Dynamic Behaviour and Energy Evolution of Granite in a Tunnel Under Cyclic Impact Loading

Drill and blast tunneling is a cyclic construction process in which the rock away from the blast hole is subjected to cyclic blast load that can influence the dynamic behaviour, energy characteristics and damage evolution of the rock. To study the dynamic mechanical properties and energy evolution of granite under cyclic impact loading, dynamic tests were performed on 100-mm-diameter granite samples at different impact velocities using a split Hopkinson pressure bar system. The stress–strain relationship, energy and damage evolution, and microscopic features of the rock are discussed and analyzed in detail. The results show that the stress–strain relationship of the granite is significantly sensitive to both impact velocity and number of impacts. At the same impact velocity, the peak dynamic stress decreases and the peak strain increases linearly with the increase in the number of impacts, while the peak stress decreases significantly and visible cracks appear or the sample is completely destroyed. Energy evolutions of granite samples during the cyclic impact process with different impact velocities present a uniform variation tendency, and the impact compressive process of granite samples can be divided into three stages: linear elastic deformation stage, plastic deformation-dominant stage and rock failure stage. The cumulative energy dissipation rate, defined as the sum of the ratio of total dissipated energy to the input strain energy after each impact loading, is proposed, and it increases linearly with the number of impacts. In addition, the damage evolution and microscopic features of the rock after cyclic impact loading are also discussed, and the microscopic fracture mode changes from the combination of intergranular and transgranular fracture to transgranular fracture with the increase of impact velocity.