Geotechnical Characterisation of Subgrade and Sub-Base Materials for Road Pavement Design in Kasese District, Western Uganda

Abstract Abstract Pre-design and construction geotechnical assessment of subgrade and sub-base materials was carried out for optimal pavement performance and sustainability in Kasese District, Western Uganda. The method involved boring 30 trial holes, sampling, and geotechnical testing. Soil profile indicates a top organic clay layer (OL/OH) from surface to 0.45 m, a silty clay layer (CL/CI) to 1.2 m, and a poorly graded sand with gravel layer (SP/GP) to 2.0 m. Subgrades classify as USCS CL-CI, AASHTO A-2-6, A-2-7, and A-7-6, with 62% rated as poor to fair and 38% good to excellent due to high seasonal moisture from nearby Rwenzori Mountains and Nyamwamba River floods. Clay activity ranges from 0.9 to 38.7, expansivity from 5.2 to 62.4, indicating presence of sensitive and expansive clays. 48-hour soaked CBR varies from 5% to 32%, surface modulus (Eₛ) from 55 MPa to 150 MPa. Sub-base fills have moisture content 14-22%, liquid limit 22-35%, plastic limit 12–20%, plasticity index 8-14, composed of 32% fines and 68% sands; AASHTO A-2-6 rated excellent to good. Soaked CBR of sub-base ranges 18-42% with 12-58% strength reduction upon soaking. Resilient modulus varies from 200-500 MN/m². Recommended pavement thickness is 350-400 mm based on CBR and group index, with coarse granular sub-bases recommended for moisture egress to improve effective stress.