Objective This study aims to investigate the deformation and energy dissipation characteristics of broken rock masses in the caving zones of goafs during compression and to reveal their impacts on the movement and failure of overlying strata and surface subsidence. Using a self-developed experimental system, this study conducted compression experiments on broken rock masses under lateral confinement under different Talbot index values and axial stresses. Then, it analyzed variations in axial displacement, void ratio, relative breakage index, fractal dimension, and strain energy density of the broken rock masses during compression. Accordingly, this study quantified variation patterns of strain energy density with axial strain and relative breakage index and revealed the influential mechanisms of axial stress and Talbot index on compressive deformation evolution of broken rock masses.

Results and Conclusions  The results indicate that with the increasing Talbot index and axial stress, the axial displacement and relative breakage index of the broken rock masses increased, while the void ratio decreased. Based on the nonlinear load-bearing characteristics of the broken rock masses, the compression process was divided into three stages: void compaction stage (σ<4 MPa), breaking and filling stage (4 MPa ≤ σ<6 MPa), and stable consolidation stage (σ≥6 MPa). With an increase in the axial stress, the fractal dimension of samples with different Talbot index values increased, albeit with a gradually decreasing growth rate. In contrast, under the same axial stress, the fractal dimension of samples showed a monotonic decrease with an increase in the Talbot index, significantly corresponding to the compression process. Moreover, the broken rock masses presented increasing strain energy density with an increase in the axial strain and relative breakage index. In contrast, under identical axial strain or relative breakage index, a higher Talbot index value corresponded to lower strain energy density. During the initial loading stage, the broken rock masses showed slight differences in strain energy density under the same axial strain, with the differences gradually increasing as loading progressed. The results of this study can provide a theoretical basis for the safety management of coal mine goafs and the prediction of surface subsidence.