Due to technology and safety limitations, the amount of coal resources overlying slopes in open pit coal mines is immense. In recent years, this problem has gradually attracted the attention of researchers. How to realize the efficient recovery of the side overburden resources with the premise of ensuring the stability and safety of the slope has become an important topic for the development of opencast mining technology in China. To study the yield failure characteristics of coal pillars and the rock mass migration law of the end slope mining field under the mining condition of the end slope shearer, 2D/3D, integrated, simulation experimental equipment is developed based on similarity theory and efficient region theory. This equipment overcomes the technical problem that the internal failure of the rock mass is invisible and that deformation data are not easily obtained during the simulation of end slope coal mining on an existing experimental platform. Based on the engineering geological conditions of the Ordos mining area in China, a typical engineering geological model of the slope near the horizontal condition is constructed to simulate the process “formation of mining cave group -failure of support coal pillars - instability of slope rock mass”. Based on laser positioning technology and multiangle, oblique photography technology, a panoramic phase 3D laser scanner, high-resolution digital camera and deep space micromonitoring system are comprehensively employed to carry out the whole process tracking monitoring and analysis of the deformation and failure of the supporting coal pillars and slope rock mass. The experiment is verified by numerical simulation. The results show that under the experimental conditions, with an increase in mining cave depth, the vertical stress of the supporting coal pillar increases linearly. At a certain distance before reaching the end of the mining cave, the peak value is reached. At this time, the depth continues to increase, and the stress value decreases sharply. The vertical stress gradually decreases to the original rock stress after a certain distance beyond the end of the mining cave. A certain length of supporting coal pillar from the end of the mining cave will never collapse, which is approximately 2.5~3 times the width of the mining cave. The triggering condition of slope deformation and failure is under the combined action of dynamic and static loads. The actual stress of the supporting coal pillar in the deep part of the geometric centre along the slope of the mining cave group is greater than the ultimate stress, and then large discontinuous deformation of multiple adjacent coal pillars around the central coal pillar is caused by compressive shear failure. The boundary of the final collapse plane range of the roadway group is approximately a closed curve formed by two paraboloids, which are axisymmetric with the No. Ⅳ coal pillar and open opposite. The parabola opening in the shallow part of the slope area is small, and the parabola opening in the deep part of the slope area is large. There is a significant space-time correspondence between the failure of supporting coal pillars and the deformation of the slope surface. According to the failure process of the rock mass structure and the movement and deformation characteristics of the slope surface, the slope after failure can be divided into three areas, and the upper part of the slope is the key area of deformation and instability of the overlying rock mass in the end-slope mining field. The research results provide a theoretical basis for scientific monitoring and stability control of slope deformation coal mining conditions in open-pit mines.
Due to technological and safety limitations, there are considerable amounts of coal resources pressed under end slopes in open-pit coal mines. In recent years, this problem has gradually attracted the attention of researchers. How to realize the efficient recovery of side overburden resources while ensuring the stability and safety of slopes has become an important topic for the development of mining technology in China. A 2D/3D integrated simulation experimental device for end slope coal mining is developed. Based on the geological engineering conditions of the Ordos mining area in China, a typical geological engineering model of a near-horizontal slope condition is constructed to simulate the whole process of “roadway group formation—supporting coal pillar failure—slope rock mass instability”. By tracking and monitoring the movement and failure processes of slope rock masses in an end-slope mining field, the yield failure characteristics of coal pillars and the movement and deformation laws of slope rock masses are revealed, and the deformation and failure zoning method of overburden rock in an end-slope mining field is proposed; in addition, the key stages, trigger conditions and key positions of slope deformation and instability are defined. The results show that under the coupling action of the “slope stress field” and “roadway group stress field”, the actual stress of a supporting coal pillar in the deep part of a geometric centre along the slope of a roadway group is greater than the ultimate stress, and then large discontinuous deformations of multiple adjacent coal pillars around the central coal pillar are caused by compressive shear failure, which is the triggering condition of deformation and instability around the mining slope. The key position of yield failure of the supporting coal pillar is located in the deep part of the geometric centre along the slope of the roadway group. At the end of the mining adit, the supporting coal pillar within a width of 2.5–3 times the mining adit will not be destroyed. The boundary of the final collapse plane range of the roadway group is approximately a closed curve formed by two paraboloids, which are axisymmetric with the central pillar and open oppositely; the parabola opening in the shallow part of the slope area is small, and the parabola opening in the deep part of the slope area is large. There is a significant space–time correspondence between the failure of supporting coal pillars and the deformation of the slope surface. According to the failure process of the rock mass structure and the movement and deformation characteristics of the slope surface, the slope after failure can be divided into three areas, and the upper part of the slope is the key area of deformation and instability of the overlying rock mass in the end-slope mining field. These research results provide a theoretical basis for the scientific monitoring and stability control of slopes in an end-slope mining field.
