Enhancing the long-term cyclic performance of clay–sand mixtures by polyurethane curing for offshore foundation stability

Mixed clay–sand deposits are widespread in offshore seabeds owing to natural sedimentation and construction-induced soil mixing, and their cyclic instability directly threatens the safety of offshore foundations. Yet, effective reinforcement strategies for clay–sand mixtures to withstand long-term cyclic loading remain underexplored, motivating this study to provide a systematic experimental evaluation of two-component polyurethane curing for this critical problem as a promising solution. A programme of 33 constant-volume cyclic direct simple shear tests showed that polyurethane curing increased the critical cyclic stress ratio (CSR) by 20%–50%, with values converging to approximately 0.18 across different clay–sand mixtures, eliminating the initial sand-content dependency. Curing also improved cyclic strength by extending cyclic lives up to 75-fold at the strain criterion, and strengthened cyclic properties by delaying modulus–damping transitions by factors of 9–20 at high CSR while maintaining their long-term stability at low CSR. Among the tested mixtures, the sand-rich one, initially the most vulnerable, exhibited the most pronounced performance enhancements. Microstructural analyses identified three reinforcement mechanisms, establishing a direct micro–macro linkage to the observed macroscale improvements. These findings demonstrate the potential of polyurethane curing to enhance the cyclic resilience of mixed clay–sand deposits.