Expansive soils such as Black Cotton Soils (BCS) present significant challenges for road subgrade construction due to their high plasticity, swelling potential, and low strength. This study explores a triphasic stabilization method using Bamboo Charcoal (BC), Quarry Dust (QD), and Lime (L) to enhance the engineering properties of BCS for rural road applications. Initial soil characterization involved standard tests, including Atterberg limits, compaction, and Californian Bearing Ratio (CBR) assessments. The soil was treated with varying BC proportions (5% to 35% at 5% intervals) in the initial phase, leading to a progressive reduction in the Plasticity Index (PI) and swell index and an enhancement in the CBR up to 20% BC content. This further resulted in a soaked CBR value of 2.7%. In the second phase, additional treatment combined with BC and QD, incorporating diverse QD proportions (4% to 24%) relative to the optimal BC content. This further improved the CBR to 7.7% at 12% QD, but the PI exhibited a non-linear trend. Finally, 5% lime was introduced in the final phase. This minimized the PI to 11.2% and significantly increased the CBR to 19%. The optimal combination of 20% BC, 12% QD, and 5% Lime achieved optimal plasticity, compaction, and strength characteristics, demonstrating the viability of this approach for transforming BCS into a sustainable and cost-effective alternative for rural road subgrade construction.

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