Damage zone around underground opening caused by combined blast loading and initial stress unloading

The formation of an Excavation Damaged Zone (EDZ) is a common issue in mining and other geotechnical engineering fields, which impacts the stability of surrounding rock mass. The excavation of deep, stressed rock mass induces stress redistribution and propagates stress waves that form an EDZ around the excavation. Modelling the complex processes of stress relief and adjustment in anisotropic stress states is challenging but essential for understanding EDZ formation mechanisms and optimizing blast design. In this study, we derive the stress response in the surrounding rock mass caused by the initial stress unloading and blast loading in arbitrary stress states. This is achieved by the application of modal superposition and Laplace transform. The impacts of the unloading period, blasting amplitude, and stress state were studied. Then a series of Finite Element Method (FEM) numerical simulations were carried out to obtain the EDZ around the opening under pure unloading and combined loading-unloading. The field observations of the distributions of EDZ in underground mining tunnels excavated by blasting were analysed in conjunction with the theoretical and numerical findings. The results showed the extent of the EDZ increases with initial stress, and at lower stress levels, blasting predominantly governs the formation of the EDZ, while at higher stress levels, unloading and stress redistribution prevail. Blast loading was suggested to be the main cause of the Highly Damaged Zone (HDZ), as the unload-induced redistributed stress and blast loading stress were linearly superimposed in the radial and tangential direction, leading to the elevation in the tangential tensile stress and the radial compressive stress in the rock mass near the excavation boundary.