Sustainable stabilization of expansive soil using Corn Cob Ash and Ferric chloride through experimental investigation and RSM optimization

Expansive soils (ES) are among the most challenging geomaterials because they exhibit significant swelling and shrinkage behaviour, which adversely affects the stability and service life of pavements, foundations, and other civil infrastructure. The paper under discussion explores the usefulness of Corn Cob Ash (CCA), a waste agricultural product, in conjunction with Ferric chloride (FeCl 3 ) as a chemical additive in stabilizing ES. A historical data design approach within the Response Surface Methodology (RSM) framework was adopted using a full factorial experimental matrix consisting of 36 predefined mixtures with CCA content (0–15%), FeCl₃ dosage (0–2%), and soaking duration (0, 7, and 28 days). Plasticity Index (PI), Free Swell Index (FSI), Maximum Dry Density (MDD), and California Bearing Ratio (CBR) have been used in evaluating the engineering performance of stabilized soil. The findings revealed that PI and FSI had significantly reduced and the MDD and CBR had improved significantly with the increase in the content of stabilizers and length of the soaking duration. The optimum stabilization condition was identified at approximately 13.14% CCA, 1.43% FeCl₃, and 22.82 days of soaking, yielding PI = 15.57%, FSI = 51.81%, MDD = 1.71 g/cm 3 , and CBR = 6.93%, with a desirability of 0.958. The predictive ability of the developed quadratic models was very good (R 2 > 0.95) and validated with three independent experimental mixtures. There was a good agreement between the predicted and experimental responses with prediction errors of less than 5% in the developed models which indicates the reliability and predictive capability of the developed model. Results showed that the use of agricultural waste derived CCA and FeCl₃ together is an effective method of stabilization for ES to improve its engineering properties, which can be sustainable in terms of utilizing the agricultural wastes and reducing the use of conventional stabilizing material.