Multipurpose Engineering Geological Mapping of Yirgalem Town, Southern Ethiopia

Abstract

In Ethiopia, many engineering works are constructed and under construction in different towns without proper and adequate geotechnical characterization of soils and rocks to know the site suitability for intended construction activities. Yirgalem Town was among these towns. Currently large engineering work including Yirgalem integrated agro industrial park constructed, highway and railway route which connect the country with neighboring are planned for construction. Lack of detail geological and engineering geological data and respective maps, poor drainage, and surface geodynamic problem are the main problems encountered at Yirgalem Town. Therefore, this study aims to address geological and geotechnical characterization and classification of soil and rocks to produce a detailed medium scale multipurpose engineering geological map of the study area. Secondary data collection, field and laboratory work, sampling data, discontinuity survey, data analysis, and interpretation, and preparation of maps were approaches and methods used to carry out this research by using ArcGIS software, Surfer, and SRTM DEM image. Results from laboratory data analysis show that moisture content ranges from 10.05 to 49.5%, the liquid limit ranges 49.5-70, the plastic index ranges 13.84 – 36.9 and from grain size analysis silt soil was found to be dominant. The activity of soil ranges 0.62 -1.53(inactive to active) and free swell test ranges 13-104% indicating low to high potential expansion. The shear strength parameters were analyzed based on Mohr coulomb failure criterion, cohesion (C), and internal friction angle (Ф) of the soil sample ranges from 9.928-39.7KN/m2 and 22.6-34.31 respectively. Engineering classification of soil based on Unified soil classification system of soil falls into poorly graded sand, poorly graded sand with silt, sand with silt, elastic silt, and fat clay. Elastic silt soil is dominant and indicates most of the soils in the study area are poor for engineering purpose. Characterization and classification of rocks in the study area were based on the empirical relation of Schmidt hammer rebound value with compressive strength, weathering, and discontinuity survey, and rock masses of Yirgalem Town were grouped as low, medium, and high mass strength. Gully erosion, landslide, and problematic soil are the main surface geodynamic problems identified during field investigation. The study conclude that most of the soils in the study area are expansive and poor for engineering workability and the different engineering structures to be built in the town require detailed site-specific geotechnical investigation. It is therefore recommended to make further slope stability assessment, and detailed surface and subsurface geophysical investigation.