| With our country coal resources development, parts of mine field such as Pingdingshan, Kailuan, Huainan and Xuzhou successively enter into the deep mining. The influence of tectonic stress on coal and rock damage has increased dramatically, and the horizontal tectonic stress is equal to or even greater than vertical stress, so coal and rock is under high triaxial stress. Besides, there is generally high content gas of coal seam in our country. More than half of the coal seam is the high gas coal seam, so the gas accidents often occurred. In fact, the gas is a kind of non-renewable clean energy. However, nowadays, in our country about two-thirds of it directly discharges into the atmosphere, wasting resources and polluting the atmosphere. Thus, to achieve reasonable and efficient gas extraction, the research in configuration features and space-time evolution of deep mining rock fracture has important significance. On the basis of systematically summarizing the mining-induced fracture of coal and rock evolution research at home and abroad, the dissertation studied the fractal-percolation characteristics of coal and rock fracture network evolution under the condition of the deep coal mining with the introduction of the fractal and percolation theory, and combining with the renormalization group method. The main research contents and conclusions are as follows:1. The similar material model experiments of tectonic stress and gravity stress were constructed on the basis of coal mining face No. 15-22060 at No.8 Mine of Pingdingshan Coal Mining Group. The physical modeling is 3.5m in length, 1.53 m in height and 0.25 m in width. Combined with geological conditions of the mine and analog simulation theory, the linear ratio is set as 1:100, the density ratio is set as 3:5, and the stress ratio is 3:500, horizontal tectonic stress and the vertical gravity stress with 1:1. Strain gauges are embedded in the 14.1 m, 39 m and 84 m height which are in relative to the prototype model making process from coal floor, to monitor the exploitation of mine pressure in the process of change. Researches have shown that with the work face pushing ahead from right to left, the upper strata stress in front of working face coal and rock is increasing until the strata caving and pressure relief. The upper strata stress of coal pillar in open-off cut increases with the working face advancing. Especially, the strata stress near the coal pillar experiences three stages: increases slowly, increases sharply and stabilize. As caving zone extended to the upper left, stress release area and vertical stress concentration area appear above the coal seam. And these two areas are constantly changing according to the working face advancing. The peak stress distributes in front of the working face within range from 0 to 15 m, increasing and moving forward with the increase of advance degree.2. As the working face advancing, coal and rock strata moving, destructing and caving, displacement of setting up observation point has been changed. Using high precision electronic theodolite observes and records the change of the horizontal angle and vertical angle during experiments. Then through the formula to calculate the subsidence and horizontal displacement and study their changing rules with the working face advancing. Researches results are as follows:(1) With the increase of advance degree, coal and rock cracks begin to extend to the ahead and above along the direction of mining working face, subsidence range has also been expanded. Strata gradually start to appear significant subsidence at the order from right to left, from bottom to top. Subsidence and the subsidence area continually decrease from bottom to top, and the shape of subsidence curve changes from inverted trapezoidal into funnel.(2) Rock fracture scope and fracture angle on either side of the goaf increase with the increased of strata height, coal and rock strata form gradually changes from caving into bending deformation from bottom to top.(3) When the working face advancing, there is slightly relative horizontal movement between the un-caving rock strata. Working face advancing leads to overlying rock strata caving. Due to the sharply movement of the strata, caving rock strata horizontal displacement significantly changes from bottom to top, the most obvious change is one of the lower strata, and slows gradually from bottom to top. Under the effect of horizontal tectonic stress, rock horizontal displacement is increased apparently; the maximum displacement is more than 1 m, it is closer to mine the movement regularity of the surrounding rock of deep mining conditions.3. During experiment, high-definition cameras are used to real-time monitor and gather data of overlying strata deformation and caving. On the whole process of working face pushing, roof caves 15 times and has six times periodic caving. Based on overlying strata caving cases of different advance degree under the effect of tectonic stress, study its correlation between roof pressures and caving regularity with working face advancing. Researches results show:(1) With the advancement of working face from right to left, overlying caving area gradually expand to the breadth and height direction, constantly generate new system of rock mass structure. The overlying strata distribution is the "three zones" from bottom to top. Pushing 160 m, bending subsidence zone height is 57.65 m; caving zone height is 14.1 m, it's about 3.9 times of the mining height; fissure zone height is 81.4 m, it's about 22.6 times of the mining height.(2) There is a corresponding relationship between the point pressure measurements in front of working face and the roof periodic caving. The roof periodic caving has significant effect on the measured pressure values in front 20 ~ 30 m of it, i. e. when roof periodic has caved, the pressure value of the measuring point within 20 ~ 30 m in front of it will occur mutation. This can provide a certain basis for predicting periodic weighting and periodic caving in the process of mining through the measured value changes of the pressure measuring point embedded in the rock formation in front of working face.