| With the enlargement and modernization of coal mine,the technology of gob-side entry driving is frequently adopted in the layout of coal face.However,the surrounding rock of gob-side entry not only has much difficulty in roadway maintence but also is sensitive to mining influence.The research on the formation law of overlying strata fracture structure and its influence on the stability of surrounding rock is essential to the gob-side entry support.Therefore,study on the lateral fracture position and the structure stability of main roof is significant to the development of the surrounding rock control technology used in gob-side entry and the production safety of the coal mine.The rock fracture position and the surrounding rock control of 20321 gob-side entry are studied in this paper by research method such as the mine pressure behavior research,theoretical modeling analysis,laboratory test,numerical simulation,field borehole analysis and mine pressure monitoring.Besides,some key issues of gob-side entry are respectively analyzed in this paper,including the plastic zone distribution of surrounding rock,theoretical calculation of roof fracture position,the influence factor and field measurement of roof fracture position,the optimum design of rock fracture observation equipment,the fracture structure stability of main roof,the stability of surrounding rock,the law of strata behavior,the asymmetric support scheme of newly excavated section,the reinforcement support scheme of elderly excavated section and the mine pressure monitoring after industrial test.The conclusions are obtained as follows:(1)Based on the field investigation of the strata behavior characteristics in the 20321 gob-side entry,it is found that 2# coal seam,which consists of developmental rock joints and rock cracks,is of soft broken rock and low strength.Besides,the mudstone of the roof is irregular,with high porosity and low strength;the siltstone is relatively complete and has high strength.The surrounding rock deformation is abnormally large and there is a large amount of coal emission in the section of the coal pillar.Although a series of short cables are added in the coal pillar,the effect is not obvious.All this indicate that the original support scheme is lack of rationality and needs to be optimized.(2)Based on the failure characteristics of gob-side entry,the factors affecting the stability of the surrounding rock are classified into three categories: geological structure,engineering structure and supporting structure.The relationship between each factors is analyzed in this paper.It is found that the supporting structure and the structure stability of overlying strata is the main controlling factors.(3)The lateral fracture structure in main roof of gob-side entry is divided into 4 basic forms according to the different relationship between the main roof fracture position and the roadway position.The 4 forms consist of strata fracture above the solid coal,strata fracture above the roadway,strata fracture above the coal pillar and strata fracture above the goaf.(4)The lateral fracture structure model of the main roof above the gob-side entry is built based on the mechanical model of elastic foundation beam.The results show that the reason of the roof fracture is that the bending moment is excessively large,and the tensile stress at the edge of the roof exceeds the rock tensile strength.The roof fracture belongs to brittle fracture.The brittle fracture occurs at the maximum bending moment.The main roof fracture position x0 is determined by the distribution characteristics of the bending moment.The analysis of the bending moment distribution shows that the fracture position of the main roof is 8.6m from the coal side along the gob.The calculation formulas are as follows:(5)The analysis of the factors affecting the fracture position in the main roof is based on the mathematical formula of the fracture position calculation.It turns out that the fracture position of the main roof is affected by some parameters such as the young's modulus,the bedding value,the overhang length and the thickness of the coal seam,of which the overhang length is the main influence factor.(6)The fracture starting point of the main roof is observed by a reasonable field observation scheme and the drilling peep.It turns out that the fracture position,which ranges from 4.4m to 9.1m,starts from the 9.1m next to the gob and extends to the gob.The fracture position drawn from the theoretical calculation is consistent with the measured data by drilling peep,which poves that it is feasible to calculate the fracture position by the mechanical model in this paper.(7)A device for three-dimensional detection of rock mass fracture structure is designed with the use of high-definition camera and electromagnetic wave,which could effectively overcome the shortcomings of existing roof fracture observation equipment.Then the attachment link equiped in this device is upgraded.The device can improve the accuracy,the operability and the range of the rock fracture observation.Meanwhile,the device has the convenient,rapid and labor-saving effect,which can eliminate the time-consuming and laborious work of the drilling peep.(8)The mechanical model of the articulated block is established based on the characteristics of the lateral strata fracture above the gob-side entry.The mechanical condition of the sliding instability and rotary instability which occurred in the fracture structure of the main roof during the period of roadway excavation and coal mining is deeply analyzed on the basis of the key block B.