Effect of wheat straw protein hydrogel on the mechanical and compressibility behavior of dispersive soil

Dispersive soils pose serious challenges in geotechnical engineering due to their high susceptibility to internal erosion upon contact with water. This study investigates a sustainable bio-stabilization approach using a natural hydrogel synthesized from gluten protein extracted from wheat straw, combined with polyvinyl alcohol (PVA) and boric acid as cross-linking agents. The hydrogel was incorporated into dispersive ML soil collected from the Jafariyeh region of Qom, Iran, at dosages of 1%, 3%, 5%, and 7% by dry soil weight. All specimens were compacted at optimum moisture content and maximum dry density determined in accordance with ASTM D698, cured for 7, 14, and 28 days, and evaluated through geotechnical index tests, dispersivity assessments (double hydrometer and total dissolved solids), mechanical tests (unconfined compressive strength, direct shear, and California Bearing Ratio under soaked and unsoaked conditions), mineralogical analysis (XRD), and baseline consolidation testing on untreated soil. The untreated soil exhibited a high degree of dispersion (63-79%), classifying it as critically dispersive. XRD analysis identified Quartz, Albite, Calcite, and Bassanite as dominant minerals, with higher carbonate-sulfate contents associated with increased dispersivity due to pore-water ionic imbalance and reduced interparticle attraction. Hydrogel treatment reduced dispersivity to below 30% for dosages up to 5%, while a rebound to 39% was observed at 7%. Unconfined compressive strength increased from 0.80 MPa in untreated soil to 1.60 MPa at a 5% dosage after 28 days of curing, and the internal friction angle exceeded 31° in treated samples. The highest CBR improvement (up to 50%) was achieved at 3%. While 3% hydrogel provided the most efficient strength-driven performance, a multi-criteria decision analysis (TOPSIS) identified 5% as the overall optimal dosage by balancing mechanical improvement and dispersivity control.