Effect of loading rate on mechanical behavior of coal samples with initial damage accumulation

In the underground tunnels, caverns or bord-and-pillar goaf, the initial damage usually occurs in the surrounding rock or in coal when mining starts. Under the frequent influence of mining stress, mechanical properties of the surrounding coal rock with initial damage, especially the long-term creep performance, are related to the safety and stability of underground engineering. However, under different loading rate, this initial damage is rarely considered during the current testing methods used for coal. In the present study, a method of preparing coal samples with initial damage by cyclic pre-loading and unloading is innovatively proposed. The mechanical and creep properties of the initially damaged coal specimen under loading rates of 0.002 mm.s(-1), 0.005 mm.s(-1), 0.01 mm.s(-1), 0.05 mm.s(-1) and 0.1 mm.s(-1) are investigated. The laboratory tests results showed that after the loading rate increased to 0.05 mm.s(-1), the compression stage of micro-cracks gradually became longer, and the elastic deformation stage began to shorten. The existence of initial damage has promoted the development and expansion of some cracks in the coal sample such that with the increase in loading rate, the deviatoric stress of initial damage samples (IDS) was flat first and then underwent a gradual increase. In the creep test, with the increase of the loading rate, the instantaneous strain of IDS showed a nonlinear change, first decreasing and then increasing, with V = 0.01 mm.s(-1) as the inflection point. As the loading rate increased, the stress threshold gradually declined and then increased, and the deviatoric stress of the IDS creep damage gradually increased nonlinearly. This study can provide insight into the underground excavations and the design of supports to the openings under such a disturbance.