Experimental study on the influence of stress disturbance path on the mechanical properties of coal specimens

During pumped energy storage using abandoned mines, coal pillars are not only affected by monotonic loads, but also suffer from frequent stress disturbances. The strength and deformation characteristics as well as the failure mode of the coal specimens under the influence of monotonic loading (stress path 1), multilevel constant-amplitude cyclic loading (stress path 2) and multilevel increasing cyclic loading (stress path 3) were studied using the coal samples from Hepan Coal Mine, Shaanxi Province, as the research object. The test results showed that the stress-strain curves of the coal specimens under cyclic loading formed obvious hysteresis loops, and the area of hysteresis loops increased significantly with the rise of stress level; compared with the axial strain, the circumstantial strain was more sensitive to the stress path; the peak strength and peak axial strain of the coal samples were the largest under monotonic loading, and the peak strength and axial strain of the specimens decreased by 22.12% and 18.35% (path 2) and 5.30% and 10.34% (path 3) under multi-level cyclic loading, respectively; the tangent modulus of the specimens under stress path 2 was significantly higher than the other stress paths; the stress paths had not changed the macroscopic failure mode of the coal specimens, but the cyclic loading caused the number and size of cracks on the specimen’s surface to become larger.