Creep Behavior of Coal after Cyclic Loading and Unloading and Its Effect on Mining-Induced Stress Boundary

Damage accumulation of coal caused by cyclic loading and creep impacts may ultimately lead to rock failure and dynamic disasters. Quantifying this behavior is crucial for evaluating the mechanical response under creep and cyclic loading processes. Here, creep tests were conducted after cyclic loading and unloading at 10 MPa confining pressure. The elastic modulus and Poisson's ratio indicated fatigue damage during the entire loading process. The elastic moduli in the unloading stage were slightly higher than those in the loading stage. The gap in Poisson's ratio decreased in the cyclic loading procedure. The axial and volumetric strains exhibited a slightly negative exponent trend. The stress–strain response indicated hardening and softening effects under constant axial stress. The hardening effect dominated the peak strength at lower axial stresses. The creep-hardening deformation was 0.17 and 0.21 under 88 and 100 MPa, respectively. However, the softening effect played a more significant role at higher applied stresses. Additionally, the peak strength decreased by approximately 2.7 MPa when the axial stress increased from 100 to 102 MPa. The coal near the mining-induced stress boundary experienced cyclic damage, and the creep stress gradually decreased. Thus, the mining-induced stress boundary evolution rate initially increased and was mitigated as the mining-induced stress reduced. Next, the boundary expanded outward and gradually stabilized until the applied loading did not lead to obvious rock failure. Finally, the mining-induced stress boundary in the entire gob may be flat during the long-term evolution. Considering the creep failure time and choosing a suitable time to determine the boundary are essential for preventing dynamic disasters.