified by using XRD (X-ray Diffractometer).  The non-clay minerals include quartz, orthoclase, plagioclase, and Mica and the clay minerals are mainly illite, chlorite, and kaolinite occur in very small amounts. The natural moisture content values of the samples range from 15.65% to 32.19% and the average is 25.67%.  The specific gravity value ranges from 2.20 to 2.93, the average is 2.48, and the values decrease with increasing depth. The obtained values are closer to the values recommended for Illite –Chlorite. For geotechnical investigations, three types of soils (sand, silt, and clay) have been categorized based on grain size distribution. The soil samples are mainly composed of sand with silt and a small amount of clay and might be defined as silty sand. The values of Cu and Cg suggest that the studied soil is well-graded. The liquid limit values range from 28.01% to 48.06%. The plastic limit is in the range of 14.72% to 23.69%. The plasticity index values lie between 7 % and 21.48%. According to the plasticity chart, the Clay soils of Rangamati can be characterized as low to medium plasticity inorganic clay and classified as CL to CM from their position in the plasticity chart which has low to medium swell potential.