Experimental study on the evolution of mesoscopic structure and macroscopic mechanical properties of typical karst fillings soil under dry-wet cycles

Jinan in Shandong Province is a typical karst development zone in northern China. When buildings are constructed above filled karst caves, the water environment of the karst fillings may be altered, and their mechanical properties gradually deteriorate under alternating dry-wet conditions. This problem is further exacerbated when the fillings are expansive, as drying induces shrinkage cracking while wetting triggers swelling. However, few studies have addressed this specific issue. In this study, laboratory physical property tests were first conducted on expansive karst fillings collected from typical sites. Subsequently, unloaded swelling ratio tests, mercury intrusion porosimetry (MIP), and consolidated undrained triaxial tests were performed on specimens subjected to different numbers of dry-wet cycles. The results indicate that the fillings exhibit moderate expansion potential, attributed to their high montmorillonite content. With increasing dry-wet cycles, the unloaded swelling ratio decreases, the total pore volume increases significantly, the pore size distribution changes, and the microstructure of the fillings undergoes progressive deterioration. Quantitative relationships between elastic modulus, failure deviatoric stress, and the number of dry-wet cycles were established based on the triaxial test results. As the number of cycles increases, the failure deviatoric stress decreases markedly, while the elastic modulus decays exponentially and tends to stabilize, with a relatively uniform deterioration rate of the failure deviatoric stress throughout the process. Based on the MIP and triaxial test results, a progressive macro-micro damage evolution model was established, revealing the intrinsic correlation between microstructural damage and macroscopic mechanical degradation of karst fillings under dry-wet cycles. This study provides a useful reference for engineering construction in similar karst sites.