Main controlling factors and characterization of overburden fracture damage based on energy conduction mechanism

The migration and fracture of overlying strata during coal seam mining is an important factor affecting the strata behaviors in working face. By studying the damage and failure characteristics of overlying strata from the perspective of energy, the behavior law and potential risk of overlying rock migration and failure under the influence of mining can be better understood, which provides effective guidance for reasonable mining parameter design of longwall face. Based on the research background of Qinglongsi coal mine in Shenfu mining area, Shaanxi province, this paper studies the post-peak deformation and failure characteristics of overlying rock fracture damage from the perspective of two-dimensional plane and three-dimensional space through the energy dissipation theory. An innovative method has been proposed for calculating the total energy of overlying rock layers with gravity potential energy imparted by coal seam excavation. On the basis, an overburden damage degree characterization system was built based on energy conduction mechanism through the integration of numerical simulation and theoretical analysis. The finite difference equation for rock dissipative energy is derived based on energy balance and finite difference theory. This equation is then incorporated into the FLAC^3D strain softening model using FISH language. This effectively supplements the software energy calculation module. This approach addresses the limitations of conventional qualitative characterization of damage degree and type resulting from engineering rock excavation due to plastic zone effects. The energy dissipation degree of the scale effect of the index parameter was quantitatively characterized by defining the damage degree index. Based on the geological conditions of the Qinglongsi coal mine longwall face, a simulation analysis was conducted to investigate the influence of longwall face length and advance speed on the damage degree of overlying strata. The overburden damage degree increases and decreases in an "S" shape with the increase of longwall face length and advancing speed, respectively. Finally, it was established that the optimal longwall face length should not exceed 303.26 m, while the appropriate range for the uniform advance speed is between 10.13 m/d and 18.00 m/d. If the parameters of the longwall face exceed the above-mentioned range, the mining-induced damage could result in the failure of key controlling layers in the overlying strata, leading to a significant increase in the damage ratio. Finally, the feasibility of the characterization system based on the energy transmission model was verified through an analysis of the advance speed and the corresponding strata pressure of the 5-20108 longwall face. It was found that the increase in the advancing speed of the longwall face resulted in an increase in the step distance of the periodical Weighting, an increase in the strength of the mining pressure manifestation, and a decrease in the overall degree of overburden damage.