Quantitative Characterization of Overburden Rock Development Pattern in the Goaf at Different Key Stratum Locations Based on DEM

The overburden rock mining fissures are the main cause of coal spontaneous combustion, gas pooling, and mine water inrush caused by goaf air leakage. Rapid and accurate determination of the development and evolution law of mining fissures have great significance for the application of coal spontaneous combustion prevention and control, gas disaster prevention and control, and water damage prevention and control measures. In this paper, a preliminary judgment of the development height of the water-conducting fracture zone is made based on the theoretical analysis, and the physical model size of the numerical simulation is determined according to its judgment result. It is judged that the development height of its water-conducting fracture zone is between 49 and 64.2 m, which is in line with the actual results. Based on this, a three-dimensional solid model was established in PFC (Particle Flow Code) software to analyze the fissure development pattern of the overburden rock and the development height of the water-conducting fracture zone when the main key stratum of the rock seam is in different positions by simulating the excavation process of the coal seam. The results show that when the main key stratum is located in the “original crack belt boundary,” the development of water-conducting fracture zone is significantly inhibited; when the main key stratum is located in the “original caving zone,” the water-conducting fracture zone is fully developed, and the crack belt finally develops to the top of the model. In order to verify the accuracy of the numerical simulation, similar material simulation experiments were performed under the same scheme. The results are consistent with the numerical simulation conclusions, effectively verifying the accuracy of the numerical simulation. Finally, the extraction of porosity of the goaf was carried out based on numerical simulation, and the permeability zoning of the goaf was performed; the results show that the development of the water-conducting fracture zone has a significant influence on the permeability of the mining area, and the more fully developed the fissure is, the greater is its permeability. In this paper, the fissure development law in the goaf under different key stratums is explored by various research stratums, and the results show a good consistency, which provides a scientific basis for the prevention and control of disasters such as water inrush and coal and gas outburst in mines, and provides theoretical guidance for safe mining.

SIMULATION OF JOINT FORMATION OF CAREER OVERBURDEN ROCK WITH PLACEMENT OF IRON ORE ENRICHMENT WASTE

Purpose. The purpose of the research presented in the article is to substantiate the possibility of dump formation during joint dumping of quarry overburden rocks and iron ore wastes during the development of the Horishne-Plavnivsky deposit of iron quartzites. Task. Perform geomechanical modeling of the behavior of the quarry dump with the placement of funnels with enrichment wastes and establish the possibility of their joint storage in the dumps of the Horishno-Plavnivskoye deposit; to establish regularities of development of deformation processes in a dump in process of construction of each layer at dump of wastes of enrichment in funnels and to calculate a factor of a stock of stability of a dump after its formation. Research methods. To achieve the goal of research used: the method of complex analysis – to summarize previous research on the establishment and justification of the most rational method of dumping of enrichment waste; modeling methods – to establish the possibility and feasibility of joint dumping of enrichment wests and overburden rocks from the quarry. Scientific novelty. For the conditions of the Horishne-Plavnivsky deposit of ferrous quartzites by the method of geomechanical modeling the expediency and possibility of placement of dehydrated enrichment wastes in funnels on dump of overburden rocks is substantiated. Conclusions and practical significance of the article. For research results the predicted behavior of a dump during all term of its use is received. The obtained values of the maximum deformations are deviated in the process of its formation by each of layer and the investigated efficiency of deformation at formation of the following layer is established. The indicator of the coefficient of safety of the stability of the corresponding deviation after its formation and building, which means that it exceeds the minimum, in accordance with current regulations (n=1,329>1,3), after it is formation with further use joint dumping of overburden rocks and enrichment wastes is possible and appropriate.

Ground Pressure under Mining Ore Bodies of Different Angles Based on Physical Simulation

Ground pressure characteristics of the ore body and the overburden deformation of the stope depend highly on the combined influence of geological conditions and mining disturbance. The ore body inclination, as a natural geological factor, has a nonnegligible effect on the underground mining. The ore angle plays a great role in the stress distribution of the overlying rock layer, resulting in the movement and destruction of the rock layer. The variation of the ore angle dominates the stress distribution of the overburden rock, the forms of movement, destruction, and the surface moving basin. Here, taking the geological mining conditions of the deep ore body mining in Jinning Phosphate Mine as the engineering background, we adopt a similar material ratio scheme of each rock layer in the mining area via the similarity theory and the principle of orthogonal experiment. We conduct systematic study on the strata movement, mining failure characteristics, and movement of the overlying rock in stope using a similar simulation test under two different ore angles of 20° and 50°. We found that, as the ore body inclination increases from 20° to 50°, the overburden unloading area of the stope extending to the deep part of the rock layer in the vertical direction is more obvious and its shape is more asymmetric about the stope center. The unloading area is more concentrated in the middle and upper part of the stope, while the upward development trend is more obvious. The relevant results can provide a certain reference for the underground mining of the mines and those with similar conditions.