According to the relationship between permeability and porosity of geotechnical materials, a finite element model representing pore and solid particles is generated randomly according to the porosity of a given finite element calculation model. According to Darcy’s law of flow distribution and steady seepage in the finite element random simulation section, the equivalent permeability coefficients at different porosities are calculated, and the relationship between the equivalent permeability coefficient and the porosity of rock and soil is studied. The results show that the equivalent permeability coefficient is proportional to the porosity with the same pore size. In order to study the seepage characteristics of structural planes of nonmaterial geotechnical materials in different strata contact zones, the formulas for calculating the deformation parameters and permeability coefficients of heterogeneous rock masses with single nonmaterial geotechnical materials are deduced theoretically, and the correctness and applicability of the formulas are verified by experiments. The rock mass sample selected in this paper is granite, which is simulated and analyzed by sandstone in the experiment. The results show that the permeability coefficients of coarse sandstone, fine sandstone, and heterogeneous rock mass are different under the same water pressure and confining pressure. This shows that the lithology on both sides of the nonmaterial geotechnical material surface has a significant influence on the permeability of the nonmaterial geotechnical material rock mass; the permeability coefficient of the nonmaterial geotechnical material rock mass decreases with the increase of confining pressure, the numerical change is limited to a certain confining pressure range, and the permeability coefficient tends to be stable when the confining pressure reaches a certain value. Comparing the theoretical calculation value of permeability coefficient of rock mass with the experimental result, it is found that the two values are in good agreement, which indicates the correctness and applicability of the theoretical calculation formula of permeability coefficient of rock mass of single intangible geotechnical material.
Due to the influence of mining technology, some coal mines will leave mined-out areas (or goafs) in the rock mass after mining. If the distribution law and influence scope of mined-out areas cannot be accurately explored and controlled, it will bring great potential safety hazards to subsequent coal mining and cause serious harm to adjacent houses, roads, bridges, railways, and so on. The unmanned aerial vehicle (UAV) aerial survey is used to accurately acquire the three-dimensional model of the slope in the key area of Yuanbaoshan open-pit coal mine, which provides basic information for the real-time visualization of the open-pit mine and the establishment of the engineering geological research profile. And the distribution of mined-out areas and water accumulation in the coal mine are investigated in detail by using the comprehensive geophysical prospecting method of the transient electromagnetic method (TEM) and controlled source audio-frequency magnetotellurics (CSAMT). Then, the borehole detection method is used to verify the goaf range obtained from comprehensive geophysical prospecting, and the results show good consistency. In addition, based on the results of borehole exploration and the comprehensive analysis of the borehole image, the instantaneous water level elevation in goaf and the development height of the water-conducting fracture zone and caving zone are concluded. Finally, the distribution of coal mined-out areas in the exploration area is obtained, and the results show that through the use of the transient electromagnetic method and controlled source audio-frequency magnetotellurics method, combined with drilling detection methods, the purpose of accurate exploration of mined-out areas in coal mines can be achieved.
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