Study On Control Mechanism Of Rock Mass Structure Of Coal Floor Mining Effects At Great Depth

The coal floor water accidents have occurred increasingly frequent in recent years with the increasing of mining depth in domestic coal mines. The crustal stress and water pressure under coal floor are getting higher while the coal mines reach deep mining. In particular, the coal floor was threatened by high confined water of Taiyuan group limestone and Ordovician limestone while mining of coal seam berried at the bottom of Shanxi group in the North China type coal field. Meanwhile, the geological tectonic unit of Huaibei mining area is special leading to the geological structural such as faults, collapse columns developed well. So, the research of water inrush form coal floor mechanism has a theoretical and practical significance for the safety mining of lower group coal seam in Huaibei mining area and even the North China mining area.In this paper, Huaibei mining area was taken as an example and the mining effects of different coal floor structures were researched. Based on the hydrogeology and engineering geology of mining area, the sedimentary characteristics of coal floor of Shanxi Group, rock mass structure characteristics of coal floor and water abundance characteristics of Taiyuan group limestone aquifer were analyzed. The different coal floor structure models were built consisting of two main categories and six sub categories. The mining effects of different coal floor structure models was analyzed by numerical simulation method and the mining effects of coal floor cutting by a fault was analyzed by similar material simulation method. Finally, combining with the grouting reinforcement and reconstruction project of coal floor, the mining effects difference of coal floor before and after grouting was analyzed based on theoretical analysis, numerical simulation, laboratory tests and filed tests obtaining the following results and conclusions:(1) Based on the analysis of the sedimentary characteristics of coal floor of Shanxi group, sedimentary association of lower coal group floor was classified obtaining the sedimentary association of lower coal group floor in Huaibei mining area can be classified as categories:hard-soft type, hard-soft-hard type and soft alternate with hard type. The complete layered structure coal floor model was built based on the classifying results. The position of water inrush from coal floor had a close connection with the rock mass structure of coal floor such as thickness thinning belt, development position of fracture, faults and collapse column by researching the water accidents happened in Huaibei mining area while the mining of lower coal group reflecting a congruent relationship between water inrush and rock mass structure of coal floor.(2) The secondary development of FLAC3D was carried out based on FISH language to research the mining effects of coal floor under the condition of fluid-solid coupling. The results show the depth of coal floor failure will be deeper with the increasing of water pressure while the thickness of aquiclude is certain, meanwhile, the in-situ fractures appear with coal mining and the range of which will expand while the water pressure become higher leading the risk of water inrush from coal floor increasing. With the increasing of mining depth, the failure depth of coal floor become deeper under the condition of high mining stress and water pressure revealing the risk of water inrush form coal floor getting larger after coal mine reaching the deep mining and the differences of coal floor failure zone between shallow and deep mining. The main differences of coal floor failure zone appear in the in-situ fractures zone developed above the aquifer under the condition of deep mining while in the shallow mining condition, there is only mining failure zone.(3) The mining effects of floor under different rock-layers combination conditions were analyzed and there was obvious difference between different mining effects under different conditions. The soft and hard alternated floor is most beneficial to block water while the hard-soft floor is the weakest combination with mining stress and permeability of surrounding rock after mining. The characteristics of mining stress of floors could be summarized to one curve, two sections, three characteristic points; the mining curve could be divided into supercharging and releasing of pressure as well as three characteristic points of peak value of releasing, changing of stress and regressing of stress. The depth of changing of stress increases when fluid-solid coupling has been taken into consideration and it shows that it is influenced by releasing of pressure more and the deep part changes from supercharging to releasing gradually, which means that the releasing of pressure exists in the deep part of floor because of the pressure of confined water in floor, and that is bad for blocking of floor. What’s more, the rangeability of depth in different points of stress changing in different rock-layers combinations is different and the range of releasing of hard-soft floor is the largest while the soft and hard alternated floor is the smallest, which reveals the controlling mechanism of characteristics of rock-layers combination on the mining effect of floor.(4) The three floor models of different structures of fracture, fault which could cut coal seams, hidden collapse column were studied from mining effect. The inrush of floor of working face caused by excitation influenced by mining and water pressure of fault and collapse column were got and the results show that the risk of inrush in floor caused by structures increases with the increasing of mining depth, water pressure of floor and crustal stress. The inrush of floor caused by fracture could be divided into three processes, which are plastic damage stage of fracture, expanding stage of plastic zone of fracture and formation of water inrush channel. The inrush of floor caused by fault which could cut coal seam is mainly because that in-situ tensile fracture belt develops upward to the upflowing of mining, which leads to that the thickness of effective water resistant layer of floor decreases obviously. The inrush process of floor caused by hidden collapse column is because the upward upflowing holing with the mining damage of floor, which reveals the inrush mechanism of floor of different structures of rock mass.(5) Based on the research of similar material simulation test of fault that could cut the coal seams, we conclude the fault has a very obvious control action on mining effects of rock mass. On the transfer law of mining stress and displacement view, the fault has an obvious barrier effect for the transition of mining stress. The mining stress will concentrate on the hanging wall near the fault zone leading to the difference of mining stress in two walls appearing while the coal pillar width is smaller than 30m. Finally, the displacement differences of two walls appear along the fault plane and the maximum vertical displacement difference is 20.5cm leading the hanging wall sliding along the fault zone and making the fault active. The results reveal fault has control action to the mining effects and the mechanism of fault becoming active.(6) Combining with the grouting reinforcement and reconstruction project of II 615 working face coal floor in Hengyuan coal mine, rock mass structure before and after coal floor grouting was analyzed based on the measurement of rock mechanics characteristics and wave velocity and the measurement of wave velocity in situ. The research results show the strength of sandstone increase 1.36 times and which of mudstone increase 1.01 times after grouting of coal floor and the grouting effects is obvious. The failure depth of Ⅱ 615 working face coal floor caused by mining is 14m by measuring of mining failure in situ while which of Ⅱ614 working face is 14.9m. The mining conditions of two working faces are similar, so we can conclude the mining effects of coal floor are different because of the differences of rock mass structure caused by grouting revealing the control mechanism of rock mass structure of coal floor mining effects.(7) The strength of sandstone and mudstone of coal floor increased after grouting by measuring of laboratory testing and in suit testing, meanwhile the aquifer was transferred to aquiclude and the thickness of aquiclude was larger than before grouting. The increasing of coal floor thickness is not beneficial for the development of in-situ fractures zone when the water pressure of aquifer is constant, while the increasing of strength of coal floor rock can make the mining failure reduce obviously. So, after grouting of coal floor, the depth of mining failure and the height of in-situ fractures will reduce synchronously with the increasing of aquiclude thickness and strength making the effective thickness of aquiclude increase and proving the ability of blocking water. Finally, the security of coal mining can realize with pressure under the coal floor of lower coal group.