The dynamic response and crack propagation mechanism of deep jointed rocks under transient unloading

Aimed at the full-face blasting excavation of a deep circular tunnel, this research explores the dynamic response and crack propagation mechanism of jointed rocks under transient unloading. Based on the plane strain assumption, the theoretical mechanical behavior of rocks under transient unloading was analyzed. Model tests were conducted using a self-developed test system to investigate the effects of joint length and joint spacing on the transient unloading process. The results show that joints have significant reflection and transmission effects on stress waves. Compared with the condition without joints, the presence of joints increases strain reduction on the reflected side by approximately 16.5%, indicating enhanced wave reflection, and decreases strain reduction on the transmitted side by approximately 25.8%, indicating weakened wave transmission. Longer joints result in stronger reflection effects at the joint center, and smaller joint spacing may induce tensile strain in the surrounding rock. As for crack propagation, two primary crack initiation modes were observed: initiation at the ends and at the middle of the joints. These findings enhance the understanding of dynamic response and crack propagation in jointed rock masses under transient unloading, and provide a theoretical basis for related engineering applications.