Soils are naturally existing materials that challenges civil engineering structures such as roads and buildings. However, not all soils are suitable materials to uses due to their changing in strength and stability under fluctuating environmental conditions. Expansive soils, in particular, pose substantial challenges due to their propensity to experience considerable volumetric changes with variations in moisture content. This instability can lead to structural damage and increased maintenance costs. The study aims on improving soil's engineering properties using highland bamboo sawdust as a stabilizing agent in Jimma City. This study considered two soil samples collected from Jimma City, around the Jimma University Institute of Technology campus and Urael area. The soil samples were mixed-up with the highland bamboo sawdust in proportions of 4%, 8%, 12%, 16%, and 20 %by weight. Tests in lab, such as Atterberg limit, specific gravity, free swell, compaction, California bearing ratio (CBR), and unconfined compressive strength (UCS) were performed for the improvement of subgrade soil. The laboratory tests were performed on mixed soil samples according to the American Association of State Highway and Transportation Officials (AASHTO) and ASTM laboratory test procedures. The designated soil samples were categorized as A-7-5 soil based on the AASHTO and CH based on the Unified Soil Classification System (USCS), which is clay soil with poor engineering characteristics. Tolerable strength was attained with 16%, which is the ideal Highland bamboo sawdust (HBSD) content in improvement of the expansive soil nature. Maximum Dry Density (MDD) improved from 1.45g/cm3 in natural soil to 1.57g/cm3 in 16% HBSD-treated soil. There is a significant change in CBR values from 1.31% to 9.8% with and without highland bamboo sawdust, respectively. Thus, the optimum values were attained at 16% HBSD.

Engineering properties, expansive soils, experimental, highland bamboo sawdust, optimum ratio