Effect of rice husk ash on strength behaviour of black cotton soil: a reliability-based approach

Reliability analysis is crucial for quantifying uncertainties in soil properties arising from spatial variability, which are further increased by the addition of stabilising agents. This study evaluates rice husk ash (RHA) as a sustainable stabiliser for expansive black cotton soil (BCS) using an integrated microstructural, geotechnical, and reliability-based framework. BCS was treated with 3–15% RHA and cured for 7, 14, 21, and 28 days. Microstructural analyses (SEM, FTIR, and XRD) revealed progressive pozzolanic reactions, formation of hydrophobic siloxane linkages, and reduction in montmorillonite content with curing. XRD peak area analysis showed maximum calcium silicate hydrate (CSH) formation at 9% RHA, identifying it as the optimum dosage. Correspondingly, geotechnical tests, including unconfined compressive strength (UCS), California bearing ratio (CBR), indirect tensile strength (ITS), and cone penetration test (CPT), also exhibited peak performance at 9% RHA. Reliability analysis using First-Order Reliability Method (FORM) in MATLAB indicated that unsoaked CBR achieved a reliability index (β) ≥3 upto a 20% coefficient of variation (COV) for curing periods of at least 21 days, while ITS and CPT showed reduced reliability beyond a 10% COV. This study demonstrates the effectiveness of RHA and highlights the importance of reliability-based evaluation in sustainable soil stabilisation.