Sustainable Stabilization of Collapsible Clay Soils Using Eco-Friendly Additives and Sarooj Mortar: Experimental Assessment of Strength and Collapse Behavior

Collapsible soils present significant geotechnical challenges due to their abrupt volume reduction and strength degradation upon wetting, which can lead to severe structural damage. This study evaluates the effectiveness of sustainable and eco-friendly additives—including rice husk ash, lime, eggshell powder, turmeric, polypropylene fibers, nanosilica, and Sarooj mortar—in stabilizing a naturally collapsible clay soil from Gorgan, Iran. A comprehensive experimental program comprising collapse potential, unconfined compressive strength (UCS), and unconsolidated undrained (UU) triaxial tests was conducted. The untreated soil exhibited a high collapse potential of approximately 11.1%, classifying it as severely collapsible. Upon stabilization, the collapse potential was significantly reduced to 1.35–4.63%, representing a reduction of up to ~88%, and reclassifying the soil into slight to moderate collapsibility. In terms of strength improvement, the UCS increased from 0.71 kg/cm2 (untreated soil) to values exceeding 3.5–4.3 kg/cm2 after 28 days of curing, corresponding to an increase of more than 4–5 times depending on the mixture composition. Additionally, triaxial test results indicated improvements of over 20% in shear strength parameters, including cohesion and friction angle, particularly after 28 days of curing. The observed improvements are attributed to the combined effects of pozzolanic reactions (lime, rice husk ash, nanosilica), cementitious bonding (Sarooj mortar), and mechanical reinforcement (polypropylene fibers), which collectively enhance soil structure, reduce the void ratio, and increase interparticle bonding. Among the tested mixtures, samples containing higher nanosilica and fiber content demonstrated superior performance in both strength and collapse resistance. Overall, the integration of traditional Sarooj mortar with modern eco-friendly additives provides a sustainable and efficient solution for mitigating collapse potential and enhancing the mechanical behavior of clayey soils. The proposed approach offers a low-carbon alternative to conventional stabilization methods, with significant implications for foundation engineering and infrastructure development in regions with problematic soils.