Experimental Study On Pressure Releasing And Permeability Improvement Of Seam Slitting With Diamond Wire Saw

Coal seam permeability is generally low in China,and gas drainage is difficult.In order to meet the requirements of drainage standard,strengthening gas drainage methods has become the focus of scientific research in the field of gas control in China in recent years.Seam slitting is a widely used method of enhanced gas drainage.At present,the range of slitting has been expanded through technologies such as new jet,increased slitting pump pressure,increased slitting times,and combined hydraulic fracturing.These technologies not only effectively expand the range of slit disturbance,but also increase the amount of gas control works and capital investment.In order to reduce the amount of seam cutting and realize pressure relief and permeability enhancement in a wide range of coal seams,we have explored a new economic and efficient method of continuously cutting coal seams in a large area,." diamond string saw in coal seam slitting".Supported by the National Natural Science Foundation Project(51974121)and the natural science foundation Project of Hunan Province(2019jj40083),this paper takes the field test of seam slitting in Tangshan mine of Hebei Kai luan Group as the engineering background,and comprehensively uses the research means of theoretical analysis,mechanical experiment,similar simulation test and numerical simulation analysis,The pressure relief and permeability enhancement effect of seam slitting by wire saw is studied.The main research work and conclusions are as follows:(1)Based on the characteristics of wire saw cutting technology and equipment and the layout of underground mining roadway in coal mine,three process methods of wire saw cutting seam with single slit trend,single slit tendency and double slit trend are designed.The engineering application of rope saw in seam cutting shows that the seam working face can be constructed by using underground roadway and borehole.the diamond bead rope saw can cut seam safely and efficiently according to the design requirements,and the working efficiency and seam depth of the rope saw are greater than the hydraulic seam cutting measures.(2)A similar simulation test materials for wire saw cutting of coal seam were prepared,The mechanical properties of similar materials were obtained through mechanical test.The analytical mechanical test results show that when the lime-plaster ratio is the same,the greater the aggregate-cement ratio,the lower the strength of the material,and when the aggregate-cement ratio is same,the smaller the lime-plaster ratio the greater the strength of the material.A similar simulation test device for coal seam cutting with diamond wire saw was developed,The similarity simulation test of coal seam cutting by wire saw was carried out.The stress field,displacement field,fracture field and their evolution law of the coal seam around the seam slit were obtained.In the process of cutting the seam by wire saw,the stress of the coal around the seam slit shows the change characteristics of "rising-decreasing",and the coal seam above the seam slit has subsidence deformation.At the same advancing distance,the closer to the slit,the greater the subsidence of the coal seam,and the more obvious the degree of stress change in the area of coal seam stress rise and stress decreasing;At the same advancing distance and the same coal seam position,when a larger diameter wire saw is used to cut a seam,the subsidence of the coal seam and the degree of stress change in the coal seam are greater than those of the small diameter wire saw.After the seam is cut by the wire saw,The seam overlying the roof of the slot forms a fracture zone and a bending subsidence zone,no caving zone was formed.The coal seam in the fracture zone generates many fractures with smaller openings and lengths.The height of fracture development is greater in the middle than at both ends of the groove,The number of seam fissures generated is more at the beginning position of seam cutting than at the end;When using a larger diameter beaded rope to cut a seam,the number and length of the cracks in the overlying seam on the roof of the slit are greater than those of the small diameter beaded rope.(3)The RLW-500 G Triaxial creep-seepage test device was used to carry out the gas seepage tests of coal samples with different damage degrees,Under the same stress path,the permeability of fractured coal sample is one order of magnitude larger than that of elastic coal sample,the permeability of coal samples with different damage degree decreases exponentially with the increase of stress.In the process of cyclic loading and unloading,if the coal sample enters the yield stage,its permeability will increase,and the permeability of the coal sample after unloading is greater than that before loading.By fitting the gas seepage test results of coal samples with different damage degrees with the ideal stress-permeability model,the stress-permeability formulas of elastic coal samples and fractured coal samples for three times of cyclic loading and unloading under constant pressure were obtained.Then the stress-permeability formulas is into the numerical simulation.