Geotechnical Improvement of Sugarcane Bagasse Ash and Calcium Chloride–Stabilised Expansive Soil Subgrade for Flexible Pavement Applications

Abstract

Expansive clays prevalent across peninsular India, particularly the black cotton soils of Telangana, pose serious challenges for flexible pavement subgrade construction on account of their high swell-shrink activity, low bearing capacity, and susceptibility to moisture-induced strength loss. This study investigates a binary stabilisation system comprising sugarcane bagasse ash (SBA) and calcium chloride (CaCl₂) for improvement of a highly plastic clay (CH) subgrade sourced from Shankarpally, Telangana (PI = 50.41%, DFS = 90%, soaked CBR = 2.2%). Laboratory tests included standard Proctor compaction, Atterberg limits, differential free swell, soaked California Bearing Ratio (CBR), unconfined compressive strength (UCS), X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). At the optimum dosage of 20% SBA + 1.5% CaCl₂ (by dry mass of soil), the treated subgrade achieved a maximum dry density of 1.77 g/cm³ at 16.20% optimum moisture content. Plasticity index declined from 50.41% to 9.30%, differential free swell decreased from 90% to 13%, and soaked CBR increased to 11.20% — a 5.09-fold improvement over the untreated baseline. UCS rose from 0.727 kg/cm² to 1.253 kg/cm². Analytical processing of the results showed a 34.3% reduction in void ratio and a 3.8-fold rise in the estimated subgrade resilient modulus (22 to 83 MPa), and regression models relating DFS to PI, CBR to dry density and PI, and UCS to CBR reproduced the measured values with mean absolute errors of 1.3–9.6%. Microstructural analyses confirmed pozzolanic calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H) gel formation, accelerated by the calcium ion supply from CaCl₂. The stabilised subgrade satisfies IRC:37–2018 requirements and offers a sustainable, low-carbon alternative for expansive subgrade improvement using agro-industrial and industrial by-products.