Subgrade Strength Indicator Model for Geopolymerized Soil Reinforced with Discrete Fiberglass: A GEP Approach

The dual nature of expansive subgrade soil exhibits high volumetric instability against the periodic moisture imbalance, causing extensive damage to highways, airfield pavements, and lightweight concrete structures if constructed on such calamitous soils. Although well-known conventional stabilizers (lime and cement) can effectively regulate the volumetric behavior and soil compressibility, their production can significantly degrade the environment by emitting greenhouse gases. This study investigates the efficiency of an envirosafe alkaline activator stabilizer (AAS) and reinforcement provided by discrete fiberglass (DF) to enhance the performance of an expansive subgrade soil layer. The proportions of coal-gangue ash (CA) replacement with silica fume (Sf: 0–20%) were varied in the alkaline solution composed of sodium silicate and sodium hydroxide. A series of swelling, compressive shear, flexural, and penetration resistance tests were performed to determine the subgrade strength properties, and X-ray diffraction and Fourier-transform infrared (FTIR) spectroscopy tests were carried out for the chemical characterization at varying soil-fiberglass mixture dosages. The study proposed an optimum dosage of DF-Sf-CA in AAS-subgrade soil. It showed a substantial improvement in the California bearing ratio (CBR) penetration (68%) and compressive shear strength tests. The Sf (> 10%) results in CA-based alkaline stabilizer soil achieving the lowest swelling reduction (56%) compared to the unreinforced subgrade soil. Finally, a gene expression programming (GEP) model was proposed to predict the subgrade strength performance of fiber-reinforced alkaline soil, and the results showed an excellent predictor of subgrade strength (R²_train = 0.9711, R²_test = 0.9705).