Experimental investigation of a special chemical additive for improving the geotechnical properties of dispersive clay soils

• The pinhole test showed that increasing the percentage of RRP235 Special additive significantly reduces soil dispersivity. • Compaction tests indicated that while the addition of RRP235 Special lowers the optimum moisture content, it increases the maximum dry density of the treated soil. • Triaxial test results demonstrated that both the friction angle and cohesion of the soil improve progressively with higher additive content and longer curing times. Dispersive clay soils present significant challenges in geotechnical engineering due to their high susceptibility to erosion and structural instability. Traditional stabilizers such as lime and cement are often costly and less eco-friendly, and may require higher percentages to achieve the desired performance. This research evaluates the effectiveness of RRP 235 Special , an eco-friendly, algae-based chemical additive, in improving the geotechnical properties and reducing the dispersivity potential of clay soils in Urmia, Iran. A comprehensive experimental program was carried out, including pinhole test, compaction test, Atterberg limits, unconfined compressive strength (UCS), California bearing ratio (CBR), consolidated undrained triaxial test, and scanning electron microscopy (SEM). Six percent of RRP235 Special (0.0115–0.0498 %) was applied, with curing times of 0, 3, and 7 days. Wet–dry cycling tests were also performed to assess durability. The optimal percentage of 0.0345 % changed the soil classification from dispersive (D2) to non-dispersive (ND4) after 7 days. The plasticity index decreased by 76 %, maximum dry density increased, while optimum moisture content decreased, and shear strength parameters improved by up to 67 %. At 0.0498 %, UCS and CBR values increased by 47 % and 82 %, respectively. SEM analysis revealed improved particle bonding and reduced pore space, while durability tests indicated less than 5 % strength loss after five wet–dry cycles. RRP 235 Special proved to be highly effective at low percentages, economically advantageous compared to conventional stabilizers, and environmentally compatible, making it a promising alternative for sustainable soil stabilization in infrastructure projects.