Protective Layer Mining Research Articles

Stress evolution and coal seam deformation through the mining of a remote upper protective layer

ABSTRACTProtective layer mining is considered to be an effective method to resolve the outburst risk of coal seams, but limited theoretical and experimental results exist for remote upper protective layer mining. To study the effect of the remote upper protective layer mining, a case study of the Renlou Coal Mine is selected. The stress evolution in the protected layer is analyzed using Flac3D and is verified by the measurement of coal seam deformation and investigation of the unloading boundary. The results indicate that remote upper protective layer mining induces an unloading of stress within the protected layer, while a stress concentration occurs outside of the unloading area. The coal seam deformation is consistent with the stress evolution, for which the maximum compressional and expansional deformation are 0.5‰ and 7.19‰, respectively. The gas pressure decreases significantly as a consequence of these applications, following which the gas pressures within the theoretical unloading boundary are 0.30 MPa and 0.35 MPa along-strike and along the tendency of the protected layer, respectively.

Protection scope and gas extraction of the low-protective layer in a thin coal seam: lessons from the DaHe coalfield, China

Aiming at promoting thin coal seam pressure relief gas extraction, as well as delimiting the protection range of the low-protective layer in thin coal seam, in this paper, we simulated the changing process of the protective layer stress field and deformation field during the low-protective layer mining and obtained the characteristics of the displacement as well as the stress varied characteristics of the overlying strata in the low-protective layer mining. According to the protection layer mining stress relief and the deformation protection criterion, the tendency and trend of the protection layer protection scope were determined, and the theoretical calculation results were basically consistent with the direction protection angle of the protected coal seam. Taking the Dahe (DH) coalmine as an example, the floor rock roadway as the gas drainage roadway to extraction gas as well as the scheme of gas extraction and pressure relief in the upper and lower layers of the protective layer were implemented. The effect of gas control scheme showed that the working gas concentration reached the mining requirement. We finally established the low-protective layer in thin coal mining integrated technical scheme of protection scope and pressure relief gas extraction.

Solid‐Gas Coupling Model for Coal‐Rock Mass Deformation and Pressure Relief Gas Flow in Protection Layer Mining

The solid‐gas coupling model for mining coal‐rock mass deformation and pressure relief gas flow in protection layer mining is the key to determine deformation of coal‐rock mass and migration law of pressure relief gas of protection layer mining in outburst coal seams. Based on the physical coupling process between coal‐rock mass deformation and pressure‐relief gas migration, the coupling variable of mining coal‐rock mass, a part of governing equations of gas seepage field and deformation field in mining coal‐rock mass, is introduced. Then, a new solid‐gas coupling mathematical model reflecting the coupling effects of gas adsorption/desorption, gas pressure, and coal‐rock mass deformation on the mining coal‐rock mass deformation and pressure relief gas flow is established combined with the corresponding definite conditions. It lays a theoretical foundation for the numerical calculation of the deformation of mining coal‐rock mass and the migration law of gas under pressure relief in the outburst coal seam group.

Numerical Simulation Study on Protective Range of Upper Protective Layer Mining

During exploitation in first mined seam, how to accurately determine the protective range of stress releasing seam and further determine burst-free zone is problematic in preventing coal and gas outburst. For judgmental standard theory in burst-free zone in first mined seam, the protective range is determined on the basis of expansion deformation in stress releasing seams. the author bases on the project of 513C15 working face in Xieyi mine and determines its protective range by COMSOL Multiphysics simulating software.