(9)The elastic plane strain mechanical model of the coal pillar during the roadway excavation is put forward on the basis of the rheological characteristics of coal-rock mass along the gob-side entry,so is the calculation formula of the coal pillar displacement field in the period of roadway excavation.The differential equation of coal pillar displacement field is solved by Ritz method.(10)Accoding to the linear Mohr-Coulomb criterion,the elastoplastic mechanical model of the plane strain in the coal pillar is established on the premise of ensuring the stability of overburden strata fracture structure.Known from the interaction principle of force,the pressure of the main roof to the coal pillar is equal to the supporting force of the coal pillar to the main roof.The creep constitutive equation of coal pillar is established based on the creep characteristics of coal-rock mass and the dilatancy theory after failure.Then the approximate solution of the plane strain problem is solved according to the stress boundary condition.The numerical model of the overlying strata structure above the 20321 gob-sied entry is established.It found that: the strata migration law is analyzed in different depth,different stages and different directions.The results show that the vertical displacement of the roof above gob-side entry is more prominent than its horizontal displacement in the process of roadway excavation and coal mining,while the vertical displacement and the horizontal displacement of the roof during the coal mining period are about 2 times as big as that of the roadway excavation period.Meanwhile,the compressional movement to the middle of the roadway occurs in the roadway roof.In the period of the roadway excavation and coal mining,the bidirectional displacement is asymmetric,and the distribution shape of the vertical displacement obviously turns out to be like a diagonal,which is inclined to the side of the coal pillar.Moreover,the asymmetric migration law is particularly obvious in the roadway roof during the period of the coal mining.(11)Based on the observation results of the plastic zone and the strata behavior law of gob-side entry,the asymmetric support scheme of newly excavated section and the reinforcement support scheme of elderly excavated section are put forward to overcome the irrationality of the original support scheme.It is pointed out that the coal pillar and the roof next to the gob are significant to the support of the gob-side entry during the period of the coal mining.Measures can effectively reduce the sensitivity of coal pillar and roof to the mining influence,such as decreasing the bolt interval,increasing the bolt diameter and length,using the threaded steel bolt and the steel truss cable,designing reasonable eccentricity and enhancing the overall strength of coal roadway surrounding rock.(12)In the process of industrial test,the axial force and the anchorage length of bolts are measured in the coal side and the roof.The test result shows that the bolts' axial force is normal in the test section.Alhough the bolts' axial force of the reinforcement support scheme in elderly excavated section is found abnomal,the anchor bolts are still in a stable state.The anchorage length in the test section is bigger than that of the reinforcement support scheme in elderly excavated section.Owing to the relatively broken coal in the coal pillar,the anchorage length of the roof bolts is bigger than that of the coal pillar side,less than that of the solid coal side.The anchorage length of the new support scheme is universally more than 950 mm,which basically meets the design requirements.(13)Stress monitoring of the coal pillar side and the solid coal side in mining stage shows that during the coal mining,the internal stress growth velocity of coal pillar changes from large to small,and the abutment pressure distribution of the coal pillar changes from gentle to steep.The 3.5-4.5m area in the coal pillar is the stress-concentrated zone,and the stress peak value reaches 15.8MPa when it comes to 15 m from the working face.The internal stress of the coal pillar is generally higher than 7.5MPa.The ascending area of abutment pressure in the solid coal is within 0-15 m from the working face,and other area is the discending area.With the larger distance to the working face,the abutment pressure gradually reached a stable state.The mining influence is relatively large within the 6-9m of the solid coal where the stress peak value is 13.7MPa,which is less than that of coal pillar.All this prove the greater mining influence in coal pillar and the rationality of asymmetric support scheme.(14)The monitoring result of the surrounding rock displacement shows that the stability period of the 20321 gob-side entry is about 20 days.The deformation is larger during the first 10 days,then turns out to be stable after 20 days.The asymmetry of the surrounding rock deformation with the new support scheme is not obvious.The displacement of the coal side and the roof during the coal mining period is greater than that of the roadway excavation period.The deformation difference between the coal side and the solid coal side during the coal mining period approximately equals to that of the roadway excavation period,and its asymmetry increases slightly.The conclusion proves that the new support scheme plays an important role in controlling the asymmetric deformation of the surrounding rock along the gob-side entry. |
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