Research on re-fracturing mechanism and cavity structure evolution characteristics of broken rock mass in goaf of closed mine

Due to the “Dual Carbon” policy impact, geothermal extraction technology in closed mines has garnered increasing attention. The efficiency of extracting thermal fluid within the closed mine goaf is related to the permeability characteristics, with the broken rock mass porosity structure playing a key role in determining the permeability characteristics of the goaf. Therefore, it is of great significance to investigate the deformation and evolution characteristics of the porosity structure of broken rock mass in the complex environment of geothermal extraction. Numerical models of broken rock mass with different size grading indexes were established using the particle discrete element numerical method under conditions of immersion and lateral-constrained compression. The deformation behavior and evolution characteristics of the broken rock mass were analyzed, and the movement rules of particles within the rock voids were tracked. The following conclusions were obtained: the stress-strain curve during the compaction process of the broken rock mass can be divided into three stages, namely the initial stage (0<ε≤0.175), the slow growth stage (0.175<ε≤0.275), and the rapid growth stage (ε>0.275). In the rapid growth stage, the stress-strain curve shows significant fluctuations, and the phenomena of secondary fracturing and stress redistribution in the broken rock mass are most pronounced. The variation value of porosity in the broken rock mass under the thermal storage environment is directly proportional to the initial porosity, with a maximum value of 0.2. When there is a large size difference between rock blocks in the broken rock mass, the contact bond strain energy is the largest, and the growth of bond breakage energy is slow. Fractures are more common in the contact part between rock blocks and particles, while fractures in the contact part between rock blocks and voids are very rare. When the particles are not separated from the rock fragments, they move with the rock fragments, resulting in a complex overall trajectory. In the instant when the particles are detached, their velocity suddenly increases and the collision with the rock fragments causes a change in velocity. When the particle's velocity decreases to a level similar to the surrounding rock fragments, it will lead to the blocking of the void space. The research results can provide a theoretical basis for evaluating the thermal storage efficiency in the goaf of closed mines.