Stress distribution characteristic analysis and control of coal and gas outburst disaster in a pressure-relief boundary area in protective layer mining

Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Furthermore, the protective coal seam mining technology featured by economic efficiency has been proven to be the most effective and widely applied method for the prevention of coal and gas outburst disasters. However, the determinations of the protective area coal and gas outburst prevention in a pressure-relief boundary area are fundamental issues that research should be focused on. The technical method for determining stress distribution in pressure-relief boundary area during protective coal seam mining is put forward in this paper. The method is based on a stress-seepage coupled relationship within a gas-containing coal seam. The method includes complex lab experiments and on-site measurements at the Qingdong Coal Mine. The final data illustrate that the permeability and vertical stress in the pressure-relief boundary area of the coal sample form a negative exponential function relationship. Additionally, the permeability of the coal sample within the abovementioned area is significantly different compared with that located at the center of the pressure-relief area. In the pressure-relief boundary area, the gas pressure distribution gradient is 0.0375 MPa/m, while the vertical stress distribution gradient registers 0.56 MPa/m. Under this condition, coal and gas outburst disasters are prone to be triggered. Therefore, effective precautions against coal and gas outburst disasters can be put forward in accordance with stress distribution characteristics within the abovementioned “boundary area.”

Analysis of Overlying Strata Movement and Behaviors in Caving and Solid Backfilling Mixed Coal Mining

Based on techniques of close upper protective coal-rock layer mining, relieved gas extraction, and underground gangue washing-discharging-backfilling, this paper initiates the concept of mixed fully-mechanized coal mining, which combines a solid backfilling method and a caving method (hereinafter referred to as “backfill and caving mixed mining”). After the principle and key techniques are introduced, a physical simulation experiment and a numerical simulation are used to study the characteristics of the overlying strata’s fracture development, the main roof subsidence, the stress field and its influence area in the transition area with the length ratios of the backfilling section and the caving section, and the advancing distance of the mixed longwall face. Thus, the lengths of the caving section and the backfilling section, the parameters of the support system in the transition section, and the design process of the mixed longwall face are presented. In practice, the mixed longwall face Ji15-31010 in Ping-dingshan No. 12 Colliery proves that the designed lengths of 120 m and 100 m for the backfilling section and the caving section, respectively, are appropriate. The monitoring results of the hydraulic support working resistance show that the supports were working well in general; the maximum growth height of the overlying strata fracture is 18 m; the gas drainage efficiency is up to 80% and the average gas concentration is 0.1 g/m3; a large quantity of gangue generated in the Ji14 seam is disposed underground; coal and gas are extracted simultaneously; and significant environmental and economic benefits are realized.

Study on the Temporal and Spatial Effect of the Relief Pressure of Upper Protective Layer Mining

In order to study the law of pressure relief of upper protective layer mining. Through theoretical analysis and numerical simulation method, studied the distance on the protection layer mining floor stress field evolution and distribution, found a protective layer on the longwall mining goaf bottom coal seam inclination direction is concave, and strike plane was “O” shape of relief ball shell, the pressure relief angle in the cut or end mining line near the corner of the middle bottom is minimum, the scope of protection of coal seam pressure relief effect than on both sides of the pressure relief effect, namely the scope of protection of inclined coal seam central drainage radius than the two larger; in the trend of protected layer pressure relief protection layer mining face lag distance based on this theory, research to guide the protected layer of gas drilling drainage is very important.

Study on the Protective Effect of Upper and Lower Coal Seams in the Protection Layer Mining of Middle Distance Coal Seam Group

Gas treatment of high gas outburst coal seam group with protection layer mining, based on the engineering geological conditions of the mine, combined with the field engineering test, study on the effect of B10 coal seam mining on the upper cover B11b and down to B8 under the protective layer in the protection layer 22108. The results showed that the protective layer mining combined with drilling pre pumping protected layer gas, the coal and gas outburst danger of the middle and lower coal seams were reduced, the coal seam gas occurrence law was changed, the permeability of coal seam was increased, and the effect of extraction was improved, which provided the technical basis and support for the regional gas control in the protective layer of the mine.