Fractal Characterization of Pressure-Relief Gas Permeability Evolution in a Mining Fracture Network

The process of underground coal mining fractures the overlying strata and may provide storage and transportation space for gas by changing the roof rock permeability, which is released by pressure after mining. This paper adopts the experimental method of physical similarity simulation and combines the fractal theory to study the permeability characteristics of the fracture network after mining, and it establishes the fractal permeability equation of pressure-relief gas. The results of the study show that the fracture opening shows a positive correlation with the overburden permeability, whereas the tortuosity of the fracture shows a negative correlation with the overburden permeability. The shape of the high permeability area in the fracture network is found to be similar to the hat-shaped elliptical parabolic zone. In the process of permeability evolution, the key layer structure of the overburden rock is considered as the main factor that affects the trend of change in permeability. Furthermore, based on the above research results, this study developed a targeted design of high-level boreholes in the experimental face and reversed the permeability changes around the drainage borehole. The average error between the actual measured value and the theoretically calculated value is found to be 8.11%. The theoretical model and the permeability evolution law obtained from the research results can provide valuable references and insights into further research on the pressure-relief gas flow model in the goaf.

Analysis for Overburden Destruction on Lateral Boundary of Stope Based on Viscoelastic-Plastic Finite Element Method

In longwall mining, the deformation and destruction of overlying strata always lag behind coal extraction. The overlying strata characteristics at the lateral boundary of the stope can be classified into four categories, i.e., Hard-Soft, Soft-Hard, Hard-Hard, and Soft-Soft. In order to analyze the effect of the above four structures, we adopt viscoelastic theory to the finite element method (FEM) and define the point safety factor to evaluate the rock damage. The accuracy of programming is verified through example verification. A modified viscoelastic-plastic FEM model is applied to analyze the performance of four overburden structures. The numerical computation results show the following: From the rupture of overburden rock to its stabilization, the duration time of four typical structures can be sorted as “Soft-Soft < Hard-Soft < Soft-Hard < Hard-Hard”. The fracture direction and dip angle of each structure vary as well. The fracture zone of the H-S structure is inclined toward the goaf, while that of the S-H structure is inclined to the lateral boundary of the stope. The fracture zone of the H-H structure is also inclined toward the lateral boundary, with a greater angle than the S-H structure, while the fracture zone of the S-S structure is inclined to goaf, with a greater angle than the H-S structure.

STUDY ON SURFACE STABILITY AND RESIDUAL DEFORMATION OF OLD GOAF IN DONGJIAGOU MINE，CHINA

Old goaf under the overpass becomes serious hidden trouble of subgrade-pavement and bridge engineering. Based on geological survey, geophysical survey and theoretical analysis, this paper studies on formation mechanism and distribution characteristics of the surface residual deformation in old goaf in No.9 Line Overpass across Rapid Rail Transit Line No.3 in Dalian city. A comprehensive analysis and evaluation has been made on the stability of old goaf. Based on the calculation principle of the probability integration method, the conception of ground residual subsidence coefficient and the predicted model of residual deformation are proposed, ground residual deformation of old goaf under the overpass is predicted. According to the zonal principles of ground stability, the stabilities of areas are divided. The results indicated that, new overpass has an important effect on the old goaf overburden rock activation in study area that the surface will be instability uneven settlement and the ground residual deformation values will exceed allowable values. Some treatment should be done to the old goaf because of the poor stability of goaf and non-goaf within influence zone in study area.

Study on Overburden Stability and Development Height of Water Flowing Fractured Zone in Roadway Mining with Cemented Backfill

In order to explore the stability of overburden rock and the development height of water flowing fractured zone in roadway filling mining, based on the movement and deformation mechanism of overburden rock, the mechanical analysis of overburden stability and failure was carried out, and the mechanical model of main roof rock beam was established, and the ultimate span and limit deflection of rock beam fracture were deduced. Combined with the mechanical model of the main roof fractured rock, the basis for the judgment of overburden failure developing to fractured zone is given in this paper. Taking a coal mine roadway backfill under water-bearing stratum as an example, based on the equivalent mining height, the theoretical calculation and analysis are carried out on the stability of overburden rock and the height of water flowing fractured zone. The reliability of the theoretical analysis is verified compared with the empirical formula and the numerical simulation results. The results showed that the water flowing fractured zone developed to the bottom of no. 7 glutenite, with a height of 32.5 m, slightly less than the calculation result of the empirical formula. The thickness of the waterproof coal pillar was 39.8 m, which was much less than the distance from the aquifer to the coal seam and can be mined safely.

