This research investigated the effect of nano-Al2O3 on the shear and hydraulic properties of collapsible soils. Direct shear, permeability, and consolidation tests were performed on samples stabilized with nano-Al2O3 at different curing times. The results showed that the addition of nano-Al2O3 to collapsible soil led to an increase in shear strength. The cohesion and internal friction angle of stabilized collapsible soil with optimum nano-Al2O3 content (0.6%) increased by 3.25 times and 18%, respectively. Ultrasonic pulse velocity (UPV) measurements demonstrate a significant reduction in void ratio with the addition of nano-Al2O3 and confirm its effectiveness in predicting soil mechanical properties. The R-2 coefficients for estimating cohesion and internal friction angle based on UPV are 0.90 and 0.87, respectively. Moreover, the strong correlation coefficient (0.905) between UPV and the collapse index indicates its significant role in determining soil collapsibility. These results highlight the potential of UPV as a reliable and non-destructive evaluation tool in geotechnical applications. Consolidation test results showed that adding 0.6% nano-Al2O3 to collapsible soil decreased the collapse index by 81%. Nano-Al2O3 with fine-filling properties reduced the permeability coefficient by 87% compared to unstabilized collapsible soil. In general, the results of this research show that using nano-Al2O3 as a stabilizer can significantly improve the characteristics of collapsible soils.