(4)Using FLAC3 D numerical simulation software,the vertical stress,swelling strain,and plastic failure,permeability distribution characteristics of the coal seam during the process of wire saw slitting were analyzed.Before the roof with a wire saw single-slit cut seam bottoms,an "inverted U-shaped " pressure relief area is formed above and below the slit,and the vertical stress of the coal seam in the pressure decreasing area is significantly reduced.The supporting stress area is formed in the coal wall in front of the working face and behind the starting position.The supporting stress area is "vertical heart-shaped",and the vertical stress of the coal seam in the supporting stress zone increases significantly.At the same gap height,the closer to the slit,the greater the degree of vertical stress change in the pressure relief zone and the supporting stress zone.and the greater the recovery degree of vertical stress in stress recovery area.With the same advancing distance,when the gap height is large,the greater the degree of vertical stress change in the pressure relief zone and the supporting stress zone.The numerical simulation results and similar simulation experimental results confirm each other.After increasing the slitting times,the first and second slit form a through pressure relief zone in the coal seam.The pressure decreasing area formed by the double-slit cutting is larger than that of the single-slit cutting,and the pressure relief area is "double-pointed heart-shaped".After the slit roof hits the bottom,the stress recovery area formed in the middle of the slit divides the original stress decreasing area into two "upright heart-shaped" stress decreasing area,one is close to the working surface side of the wire saw and the other is close to the start cutting position side.The relationship between the advancing distance of the wire saw working face and the pressure decreasing area is quantitatively analyzed,During the advancing process,the pressure decreasing area of the coal seam first increases and then drops to a certain value,and the critical condition for the coal seam pressure relief area from increase to decrease is the first bottom contact of the coal seam in the middle of the seam slit.At the initial stage of slitting,the expansion deformation of the seam in the middle is greater than at both ends above the seam slit.In the mid-late stage,the expansion deformation of the coal seams at both ends of the slot is greater than the middle,and the expansion deformation of the coal seam with a large gap is greater than that with a small gap.Wire saw slitting will cause plastic damage to the coal seams.In the transverse direction,the plastic damage range of the coal seam will become longer as the working face advances.In the vertical direction,the plastic failure range of the coal seam is related to the thickness of the wire saw slitting and the number of slitting.Increasing the diameter of bead rope and the times of sawing seam can improve the plastic failure range in the thickness direction of coal seam;Increasing the dip length of the working face and the advancing distance of the strike seam can significantly improve the plastic failure range of the dip and strike direction of the coal seam.When seam is cut by wire saw,the permeability of coal seam is the lowest in the support stress zone at both ends of seam groove,and the highest in the pressure relief zone.The stress recovery zone in the middle part of the seam increased with the advance distance of the wire saw seam working face,The permeability of coal seam decreases in the stress recovery zone in the middle section of the seam;At different heights of the coal seam,the permeability of the coal seam is higher the closer to the seam,and the permeability of the coal seam is lower the farther away from the seam;The permeability of horizontal coal seam at the same height is greater than that of single slot seam when double slot seam has the same thickness.(5)According to mine pressure and rock strata control theory and elastic mechanics theory knowledge,The stress of coal rock on seam roof in the process of coal seam pressure relief and reflection improvement by wire saw was analyzed.The calculation formula of coal and rock subsidence above the slot is obtained.The deformation and subsidence characteristics of overlying coal rock on seam roof cut by wire saw are explained.The mechanical balance equations of the middle and both sides of the coal face cut by wire saw before and after the seam roof touches bottom are established,The formula for calculating the stress component of coal and rock under the seam is obtained,The dynamic evolution of the vertical stress of coal seam under the seam with the advance of wire saw cutting coal face is analyzed.According to the theoretical analysis,the smaller the elastic modulus of the overlying coal seam roof cut by wire saw,the closer to the cutting horizontal plane and the farther the advance distance of the coal cutting working face,the greater the subsidence of the overlying coal seam roof cut by wire saw.Before the bottom is reached,the farther the advance distance of the wire saw cutting coal face is,the greater the stress concentration coefficient of the coal seam in the support stress zone at the same position below the cutting level is,and the higher the maximum stress release rate in the pressure relief zone is.The advancing distance of the rope saw cutting coal face is the same,and the closer it is to the cutting horizontal plane,the greater the stress concentration coefficient of the coal seam in the supporting stress area is,and the higher the maximum stress release rate in the relief area is.After the bottom is reached,the wire saw cutting coal working face continues to advance.The stress concentration coefficient of coal seam in the support stress zone below the cutting horizontal plane and the stress release rate in the pressure relief zone change little,but the stress of coal seam in the stress recovery zone gradually increases.The advancing distance of the wire saw cutting coal face is the same,and the closer it is to the cutting horizontal plane,the greater the stress concentration degree in the coal seam supporting stress area,the greater the stress release degree in the pressure relief area and the greater the stress recovery degree in the stress recovery area..