Study on the Development Height of Overburden Water-Flowing Fracture Zone of the Working Face

Mining-induced fractures in underground coal mining face affect the stability of overburdens and provide preferential channels for water and material transfer in the underground environment. Therefore, to study the development of water-flowing fracture zones in overburdens of working face and goaf is of great significance for roof control, gas drainage, water resistance, disaster reduction, and efficient mining from the mining. In this study, a new method for predicting the development of overburden water-flowing fracture zone height (DHOWFFZ) was proposed based on the characteristics of overburden rock in No. 3 coal seam of Xin’an Coal Mine. First, the stope of No. 3 coal seam exhibits a rock stratum structure of mudstone and sandstone overlapping. Considering this characteristic, the overburden strata of No. 3 coal seam are divided into several “mudstone-sandstone” rock stratum groups. Furthermore, the ultimate tensile deformation of soft rock is greater than that of hard rock. It is proposed to judge the development degree of penetrating fracture in each rock stratum by adopting the elongation rate of mudstone intermediate layer. Meanwhile, the DHOWFFZ of “mudstone sandstone” composite rock stratum structure in the 3402 working face of No. 3 coal seam is calculated to be smaller than 43.1 m according to the actual situation. Finally, the DHOWFFZ in the 3402 working face was measured in the field, which verifies the rationality of the new DHOWFFZ prediction method. The research results provide new ideas for the prediction of DHOWFFZ and are helpful for future research in related fields.

Cosmic-ray muography applications in underground tunnelling

&lt;p&gt;The novel geophysical remote imaging method of muography is based on cosmic-ray induced muon particles that are detected after passing through the media of interest. If the studied objects are solid, their sizes can vary from meters to up to kilometres. In terms of penetration capability, muography can be placed between methods based on X-rays and those using seismic waves. The most famous objects imaged with muography are pyramids (e.g., Khufu's Pyramid at Giza in Egypt) and volcanoes (e.g., Mt Etna in Italy). One clear advantage of muography compared with seismic methods is that muons, unlike seismic waves, do not reflect from geological interfaces. In addition, the scattering phenomenon is a minor issue and needs consideration only at low-energy muons. Raw data must be corrected according to topography. On the basis of extensive numeric simulations of Hivert et al. (2017), the lowest density variations observable for muography with a significant level of 3&amp;#963; (a typical significance level in physics) are around 2% at 150 m, 4% at 300 m, and 10% at 700 m of depth, respectively. If these numbers are extrapolated to depths below 100 m, the mean density differences in the range of 1% are likely within the observation capability of muography. It is also worth to note that the 1% difference in a mean rock density results in an approximately 3% difference in the muon flux. This indicates that muon flux measurements are very sensitive to the density variations of rocks.&lt;/p&gt;&lt;p&gt;In underground tunnelling, muography has at least four applications: (1) muography can be used to detect a potential risk (such as a water reservoir, a weak zone with loose rocks, boulders, etc.) before or during tunnelling, (2) muography can be employed to monitor overburden rock behaviour during tunnelling operation to avoid risks like the roof cave-ins, (3) muography can be applied to monitor the overburdening rock masses in tunnels after they are excavated to predict and avoid the collapse of rock mass, and (4) muography can be used to estimate the size and volume of a rock mass collapse in a tunnel since the volume of the collapsed rocks must have markedly smaller density than original overburden rock mass. In an excavating tunnel project using a tunnel boring machine (TBM), a muon detector can be installed in the TBM during tunnelling. If there occurs a tunnel cave-in, muography can be employed in undamaged tunnels nearby (sideways or below) the collapse. If possible, the collapse can also be approached safely via an undamaged part of the collapsed tunnel. If none of these are available, borehole muography can be applied as a substitute solution. Whereas an undamaged underground tunnel is either filled by air or water, a collapsed tunnel segment is characterized by air and rock, or water and rock. In either case, the average density of the tunnel segment is increased. We are currently planning simulations and real-world tests to validate these assumptions.&lt;/p&gt;