Mechanical behavior and permeability evolution of gas infiltrated coals during protective layer mining

This study investigated the stress–strain–permeability relationship of a Chongqing coal under the stress path during the mining process. The abutment stress was first measured at the longwall (LW) face 3211 of Songzao Mine in Chongqing, China. The field monitoring results revealed that the concentration coefficient of the abutment stress was approximately 1.5–2.0 during protective layer mining. Then, triaxial compression tests for the gas-infiltrated coals were conducted under the above stress path and different gas pressures. These tests, with the simultaneous actions of unloading confining stress and loading axial stress, are called SUL tests. The triaxial compression tests revealed that the peak deviatoric stress and the corresponding strain of coal under SUL tests were lower than those under conventional triaxial compression (CTC) tests. Poisson's ratio was higher, but the elastic modulus was lower in SUL tests. The permeability evolution of coal under the SUL tests underwent four distinct stages: the increasing stage in the process of SUL, decreasing stage, slowly increasing stage beyond the yield point, and sharply increasing stage after the peak stress. With the increased gas pressure, the peak deviatoric stress and corresponding axial strain decreased, Poisson's ratio increased, and elastic modulus decreased. Further, the permeability of coal increased with increasing gas pressure in the complete deformation process.

A field investigation of the deformation of protected coal and its application for CBM extraction in the Qinglong coalmine in China

Protective coal layer mining with pressure-relief gas extraction from the protected coal layer is an effective and economical way of controlling coal and gas outbursts in underground mines. According to a field test of the deformation of the protected coal layer in the Qinglong coal mine, during the mining of the protective coal layer, the floor of the protective coal layer can be divided into five areas in the horizontal direction: the original stress area, the stress concentration area, the stress relaxed area, the stress recovery area, and the recompaction area. From coal bed methane (CBM) extraction with pressure relief, the floor strata can also be divided into five areas: the original gas flow area, the gas flow reduction area, the initial pressure relief area with increasing permeability and flow, the full pressure relief area with high permeability and flow, and the pressure recovery area with decreasing permeability and flow. Mining of a protective coal layer will result in the breakage and movement of the overlying strata as well as floor heaves in the floor strata. In these unloaded coal strata, relieving the stress leads to the production of new cracks, which increases the permeability. The coal deformation and gas desorption are remarkable in the protected coal seam. The maximum dilation and dilation rate were 13 mm and 4.19‰, respectively. The gas flow and permeability increased significantly. The maximum natural emission flow and permeability reached 0.194 m³/min and 19.75 m2/MPa2.d respectively, There were 27- and 42-fold increases in permeability, which indicated that the pressure relief in this region was complete and that fractures in the coal body developed. This emission is conducive to CBM migration and extraction.

Study of Coal and Rock Floor Failure Characteristics of Upper Protective Layer Mining

For less study on coal rock floor fracture distribution of upper protective layer mining process, by numerical simulation and similarity simulation combined to get failure characteristics of distribution on the floor with the protective layer mining ,divided into deformation-failure zone and plastic deformation zone of seam floor, with the same time divided each zone height of "upper three zone". Regional outburst prevention measures as the most effective, a mine protective layer mining practice, investigated the protective effect of the protective layer on the protected layer after mining coal seam gas pressure, gas content, permeability coefficient changes, Because of the protective layer at the plastic deformation zone of seam floor, the gas release have a good effect .By numerical simulation and similarity simulation test combined, get protective layer mining floor failure distribution. Keywords: upper protevtive layer;Similar simulation;numerical simulation; failure characteristics ;protection effect.

The evolution of permeability and gas composition during remote protective longwall mining and stress-relief gas drainage: a case study of the underground Haishiwan Coal Mine

