| In the important stage of energy consumption for low-carbon transformation mode,the security function of coal resources in the realization process of carbon neutralization and carbon peaking,is an important support to promote high-quality economic and social development.In recent years,non-pillar coal mining has become an important direction to achieve the high-quality development of coal resources.However,in the process of mining coal resources under the condition of hard roof for Carboniferous grounds,the dynamic ground pressure and serious deformation of roadway retaining by roof cutting have led to the significant deterioration of the surrounding rock control environment.Additionally,a large number of support structures have been damaged and even the roof has collapsed.These powerful ground pressure appearances have posed a severe challenge to the safe and efficient development of coal resources.Therefore,it has important theoretical guidance and practical significance to systematically study the failure mechanism and key technology of roadway retaining by roof cutting under the condition of hard roof for Carboniferous grounds.Taking the typical roadway retaining by roof cutting of Dadougou Coal Mine as the engineering research background,and taking the roadway retaining by roof cutting and the following mining states control of hard roof for Carboniferous grounds as the main research line,ground pressure features of roadway retaining by roof cutting were explored.The evolution law of the surrounding rock situation in the primarily used stage and the secondary reused stage was clarified.A three-way pressure relief blasting device that is suitable for the transformation of the suspended roof in the Carboniferous grounds was developed.The key technology of pre-splitting blasting was proposed.The surrounding rock failure mechanism of roadway retaining by roof cutting was studied.The control method with following mining states of roadway retaining by roof cutting under the condition of hard roof for Carboniferous grounds was put forward,and the engineering practice of following mining states control was field implemented.The main conclusions are as follows:(1)Rock mechanics experiment results show that the rock mass basic quality indexes of coarse-grained sandstone,fine sandstone and silty mudstone of the roof for coal seam 2# are 349.87,621.5 and 392.72 respectively,and the basic quality grades are Ⅳ,Ⅰ and Ⅲ respectively,which means they belong to the hard roof.The main reasons for the asymmetric deformation of the surrounding rock and the extrusion of gangue wall are the multiple activities of the roof cutting structure,the complex stress environment and the mismatch of the support structure.(2)The surrounding rock deformation is characterized by asymmetric failure along the vertical and horizontal directions based on the central axis of the roadway.Along the vertical direction,the convergence of roof-to-floor near the roof cutting side is greater than that near the virgin coal side,resulting in the fracture of the steel belt and the failure of the anchor bolt(cable).Along the horizontal direction,the extrusion deformation of the gangue side is greater than that of the virgin coal side,and the phenomena such as splitting of metal mesh and the bending failure of gangue retaining structure occurred.The measured results indicated that the ground pressure behavior of the working face with roadway retaining by roof cutting also presents asymmetric characteristics.The end cycle resistance and the maximum working resistance of the hydraulic support near the roof cutting side of the working face are less than those near the non-roof cutting side of the working face.(3)From the perspective of physical similarity simulation,the influence process of roof cutting on its instability is analyzed.Roof cutting significantly accelerates the collapse speed of the roof near the roof cutting side,and also enhances the lateral impact strength on the gangue wall.Taking the second invariant of deviatoric stress and the third invariant of deviatoric stress as assessment indexes,the accumulation and release characteristics of surrounding rock distortion energy and the transformation characteristics of surrounding rock strain type in the primarily used stage and the secondary reused stage are analyzed by numerical simulation.The variation zoning of deviatoric stress invariants can be divided into continuous growth area(within 10 m of virgin coal)and rapidly declined area(roadway area and gangue wall area),which shows a changing trend of gradually increasing → rapid decline → tending to stability.The failure type of surrounding rock within the range of 2 m from the virgin coal,1.4m from the gangue wall and 6 to 8m above the roof is mainly tensile failure.The peak value of deviatoric stress invariants is transferred to the deep part of the surrounding rock due to roof cutting,which promotes the release of distortion energy.(4)The damage process of rock mass and detonation stress propagation process are analyzed by LS-DYNA numerical simulation software.Energy accumulation blasting has the characteristic of the high-speed concentrated release of detonation energy in a very short time.The crack penetration of rock mass is closely related to the propagation process of detonation compressive stress ring during double-holes energy accumulation blasting.The fracture of rock mass between two holes mainly occurs in the stage of rapid formation and stable development of detonation compressive stress ring.The process of rock mass crack penetration can be divided into crack initiation stage,continuous extension stage and extension suddenly increased stage.