Abstract
Buried depth is an important factor affecting the deformation and failure of gob surrounding rock. Basing on triaxial compression unloading test and acoustic emission (AE) of limestone under three different initial confining pressures (5 MPa, 10 MPa, and 20 MPa) and three different stress paths, we analyzed the deformation and failure characteristics and energy releasing process of the virgin rock, the gob overburden rock, and the gob sidewall rock with different buried depths (within 1000 m). The results showed that, with the increase of buried depth, the shear fracture mainly propagated and the failure mode of the gob sidewall rock changed from brittleness to plasticity; however, the gob overburden rock was all brittle failure. Compared with shallow buried depths, the energy releasing triggered by confining pressure reduction in deep buried depths was more concentrated and intense. Under the background of deep buried depths, the peak strength and residual strength of the gob sidewall rock were the lowest, and the damage variable was the largest. We proposed that the first “acute phase” of AE can be wielded as the precursor information of the gob sidewall rock failure and crack propagation of hydraulic fracturing. The findings of the study are beneficial for the disaster prevention and control of deep mining in mountainous area, as well as fracturing evaluation.
Highlights
With the consumption and utilization of coal resources, deep mining gradually tends to be normalized, and the coal mines with mining depth close to 1000 m are increasing yearly [1, 2]
We investigated the mining forms and slope structure conditions of some mines in mountainous areas of Guizhou (Table 1), which have the following common characteristics: (a) large buried depth span of coal seam (160~1100 m); (b) small dip angle of coal seam (8~12°); (c) medium-thick coal seam (1.59~3.6 m); (d) limestone, sandstone, siltstone, and other hard rocks exist in the roof; and (e) longwall mining method adopted
In the process of compression deformation of rock specimens, the initiation, expansion, and coalescence of microcracks often result in energy releasing, and the magnitude of which can be directly characterized by acoustic emission (AE) counting
Summary
With the consumption and utilization of coal resources, deep mining gradually tends to be normalized, and the coal mines with mining depth close to 1000 m are increasing yearly [1, 2]. In mountainous areas of Guizhou, China, the stress redistribution of gob surrounding rock after mining further induces deformation of overburden rock and coal pillar failure and even the whole instability of slope [4,5,6]. We investigated the mining forms and slope structure conditions of some mines in mountainous areas of Guizhou (Table 1), which have the following common characteristics: (a) large buried depth span of coal seam (160~1100 m); (b) small dip angle of coal seam (8~12°); (c) medium-thick coal seam (1.59~3.6 m); (d) limestone, sandstone, siltstone, and other hard rocks exist in the roof; and (e) longwall mining method adopted. The buried depth of coal seam determines the initial in situ stress conditions of gob surrounding rock, affecting the deformation of gob overburden rock and gob sidewall rock [7]. The deformation and failure characteristics and instability prediction of gob surrounding rock affected by buried depth were rarely studied by scholars
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