(3) VII With working face advancing, before the main key stratum damage, coal and rock destructing and caving under the effect of horizontal tectonic stress is later than which under the gravity stress effect, caving height and range is smaller. After the main key stratum damage, coal and rock periodic caving under the effect of horizontal tectonic stress is later than which under the gravity stress effect, but caving height and range is larger. This fully demonstrates that tectonic stress has a significant influence on coal and rock caving regularity, and the quality of coal and rock strata plays a crucial role of its effects.4. With the continuous advance of working face, new horizontal delamination fracture and vertical breakdown fracture constantly generate, so does new rock mass structure. Mining-induced fracture network distribution area gradually expands to forward and height of the working face. The evolution process is of great randomness and complexity, but it is in accordance with fractal law. Crack network images in different stages were extracted through image processing software, and then the fractal dimension was figured by box-counting method. The relationship between the mining field pressure at both sides of the working face and fractal dimension was discussed. The periodic evolving characters of fractal dimension with advancing lengths of coal mining face were summarized by analyzing crack network of coal and rock mass under the action of tectonic stress and gravity stress with fractal theory. It shows:(1) dimension reduction point is the demarcation point both under the action of tectonic stress and gravity stress, the transformation law of the fractal dimension can be divided into three increment stages and two small reduction stages, the three increment stages and the whole process share the same mathematical version but different fitting coefficient. At the first stage, the exponent increases rapidly, the slopes of second and third stages are 0.00117, 0.0006 and 0.00126, 0.001 respectively. The slope of the third stage is smaller than that of second stage. At the whole process, the increase extent tends to slow, the relationship between fractal dimensions and advancing lengths shows the negative exponential relation. But the negative exponential relation of the whole process cannot be adapted by every stage. This demonstrates the uncertainty of nonlinear and the second and third stages are only approximate linear relationship.(2) With the working face advancing, the mining field pressure at both sides of the working face gradually enlarged through the fractal dimension three increment stages. But it also magnified at the reduction stages. This is because that the roof breakage and caving makes mining space to expand, therefore the overburden pressure transferees to both sides of the mining space. Thus the pressure at the open-off cut and ahead of the working face increases and there is always a pressure increasing trend on both ends of the field measuring point. However, many factors like roof breakage and caving can affect fractal dimension, besides, the crack propagation and coalescence have influence on it. So the fractal dimension has increment and reduction with advancing lengths of coal mining face.(3) In the whole experiment process of coal seam mining, fractal dimensions of crack network under the action of tectonic stress is bigger than it under the action of gravity stress, but it also appears they share same fractal dimension in some occasions. This demonstrated that the rock strata bend significantly under the action of tectonic stress and there are a highly propagation of vertical fracture. In addition, the influence of tectonic stress on fractal dimension is also related to the quality of coal-rock mass and the location of key strata.5. In coal and rock mass, the propagation and evolution of fracture is complex, unordered and random. Percolation theory has its unique advantage in the study of random distribution system and disordered structure. This dissertation is based on HK arithmetic and MATLAB to calculate percolation probability and study the change rule of percolation probability with advancing length of coal mining face under the action of tectonic stress and gravity stress. Studying the corresponding relationship of percolation probability and fractal dimensions, and finally to reveal the influence of tectonic stress to crack network in coal and rock. It shows:(1) dimension reduction point is the demarcation point both under the action of tectonic stress and gravity stress, the change law of percolation probability can be divided into three linear increment stages with the working face advancing, the three stages share the same mathematic version, but different fitting coefficient. In the two cases, the slopes of the three stages are 0.01713, 0.04068, 0.03419 and 0.00672, 0.01089, 0.02716 respectively; the increment amplitudes are different at different stages. The increment amplitude of percolation increases first and then decrease under the action of tectonic stress; nevertheless it always has an increased trend under the action of gravity stress.(2) With the working face advancing, the vertical fractures become more and more because of tectonic stress, therefore bigger cluster generates. Obviously, the number and density of fracture in the biggest group under the action of tectonic are larger than those of gravity stress, so is the percolation probability. However, in both cases the crack network of coal and rock mass is relative small with respect to the entire study area. This determined the percolation probability values are small, too. In the two cases, the percolation probability are 0.04768 and 0.02522 respectively, they are lower than percolation threshold of two-dimension percolation model, so there is no probability of percolation change and osmosis phenomenon.(3) With the working face advancing, the percolation probability of crack network of coal and rock mass is getting larger along with the three fractal dimension increment stages. The first stage satisfies a relationship of power function and the next two stages satisfy two linear functions, with very good correlation. These mathematical models provide tools for predicting percolation probability of crack network through fractal dimension and lay a foundation for studies on equivalent permeability of crack network in coal and rock.6. On the basis of percolation lattice model of crack network of coal and rock mass, using renormalization group method, the author figured out the critical probability of system forming connected fracture network through numerical iterative when the shape parameter of rock m takes integer value from 1~10 respectively, and revealed its correlation with shape parameter changes. Taking at 180 m working face advancing fracture network as an example, the author analyzed the connectivity of horizontal and vertical direction on coal and rock fracture network plane by using renormalization technique. Researches results are shown as follows:(1) On one, two, three, four levels of coal and rock fracture network planes, the last four level matrixes of renormalization analysis is zero. That shows at all levels of fracture network in plane are disconnected, and are not penetrating network. This is the same as fracture network percolation characteristics analysis results.(2) In the horizontal direction of fracture network connectivity renormalization analysis, fracture unit of "1" shape and separation fractures distribution is consistent. Only when the separation fractures are distributed throughout all the study area, it is possible for the final renormalization matrix becoming [1, 1]. At this time, fracture network is connected at the horizontal direction.(3) One, two, three, four levels of coal and rock fracture network are impermeable in vertical direction. Vertical fracture scattered distributes in the area, this is the main cause that fracture network is not connected in vertical direction. |
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