Expansive clays pose significant challenges against transport infrastructures, particularly for lightweight structures such as roads and culverts. The cost of construction increases due to the need to transport selected materials from quarries to the construction sites for the replacement of the expansive soils with qualified quarry materials. On the other hand, excessive utilization of quarry sites harms the environment and is not a sustainable practice. This study evaluates the use of sugarcane bagasse ash (SCBA) and nonwoven geotextile (NWG) to improve the engineering characteristics of expansive subgrade clay soils. Mineralogical and mechanical tests were conducted on two soil samples taken from Jimma City. The specimen preparations and tests of the samples were carried out based on the ASTM and AASHTO standard procedures. X‐ray diffraction (XRD), moisture content, grain size distribution, specific gravity, free swell, Atterberg limits, compaction, California bearing ratio (CBR), and unconfined compressive strength (UCS) tests were performed. The trial mixes were prepared by combining the soil + SCBA, soil + NWG, and soil + SCBA + NWG. The SCBA was thoroughly mixed with the soil, while the NWG was applied in layers at different depths in the compacted soil in the CBR mold. Both descriptive and analytical methods were used to analyze the laboratory results. For the optimum amount of soil + SCBA mix, the 96‐h soaked average CBR values were increased by 215%, while for the soil + SCBA + NWG mix, the values were improved by 805%. Generally, the use of SCBA has improved both the physical and mechanical characteristics of the expansive clays to a moderate level, whereas SCBA plus NWG has enhanced the soil properties profoundly. The evidence shows that the improvements can be attributed to the chemical alterations, such as the reduction in plasticity and the increase in strength, and physical changes, like the formation of a stable soil structure, within the soil, SCBA, and NWG matrix. Utilization of SCBA has also both economic and environmental advantages.
