Failure Mechanism Of Surrounding Rock And Applicable Mining Depth Of Double Roadway Layout In Inner Mongolia-Shaanxi Mining Area

Based on the engineering practice,this thesis investigates the general situation of roadway engineering in Inner Mongolia-Shaanxi mining area and consults relevant re-search literature.On this basis,we comprehensively use the research means of field monitoring,theoretical analysis,numerical simulation,experimental analogy discus-sion,case verification.We obtain the deformation and failure characteristics of sur-rounding rock,the vector evolution law of stress field and plastic zone of surrounding rock and the influencing factors of repeated mining in entry retaining area.The theo-retical formula of plastic zone of surrounding rock is derived by considering the influ-ence of axial stress of roadway.The sensitivity of stress state-plastic zone of sur-rounding rock is analyzed deeply,and the characteristics of mining failure in retaining roadway are obtained.Based on the relevant theories,the criterion of the applicability of retaining roadway is obtained,and the control idea of surrounding rock stability in mining roadway is put forward.Finally,an engineering case is carried out based on the field monitoring of double roadway layout with different buried depths.We have made the following main conclusions and innovative achievements:（1）Field observation and numerical analysis were carried out for 5 entry retention in 5 working faces of two coal layers in Shendong Buertai Coal Mine with typical double roadway layout mode.The difference of mine pressure appearance in 2-2 coal seam reserved roadway is the butterfly expansion of plastic zone caused by the inconsistency of stress reflected by different surrounding rock conditions.The difference of mine pressure appearance in 4-2 coal seam reserved roadway is that the stress concentration degree of reserved roadway is different under the influence of the same geological conditions and different mining activities,which leads to different development sizes of plastic zones.（2）The distribution characteristics of regional stress field and plastic zone of re-served roadway in the process of working face advance show dynamic evolution law.Under the influence of the first mining,the axial stress of the reserved roadway is symmetrically distributed in the middle of the goaf with the increase of the advance distance of the working face.When the working face advances twice the width of the working face,the stress value tends to be stable.The maximum principal stress within50m on the mining side is close to the vertical stress,the intermediate principal stress is close to the axial horizontal stress of the roadway,and the minimum principal stress is close to the horizontal stress of the roadway section.Confining pressure ratio is con-sistent with stress distribution,while lateral pressure coefficient is opposite.The lateral stress of the goaf is close to vertical within 10m away from the goaf.The deflection is severe within 20m（about 60°）,and the deflection is mainly concentrated in the range of 60m on the side（35°）.During the second mining,the stress in advance section in-creases with the working face advancing to a square period,and the lead influence dis-tance is about 150m.（3）Due to the limited redistribution of the regional stress field during tunneling,the plastic zone of surrounding rock is generally small,and the stress cloud and plastic zone are symmetrically distributed.During the first mining,the change of stress and plastic zone in the reserved roadway area is limited at 100m away from the advance working face.Within 50m ahead of the working face,the change of the plastic zone gradually appears to be asymmetrical.The plastic zone in the whole advance section can be increased by 36%～50%compared with that in the excavation period.The plastic zone of the working face section with first mining lag is seriously extended.When the hysteretic working face is the width of the working face,the plastic zone is the largest and tends to be stable thereafter.During the second mining,the plastic zone begins to expand 100m ahead,the severe influence range is 50m ahead,and the plastic zone is basically in the overlying rock of coal pillar about 20m ahead.（4）Numerical analysis is made on the side stress of goaf and the plastic zone of20m coal pillar under different influencing factors.The main controlling factors of surrounding rock instability under different conditions are obtained.The deformation instability of surrounding rock can be regarded as the external reflection of the strength property of surrounding rock to the mining stress.The mining stress is firstly determined by the stress foundation controlled by the buried depth of coal seam,and secondly by the strength of surrounding rock（based on the collapse range and boundary bearing capacity）.When a certain stress base is reached,a small change in the regional stress field can cause a large difference in the plastic zone.Therefore,the main factors to consider the applicability of reserved roadway are the buried depth of coal seam and rock strength.（5）The shape evolution trend of reserved roadway from excavation to abandonment is"round-ellipse-butterfly shape".Under any circumstances,the maximum or minimum confining pressure ratio of roadway section will lead to the butterfly development of roadway surrounding rock.The theoretical correctness of the butterfly plastic zone is verified by the analogy of the existing experiments.In addition,it is concluded that the shape size of plastic zone of roadway surrounding rock is affected not only by confining pressure ratio but also by axial stress under the action of non-uniform stress field.The axial stress only changes the size of the plastic zone of the roadway when the section pressure is uniform.When the confining pressure ratio of the stress of the roadway section is≠1,the butterfly deterioration will still occur when the axial stress is large enough or small enough.（6）Five factors（absolute stress parameters of roadway,environmental stress parameters,surrounding rock strength parameters,rock deformation parameters and roadway engineering parameters）were simulated by standard orthogonal test at 4levels and 25 groups.By using range analysis and variance analysis,the sensitivity of main control factors of roadway surrounding rock deformation and failure under given conditions was obtained.Its order is:surrounding rock strength parameter>environmental stress parameters>absolute stress parameter of roadway.Three main sensitivity factors were studied and three characteristics were obtained,i.e.,R_maxmutability of plastic zone of confining pressure ratio,R_max resistance of roadway plastic zone of rock strength and R_max strengthening of roadway plastic zone of buried depth stress.No matter what kind of rock there is a certain ratio of confining pressure which leads to the sharp butterfly failure in the plastic zone of the pore.Rock strength attribute can weaken the influence of confining pressure ratio but cannot prevent the surrounding rock butterfly.The increase of foundation stress caused by the buried depth of roadway will strengthen the influence of confining pressure ratio and promote the butterfly deterioration of surrounding rock.（7）The roadway reserved mode is redistributed by macro stress field during lateral mining,while the roadway excavation mode only changes the stress distribution in a small range under the premise of stable macro stress field.This results in the obvious difference of roadway surrounding rock damage in the case that the final stress distribution is basically the same.For the layout mode of reserved roadway,it is necessary to set large coal pillars only from the perspective of plastic failure of surrounding rock.However,for the complex stress environment of deep mining,the influence of other derived disasters such as rock burst should be fully considered.（8）Based on field investigation,theoretical analysis,data summary and numerical simulation,the suitable mining depth basis of the double lane retaining mode in the Inner Mongolia-Shaanxi mining area is obtained.Firstly,the limit of deep mining in Inner Mongolia-Shaanxi mining area should be satisfied.Secondly,the identification formula of butterfly instability and the starting depth formula of impact disaster should be satisfied.Finally,the suitable mining depth of roadway retention should be judged comprehensively with the control effect of conventional support of surrounding rock with different buried depths.（9）Combined with the west mining area of 5 coal mines with different geological conditions（Wanli No.1 Coal Mine,Talahao Coal Mine,Baode Coal Mine,Buertai Coal Mine,Menkeqing Coal Mine）in the Inner Mongolia-Shaanxi mining area,engineering verification was carried out.The following conclusions have been drawn based on the ore-pressure appearance and maintenance of retaining roadway with different buried depths of shallow,medium and deep.The stability of surrounding rock of reserved roadway is good in double roadway arrangement of shallow coal seam in Inner Mongolia-Shaanxi mining area.The surrounding rock of reserved roadway is still controllable for double roadway layout with medium buried depth,but its practicability is not good.When deep mining double roadway layout,the stability of reserved roadway tends to be unstable,so it is no longer suitable for double roadway layout mode.