This study examines the application of nano-colloidal silica (NCS) in enhancing the mechanical properties of sandy clay soils. Consolidated undrained (CU) triaxial tests were performed on specimens containing varying percentages of NCS (0 %, 5 %, 10 %, and 20 %), which were then cured for different curing periods (1, 7, and 28 days) and subjected to three different confining pressures (50, 100, and 200 kPa). The findings revealed that the inclusion of 10 % NCS resulted in a significant 65 % increase in strength after 28 days compared to the untreated sample. However, higher NCS percentages, exceeding 10 %, led to a decline in strength as the excess NCS was not effectively utilized. The inclusion of NCS and increased curing time led to an increase in the brittleness of the soil and the application of confining pressure was able to reduce this brittleness.Furthermore, the use of 10% NCS and a curing period of 28 days significantly increased the stiffness and absorbed energy of the soil. Despite boosting the peak shear strength, NCS reduced the residual strength. Finally, polynomial modeling (Poly4) provided an excellent fit, enabling the characterization of the stress-strain and pore pressure-stain responses from the triaxial test. •A series of CU tests were conducted to investigate the strength behavior of sandy clay soil with NCS additive.•10 % was the optimal content of NCS to modify sandy clay soil, which can improve strength.•The addition of NCS to the sandy clay soil enhances the strength and reduces the failure strain.•Empirical models were developed to predict and satisfactorily simulate q-ɛ and u- ε curves.