Multi-Scale Improvement of Collapsible Clay Through Recycled PET, Nano-TiO2, and Sarooj Incorporation: Mechanical Response, Collapse Mitigation, and Bearing Behavior

Collapsible soils pose severe geotechnical risks to foundation systems and transportation infrastructure due to sudden structural collapse and substantial strength degradation upon inundation. This study proposes a sustainable and multi-scale stabilization framework for a highly collapsible clay from Gorgan, Iran, using a hybrid system comprising a modified Sarooj-based binder system comprising rice husk ash, lime, eggshell powder, and turmeric reinforced with nano-TiO2 and recycled polyethylene terephthalate (PET) fibers. The engineering performance of the treated soil was systematically investigated through double-oedometer testing for collapse assessment, UCS and UU triaxial testing for mechanical characterization, and CBR testing for bearing performance. UCS and CBR specimens were evaluated after 1, 7, and 28 days of curing, whereas UU triaxial tests were conducted after 7 and 28 days. In its untreated state, the soil showed a pronounced susceptibility to collapse, with a measured collapse potential of 11.13%. The incorporation of the stabilization system drastically mitigated collapse potential down to 1.66–4.48%, corresponding to a 60–85% overall reduction. The best-performing formulation within the investigated experimental matrix—incorporating 3% rice husk ash, 0.1% lime, 0.1% eggshell powder, 0.1% turmeric, 2% nano-TiO2, and 1% recycled PET fibres (designated as Sample 4)—achieved the lowest collapse potential (1.66%), shifting the soil classification from severe to slight collapsibility. Furthermore, the 28-day undrained shear strength parameters improved substantially, with cohesion increasing from baseline values up to 90.2 kPa (0.92 kg/cm2) and the internal friction angle reaching 7.10°. The formulation identified as the best-performing mixture within the investigated experimental matrix yielded a peak CBR of 21.14% after 28 days of curing.