Assessment of Environmental Impacts on Bearing Capacity (CBR) of Sisal Fiber-Reinforced Black Cotton Soil

Expansive black cotton soils pose severe challenges to infrastructure due to their low bearing capacity and high swell–shrink behaviour. This study investigates the efficacy of randomly distributed sisal fiber reinforcement as a sustainable stabilization technique. Sisal fiber was mixed with black cotton soil at 0%, 0.25%, 0.5%, 0.75%, 1.0%, and 1.25% by dry weight. The optimum fiber content of 1.0% increased soaked CBR by 551.5% (1.71% → 11.14%) and unsoaked CBR by 220.9% (2.54% → 8.15%). At this dosage, plasticity index reduced by 48.8%, free swell index by 44.6%, and swelling pressure by 44.8%. Compaction characteristics exhibited a modest 5.3% decrease in maximum dry density and 9.9% increase in optimum moisture content. Durability tests under 28-day exposure to water, acid (pH 4), leachate, and freeze–thaw cycles revealed excellent freeze–thaw resistance (only 10.2% CBR loss), good performance under water and leachate (24.9–31.5% loss), and moderate resistance in acidic conditions (40.1% loss). Most degradation occurred within the first 14 days. The results confirm that 1.0% sisal fiber reinforcement transforms weak black cotton soil into a competent, durable subgrade material suitable for pavements across diverse Indian climatic zones while offering significant environmental and economic advantages over conventional stabilizers.