Subsection amplification cyclic deterioration mechanism of creep damaged coal

Under the excavation effects of deep coal and rock, obvious creep damage is produced under the action of high stress for a long time and the sudden destabilization of the creep-damaged coal body induces rock burst disaster under the mining action in working surface, which poses a serious threat to the safety of deep coal pillar mining. Through long-term creep-cyclic loading-unloading confining pressure test, the evolution law of deterioration characteristics of coal rock under the superposition of initial creep damage and cyclic load is studied. The strength characteristics and cumulative damage characteristics of coal in different stress intervals under cyclic load are analyzed. The coupling mechanism of cyclic load times on creep damage effect of coal is explored, and the energy conversion mechanism in the process of deformation and failure of coal under creep damage effect is revealed. The research results show that: coal samples are less affected by creep pre-damage effect within the low stress range of cyclic loading, with the increase of the stress level range, the larger the long-term damage of coal samples, the more obvious the nonlinear deterioration, and the more severe the degree of rupture after sudden destabilization; When the creep stress is in the elastic stage, the time between strengthening and deterioration is generally less than 16 h, and in the high stress range, the deterioration effect is significantly; At the later stage of cyclic loading and unloading, the loading and unloading deformation modulus fluctuates sharply with the number of cycles, and the irreversible deformation increases steadily, indicating that the coal samples is at the “critical point” of sudden destabilization. Based on the theory of damage mechanics, a damage evolution model of creep and cyclic loading is established. Combined with the test results, it is found that the internal deterioration of coal samples after long-term creep is high, the stored energy and stress release are less, and the sudden instability is less evident; The energy storage capacity of coal samples also has a “critical point” in terms of creep damage duration. Due to the longer creep damage duration, the releasable elastic energy stored in the specimen is reduced, which retards the release of elastic energy to a certain extent, reducing the scope of the occurrence of dynamic disaster damage. The research results will be of great engineering significance to reduce the long-term creep induced disaster of residual coal, improve the recovery efficiency of residual coal in goaf, and promote the construction of ecological civilization in mining area.