(5)Field tests of “deep-shallow” hole combination blasting,sectional creation guidance hole blasting,new-type D-shaped energy tube blasting and three-way pressure relief blasting were implemented.A three-way pressure relief blasting device that is suitable for the transformation of the suspended roof in the Carboniferous grounds was developed.This device can effectively presplit the rock mass along the direction of the groove and the annular hole to form the fissure with “⊥” shape,which significantly destroys the integrity of the suspended hard roof in the gob.Hence,the problem of the large-area suspended roof in the gob in the process of roadway retaining by roof cutting for Carboniferous grounds is solved.(6)By means of theoretical calculation and numerical simulation,the characteristics of the stress field and the displacement field were explored when the roof cutting height was 7 m,9 m and 11 m respectively and the roof cutting angle was 5 °,15 ° and 25 °respectively.The processes of crack penetration and detonation compressive stress propagation in the rock mass with the spacing of 400 mm,500 mm and 600 mm were simulated and analyzed.The roof cutting blasting tests with different charge structures,charge quantities and hole sealing lengths were implemented.The values of key parameters for roof cutting blasting are obtained,that is,the roof cutting height is 9 m,and the roof cutting angle is 15 °,and the spacing of roof cutting holes is 500 mm,and the charging structure is "4 + 4 + 3 + 3 + 2",and the sealing length is 2.8 m,and the number of single ignition is 3.The field test results show that this roof cutting blasting scheme can effectively pre-crack the rock mass into joints,and it has achieved a better effect.(7)The mechanical models of virgin coal side and gangue wall of roadway retaining by roof cutting are established,and the failure characteristics of coal and rock mass are analyzed.Surrounding rock mechanical environments in which the fracture positions of the basic roof structure are located above the roadway,above the virgin coal and above the gob near the roof cutting side are expounded respectively.The influence process of the roof structure movement on the surrounding rock failure during the mining of the working face is theoretically analyzed.Based on the theoretical calculation results,it can be seen that the fracture position of the basic roof in Dadougou Coal Mine is located above the gob near the roof cutting side.The failure of the surrounding rock of the roadway is mainly caused by the roof subsidence near the roof cutting side and the extrusion and bulging deformation of the gangue wall.(8)The influence process of the key roof cutting blocks movement on the surrounding rock before mining of this working face,after this working face mining and after adjacent working face mining is theoretically analyzed.The asymmetric failure of the surrounding rock originates from the rotary subsidence of the key roof cutting blocks B1 and B2.The active period of roof and wall united movement mainly occurred in the mining period of this working face.The key point of surrounding rock control during this working face mining is the roof and gangue wall,while the key point of surrounding rock control during adjacent working face mining is the virgin coal side.In the advanced support region,it is necessary to strengthen the support of the upper shoulder angle of the roof near the virgin coal side.(9)According to the analysis results of the surrounding rock stability,it is concluded that the multiple movements of key blocks,the complex and changeable stress environment and the poor matching degree of the support structure are the main reasons for the failure and instability of the surrounding rock.It is a scientific strategy to effectively control the asymmetric failure of surrounding rock,the extrusion deformation of gangue wall and the roof failure under repeated mining disturbance.(10)Based on the plastic crack closure deformation characteristics of crushed extrusion,dislocation and embedding near the roof cutting side,and based on the closure deformation law of the structural plane,the constitutive equation of gangue wall is deduced and the constitutive relationship is clarified.The overall stability analysis mechanical model of "roof subsidence-channel steel gangue retaining-asymmetric support" that considers plastic softening foundation boundary is established.The composite reinforcement technology of gangue retaining with "metal mesh-cement blanket-channel steel-single hydraulic prop" as core units is proposed.Moreover,the advanced support technology of roof cutting retaining roadway based on the four degrees matching of "stiffness-flexibility-density-strength" is also put forward.(11)Combined with the practical experience of surrounding rock control,the control principle with following mining states,region division principle and differentiation principle are put forward.Based on the zoning characteristics of surrounding rock failure in the advanced dynamic pressure influence area,mining dynamic loading action area and post-mining stability area,as well as the phased characteristics in the period of the primarily used stage and the secondary reused stage,the control method with following mining states is proposed.Taking the typical roadway retaing by roof cutting of Dadougou Coal Mine as the research background,after the application and implementation of the following mining states support scheme,the damage of the roadway surrounding rock is effectively controlled,and the long-term safety of the roadway is realized. |
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