Evolution Of Stress Field In Coal-rock Combination System With Reverse Fault

Although China's energy demand began to transform to green energy such as clean energy.The national coal consumption still accounts for more than 50%of the primary energy consumption.Coal is still the main energy in China.Therefore,ensuring safe and efficient coal mining can not only promote the steady development of China's economy and society,but also ensure national energy security.In recent years,with the exhaustion of shallow resources,the depth of coal mining is increasing.In the process of deep mining,we will face the problems of high ground pressure,high ground temperature and high gas,which will seriously threaten the safe and efficient mining of coal.In particular,there will be abnormal high ground pressure and high gas geological structure areas in the mine.These areas are the high incidence areas of underground coal and gas outburst,rock burst and other coal mine dynamic disaster.This coal mine dynamic disaster seriously restricts mine production activities.In this paper,the stress distribution characteristics in the structure affected area are studied in the coal mine reverse fault structure area which is most likely to induce coal mine dynamic disaster.This paper uses physical similarity simulation and numerical simulation methods.First of all,uniaxial compression tests were carried out according to nine similar ratios,and the similar materials of roof and floor coal rock conforming to the engineering prototype were selected.Secondly,we use the similarity principle to establish the physical similarity model of“mudstone-coal seam-sandstone”roof-floor combination with reverse faults,and carry out the physical similarity simulation test of reverse fault evolution under different dip angles.Then,we established the inverse fault numerical simulation model of“mudstone–coal seam–sandstone”roof and floor combination by PFC^3D discrete element software,and verified the PFC numerical model by physical simulation test.Finally,we carried out numerical simulation tests of different roof and floor lithologic combinations,coal seam thickness and reverse fault dip.The main conclusions are as follows:(1)The control effect of different reverse fault dip angles on the internal stress field of the model.By constructing the coal-rock combination model of roof and floor with reverse faults and carrying out unidirectional loading on the model,the distribution law of stress field in the evolution process of reverse faults is studied.The test results show that the stress concentration area will first appear in the coal seam and weak rock strata.With the increase of dip angle,the stress concentration area begins to gradually shift to the rock with high rock strength.From the specific analysis of three different coal-rock strength combinations,it can be seen that in combination 1,that the strength of roof rock is lower than that of floor,with the increase of dip angle,the stress concentration area will transfer to the floor with higher strength.In combination 2 and 3,the strength of the roof rock is higher than that of the bottom plate.With the increase of the dip angle,the stress concentration area will still transfer to the rock which has higher strength,and the rock with higher strength is the roof,that is the stress concentration area transfers to the opposite direction,and focuses on the roof rock.(2)60°dip angle is the inflection point of reverse fault failure mode.From the physical similar simulation test results,it can be found that in the process of unidirectional loading,when the dip angle is 45°,the upper and lower plates of fault are mainly sliding and staggered along the fault fracture zone.When the dip angle is 75°,the reverse fault is not activated.The two failure modes of 60°inclination model occur.(3)Effect of coal seam thickness on stress field evolution of model.In this paper,the numerical model is verified according to the physical simulation test,and then the numerical simulation tests of different coal thicknesses are carried out.The results show that the greater the thickness of coal seam,the lower the stress concentration of coal seam.And we can find that the stress is mainly concentrated in the roof and floor.Stress distribution strip of'double Z'shape in roof and floor due to thick coal seam.The greater the thickness,the farther the stress concentration strip away from the coal seam.The distribution of stress tends to be gentle and the continuity of regional distribution decreases.(4)The coal seam boundary with thickness of 1 m is an important basis for predicting the stress concentration position in fault fracture zone.When the thickness of coal seam is 1 m,the stress in the roof and floor of the strip distribution will breakdown the fault.At the same time,the stress will connect the bedding planes at the top and bottom of the upper and lower coal seams as stress paths.The stress distribution on both sides of the stratification plane presents a'double Z'shape.Under this thickness,the intersection of the two bands and the fault fracture zone is the stress concentration area,that is the fault fracture zone of 0.5 m above and below the coal seam center line is the stress concentration area.Even if the thickness of coal seam increases to 2 m,the stress concentration area always exists in the fault fracture zone.(5)The strength of roof and floor strata will affect the evolution of the internal stress field of the model.It can be seen from the numerical simulation experiment that the roof and floor of the model are mainly compressed shear failure.The tensile failure area is generally the junction area of coal seam,fault and roof and floor.Therefore,in the case of the same other factors,the rock formation with larger uniaxial compressive strength are more resistant to shear failure deformation.This will make the stress unable to be transmitted to the coal-rock interface area in large quantities.It not only reduces the tensile failure of the coal seam and its nearby area,but also protects the stability of the coal seam itself.The rock with lower uniaxial compressive strength has weak ability to resist shear deformation.After rock failure,the stress will transfer to the coal seam.Finally,the stress concentration inside the coal seam is increased.