Cyclic Dynamic Behaviour of Reconstructed Soil–Rock Mixtures: Hysteresis Response, Normalized Shear Modulus, and Damping Evolution

Protective coatings and surface-protection systems improve structural durability, but the long-term performance of durability-sensitive infrastructure also depends on the cyclic stability of supporting soil–rock mixture (SRM) foundations. In this study, undrained multistage strain-controlled cyclic triaxial tests were conducted on reconstructed SRMs with rock block contents of 0%, 10%, 20%, 40%, and 60% under confining pressures of 100, 200, and 400 kPa. Hysteresis-loop morphology, secant shear modulus, normalized shear modulus ratio, damping ratio, normalized damping ratio, and fitting parameters were evaluated. The results show that hysteresis loops evolved from narrow and steep to wider and fuller forms as strain amplitude increased, indicating stiffness degradation and enhanced hysteretic dissipation. The secant shear modulus decreased from 35.835 to 158.871 MPa to 3.296–12.854 MPa, corresponding to an overall reduction of approximately 85%–94%, while the damping ratio increased from 0.036 to 0.063 to 0.195–0.268. Higher rock block content and stronger confinement increased absolute stiffness, but rock block content advanced normalized degradation and damping development, whereas confinement delayed these normalized responses. These findings provide experimental evidence for dynamic-parameter selection, deformation-compatibility evaluation, and cyclic stability assessment of complex SRM foundations.