Failure characteristics and reasonable width of water-resisting pillar under the coupling effect of mining and seepage

The coupling failure induced by the influence of mining practice and the water immersion softening of the water-resisting coal pillars in old goaf is one of the common causes of water inrush accidents in the same seam working face. Exploring the failure characteristics and reasonable width of the water-resisting coal pillars in the old goaf is of great significance for the prevention and control of mine water damage. The safe width of the water-resisting coal pillars in the closed overlying goaf in the same layer of the No.250209 working face of the Yanbei Coal Mine in Huating, Gansu Province is taken as the research object. The theoretical analysis and the FLAC^3D numerical simulation are conducted to analyze the whole instability disaster process of water-resisting coal pillars as the occurrence of partial failure of the immersed coal blocks, collapse, and insufficient total width. The coupling characteristics of the stress field, plastic area and seepage field in the water-resisting coal pillars under the complex influence of water immersion weakening, mining and seepage have been revealed and the water blocking capacity and stability evolution law of water-resisting coal pillars with different coal pillar widths are obtained. Accordingly, a method for determining the width of the water-resisting coal pillars of three-zone combined type of “water seepage zone + elastic compaction water resisting zone + plastic zone” is proposed. The results show that ① the plastic failure firstly occurs in the lower coal at the immersed side of the water-resisting coal pillar under the superimposed action of overburden load and water pressure in the goaf. And with the expansion of the seepage scope of water erosion and the gradual immersion-induced weakening, the deterioration and collapse of the bearing capacity of the coal in this area are induced, which ultimately leads to the eccentric axial compression and collapse of the water-resisting coal pillar. ② In the three stages, the development width of the plastic zone at the upper, middle and lower parts of the water-resisting coal pillar is different, showing a gradual increase from top to bottom along the increasing height of the coal pillar. That is, the extension range of the plastic zone at the lower part of the water-resisting coal pillar at the soaking side is larger than that at the middle and upper parts, indicating that the bottom area of the water-resisting coal pillar is more likely to form a water channel. This practical condition is consistent with the theoretical analysis. ③ The volume of plastic zone of water-resisting coal pillar, accounting for more than 83% of the total volume of water seepage zone, is the main area for water transmission of water-resisting coal pillar. Although the volume of elastic zone only accounts for 17%, a relatively small proportion, of the total volume of water seepage zone, it determines the maximum expansion boundary of the range of water seepage zone. Also, the seepage coupling characteristics of “small range, high stress, and low permeability” and “large range, low stress, and high permeability” have been shown in the elastic water seepage zone. ④ The water blocking capacity of the water-resisting coal pillar depends on the range and connectivity between the water seepage area and the plastic area on the mining side. When the width of the water-resisting coal pillar is 110 m and 120 m, the water seepage area and the plastic area are completely connected. And when the width of water-resisting coal pillar become 130, 140 and 150 m, the water seepage area and plastic area are not connected, and the width of elastic compaction water resisting area between them is 5.5, 11.5 and 23.5 m, respectively. Based on this, the connection between the seepage zone and the mining plastic zone is regarded as the critical condition for the mine water to break through the water-resisting pillar. And a method for determining the width of the three zones combined water-resisting coal pillar of “water seepage zone + elastic compaction water resisting zone + plastic zone” with the condition that width of the elastic compaction zone is no less than 20 m is proposed. It is pointed out that not only the conventional stability index of support coal pillar, such as more than 31% proportion of the elastic core zone and the unconnected plastic zone, but also the water resisting performance of the water-resisting coal pillar should be taken as the criteria to determine the stability of the water-resisting coal pillar.