Unloading-induced activation mechanisms and slip characteristics of fine-sand-filled fractures under varying grouting conditions

Understanding the activation mechanisms and slip characteristics of fine-sand-filled fractures under different grouting conditions is crucial for ensuring the safe excavation of tunnels. This study investigated the effects of grouting with superfine cement grouts and acrylate grouts on the slip behavior, frictional properties, energy released patterns, and damage modes of filled fractures compared to a natural-state (ungrouted) filled fracture. Triaxial unloading-induced slip experiments were subsequently conducted on all three types of samples. The results indicate that the stability of grouted filled fractures was higher than that of ungrouted fracture. For the ungrouted filled fracture, the slip process was primarily governed by the accumulation and dissipation of sand particles. In contrast, the slip process of grouted filled fractures was mainly controlled by the failure of bonding interface between the grouted structure and sandstone matrix, as well as the damage to the grouted structure itself. The average slip velocities were measured at 9.55 × 10−3, 7.27 × 10−3, and 1.74 × 10−3 mm/s for the ungrouted filled fracture, grouted filled fracture using superfine cement grouts, and grouted filled fracture using acrylate grouts, respectively. The corresponding total released seismic moment (M0) was 761.59, 682.23, and 605.59 N m, respectively. The grouted filled fracture using acrylate grouts exhibited the longest slip duration and the largest magnitude of unloaded confining pressure among these samples. The large ductility of grouted structure with acrylate grouts allowed it to absorb deformation energy during slip. These findings highlight the effectiveness of grouting using acrylate grouts to mitigate the activation of filled fractures.