The mining of protective coal seams can cause changes in geostress, leading to changes in the permeability of coal rock and creating favorable conditions for gas extraction from coal seams. At the Haishiwan Coal Mine, field tests using remote protective coal seam mining were performed in the protected layer, which is rich in CO2 gas. In remote protective longwall mining, the permeability and composition of extracted stress-relief gas can vary. Under the conditions of remote protective longwall mining, the permeability of a protected coal seam can be generally described by the Liu model. During protective layer mining, the permeability of the protective layer increases rapidly with the release of stress, then decreases gradually with the recovery of the geostress. However, matrix shrinkage and decreased pore pressure caused by CO2 desorption from coal seams also cannot be ignored when considering the factors that affect the permeability. Thus, it is necessary to appropriately configure the cross-measure boreholes in advance to drain the stress-relief gas during remote protective layer mining. Stressrelief CO2 gas extraction presents multiple consecutive peaks. The No. 2 coal seam has different trap pressure systems as CO2 migrates into the coal seam. The protected seam experiences different effective stresses during protective layer mining, and the permeabilities appear to periodically increase due to differences in the original permeability. The various permeability and diffusion coefficients for CO2 and CH4 in coal induce CO2 and CH4 fractionation in the roof and floor of the No. 2 coal seam.

Study on Underlying Coal and Strata Pressure-Relief Principle and the Gas Extraction Technique under Upper Protective Layer Mining

By study on underlying coal and strata pressure-relief principle and the gas extraction technique under upper protective layer mining, we obtain the stress change and distribution law of underlying coal-rock mass. We analyze the deformation law and fracture development characteristics of underlying coal-rock mass movement. With mining proceeding ahead, the total floor coal and rock experiences compression deformation first, then expansion deformation and re-compaction of the continuous periodic destruction. Based on different development characteristics and status of underlying coal-rock mass, the underlying coal-rock mass under an effect of upper protective layer mining was divided into the floor heave fracture zone and the floor heave deformation zone in this paper. The permeability coefficient of change law of underlying the coal seam as follows: the original value-small decreasing-increasing greatly-reducing-stability at last. The field test for upper protective layer mining of Zhang-ji coal mine of Huainan shows that the effect of pressure relief of protected seam is very good. So it eliminates the risk of gas outburst, ensuring safety mining of the protected seam. The research has an important significance for safety and efficient production under similar exploitation conditions of low-permeability with high gas and outburst risk coal seam.

Mining thin sub-layer as self-protective coal seam to reduce the danger of coal and gas outburst

In response to the severe situation of coal mine gas disaster in China, a new method of reducing the danger of coal and gas outbursts and improving gas drainage and utilization in coal mines was introduced in this paper. The main idea of this method is to mining thin sub-layer as self-protective coal seam to eliminate or reduce the danger of coal and gas outburst. This method can be implemented by drills along seam and hydraulic jet when the mined seam with a relatively weak risk of coal and gas outbursts is soft or has a soft layer. This method was first applied in the Yian mine to verify its effectiveness. The results of application showed that mining thin sub-layer as self-protective coal seam can effectively eliminate the danger of coal and gas outburst and improve gas drainage and utilization. As this method needs less time and lower cost than conventional protective layer mining, it is of great significance for mining coal seam with the danger of coal and gas outburst.

Study on Pressure Relief Scope of Underlying Coal Rock with Upper Protective Layer Mining

In this paper,in the case of upper protective layer mining of a mine for the engineering background, based on the model of semi-infinite plane body in elastic mechanics, the theoretical calculation model of verticalhorizontalshear stress of floor coal rock is established, and it is concluded that the stress expression of any point in the impact of distributed force. By introducing the rock strength index I and generalized Hooke's law, it is concluded that the discrimination criterion of guide gas fissure zone and pressure relief desorption belt of underlying coal rock with upper protective layer mining. The pressure relief scope of protection with the mining of upper protective layer is determined. Theoretical analysis shows that: the "pressure relief desorption zone" of the underlying protected coal seam that the vertical distance to the protective layer is 40m lags behind 22.2m.Through field test, the results show that the obtained pressure relief scope of protection is 26.98m,and the theoretical analysis results are basically similar.

