Enhancing Expansive Soil Stability Using Agricultural Waste: Soybean Pod Ash and Banana Fiber

This study evaluates the efficacy of agricultural waste—soybean pod ash (SBPA) and banana fiber (BF)—as sustainable stabilizers for expansive soils in Jimma Town, Ethiopia. Soil samples from Kebele 5 and Ajip Kela (1.50 m depth) with the highest plasticity indices and lowest California bearing ratio (CBR) values were selected. SBPA and BF were sourced locally, and various geotechnical tests assessed the natural and stabilized soils with SBPA (3%–15%) and BF (0.25%–1.25%, 15 mm length), including hybrid mixes (e.g., 3% SBPA + 0.25% BF). Sieve analysis revealed clay‐rich soils: 97.34% (Ajip Kela) and 95.9% (Kebele 5) passed the number 200 sieve (0.075 mm). Classified as A‐7‐5 (AASHTO) and CH (Unified Soil Classification System [USCS]), the soils exhibited improved CBR and unconfined compressive strength (UCS) with SBPA‐BF blends, peaking at 9% SBPA + 0.75% BF. Beyond this ratio, performance declined, indicating an optimal mix. Compaction tests showed reduced maximum dry density (MDD; 1.38–1.29 g/cm 3 ) and increased optimum moisture content (OMC; 26.8%–31.8%), suggesting enhanced water retention and lower compaction effort. The optimum soil sample (9% SBPA + 0.75 BF) shows improved properties with reduced compression index (Cc), swelling index (Cs), and recompression index (Cr) compared to the natural soil, this indicating enhanced stiffness, lower swelling, and better stability for subgrade soil construction. The results demonstrate that SBPA‐BF stabilization enhances engineering properties of soil, offering an eco‐friendly, cost‐effective solution for mitigating expansive soil risks. This approach aligns with sustainable waste utilization, particularly in regions like Ethiopia, where expansive soils dominate landscapes and agricultural byproducts are abundant.