Numerical simulation on pressure-relief deformation characteristics of underlying coal-rock mass after upper protective layer excavation

Based on the occurrence features of Group B coal-seams at a coal mine in the Huainan coal mining area, the elasto-plastic mechanical damage constitutive functions and numerical model for the protective layer excavation were established. With the UDEC2D computer program, after the upper protective layer was mined, the stress field change trends, crack development, and expansion deformation trends of underlying coal rock seams in the floor of the working face were simulated and analyzed. The simulation results show the stress changes in coal rock seams, the evolution process of pre-cracks during the process of upper protective layer mining, the caved zone and fractured zone of the underlying coal rock seams. At the same time, the results from the actual investigation and analysis of protected layer deformation match the simulation values, which verifies the validity and accuracy of numerical simulation results. The study results have an important guiding significance for gas management in low permeability and high gas coal seams with similar mining conditions.

The Preventing Coal and Gas Outburst Technology of Middle Distance and Low Permeability and Upper Protective Layer

Protective layer mining is the main regional measures for control coal and gas outburst . According to the defects of the preventing coal and gas outburst technology of middle distance and low permeability and upper protective layer in western Shuicheng mining area, the coal seams coal and gas outburst original danger are analyzed and protective layer is choosed initially using gas geological theory,the choice upper protective layer feasibility is analyzed using protection layer theory. the preventing coal and gas outburst technology of middle distance and low permeability and upper protective layer control effect is studied by numerical simulation and spot investigation. The conclusions from this research：the preventing coal and gas outburst technology of middle distance and low permeability and upper protective layer can decrease coal and gas outburst prediction index and control coal and gas outburst; the coal seams coal and gas outburst original danger can be preliminarily evaluated and the evaluation result can provide the basis for protective layer choice; the preventing coal and gas outburst technology of middle distance and low permeability and upper protective layer can be applied in western marine-continental transitional facies coal measure strata.

An analysis on the effect of mining height and floor lithology on pressure relief of upper protective layers

In order to understand the effect of mining height and floor lithology at the upper protective layer face on the pressure relief of protected coal seams, this paper uses a numerical simulation method to model the pressure changes at protected coal seam during mining upper protective layer. The results show that the taller the mining height at the upper protective layer face, the greater the protection on protected coal seam due to the higher level of pressure release; the upper protective layer face with hard rock floor impedes the pressure release at the protected coal seam, which affects the overall effect of the pressure release at protected coal seam using the protective layer mining method.

Numerical Simulation of Pressure Relief for Extremely Thin Coal Seam as Protection Seam Based on UDEC

Overlying strata stress distribution, pressure relief region and crack distribution characteristics are analyzed based on numerical simulation of pressure relief with extremely thin coal seam as protection seam used by UDEC. The result is a true reflection of overlying strata movement and fissures for extremely thin coal seam as protection seam. Studies have shown that the roof displacement in the face of protective layer promoting is big, and as far away from the protective layer, the roof displacement is gradually reduced, the base plate displacement is almost the same, but the roof and floor to stress have been fully release; after protective layer mining, an "O" type pressure relief crack area is formed above the goaf, which is gas rich region and gas channels at the same time and suitable for extracting high concentrations gas.

Study on the Terminal Line Reasonable Position of Two Coal Seam under the Down-Protective Layer Pressure-Relieved Mining

In order to study the stress distribution and its dynamic influence law while the protective layer mining, based on the transfer law of mining-induced stress in the coal seam floor and in front of the working face, using numerical simulation software to simulate the surrounding rock stress under the different pillar width mining conditions, and carried through the roadway deformation engineering practice observations. It is shown that reserved 110m coal pillar could weaken the impact on the front of the floor tunnel under the protective layer mining process. When the top liberated layer mining to reduce the impact of mining stress superposition, it should avoid the terminal lines on the two coal seams at the same location and may be staggered at least about 30m ~ 50m. And it obtained that the roadway deformation not only by mining impact, but also considering the geological environment surrounding rock conditions, tunnel position in which layers of rock, rock properties and other factors. The research guided the engineering practice successfully.