Structure Charactaristics And Control Technology Of Surrounding Rocks In Non-pillar Mining By Roof Cutting

China is the largest coal producer and consumer in the world,and coal production has a vital position in national economy.At present,in China’s primary energy consumption structure,coal consumption accounts for more than 60%.It is expected that in the next twenty or thirty years,coal will still occupy a dominant position in China’s energy system.For a long time,the coal production usually adopts the traditional long-wall mining technology that at least two entries should be excavated in advance and one section coal pillar must be retained between adjacent coal panels to protect the entry.Two theoretical systems of mine pressure,represented by Qian Minggao’s "masonry beam" theory and Song Zhenqi’s "transfer rock beam" theory,have been formed,which laid the theoretical foundation of longwall mining with section coal pillars in China and made indelible contributions to coal mining in China.However,coal is non-renewable energy and its reserves are extremely limited.With the continuous extraction of underground resources,the reserves of coal resources are gradually decreasing,and the waste of resources caused by this traditional mining technology is becoming increasingly prominent.Therefore,improving the recovery rate of coal resources is one of the important problems that need to be solved urgently in the field of coal mining in China.In 2009,with the introduction of the "cutting cantilever beam " theory,a new type of pillarless mining technology was formed,that is,the non-pillar mining by roof cutting(NMRC).This technology can reduce the surrounding rock pressure of the roadway by initiatively releasing the pressure on the roof through pre-splitting slits.When the coal is mined out,an entry can be formed automatically.Through the corresponding technical means to maintain the roadway,it can be used for the next working face,thus realizing nining without coal pillars.It is not only in line with the important development direction of sustainable exploitation of coal resources,but also an important guarantee for solving coal mine gas and power disasters,reducing roadway excavation rate and realizing scientific mining.In this paper,based on the principle of the NMRC,the stress evolution laws,the structure characteristics and the stability principle of surrounding rocks for NMRC are systematically studied by means of theoretical analysis,numerical simulation analysis and field test.As research results,some key technologies for controlling the stability of surrounding rocks are proposed,and a control system for surrounding rocks is preliminarily formed.The main research contents are listed as follows.(1)The stress field characteristics of surrounding rocks in the stages of roadway excavation,roof pre-splitting,primary mining and secondary mining are studied by means of numerical simulation,and the stress distribution characteristics and evolution laws of surrounding rocks in the whole mining process are obtained.The results show that the surrounding rock stress distribution is changed by the roof pre-splitting.When the roof is cut off,almost all the direct roof strata are in the state of pressure relief.(2)An evolution model for stress arch in overlying strata is put forward based on the movement characteristics of overburden rocks in the stope.It is considered that the rock masses constituting the stress arch are in the state of stress concentration,which bear and transfer the rock loads above the arch,while the rock strata in the fracture zone below the stress arch are in the state of pressure relief as a whole.Meanwhile,the morphological character of the stress arch is studied,and the form equation of the arch is derived by establishing the mechanical model of three-hinged arch.The results show that under certain burial depth and goaf size,the shape of the stress arch is only related to the height of the fracture zone,that is,the higher the height of the fault zone,the thinner and slimmer the stress arch is,on the contrary,the short and fat it is.In addition,based on the form equation of the stress arch,the loading range of the roof of the gob-side entry is obtained,and the magnitude and scope of the load increment causing the supporting pressure of coal side are defined.Furthermore,the distribution function and distribution width of abutment pressure in coal seam are derived.(3)Through analyzing the movement characteristics of surrounding rocks for the NMRC,it is proposed that the surrounding rocks can be divided into three kinds of structures,namely the "stress arch structure",the "composite bearing structure" and the"roof-cutting and pressure-relief structure".Then,the stability principle of the different structures are analyzed.It is concluded that the "stress arch structure" is a self-balanced structure existing in overburden strata of the stope,while the "combined bearing structure" and the "roof-cutting and pressure-relief structure" are both located in the stress reduction zone below the "stress arch structure".The "composite bearing structure" maintains its self-stability mainly through the "masonry beam structure"formed by the key blocks of the basic roof,while the "roof-cutting and pressure-relief structure" maintains its stability mainly by artificial support in the roadway.(4)Based on the characteristics of the "composite bearing structure",two main instability modes of the "composite bearing body" which constitute the "composite bearing structure" are put forward,namely the "compression-shear" instability mode and the "shear-shear" instability mode.The instability conditions of the two modes are obtained by establishing mechanical models.The results show that for a specific mining project,the approaching instability degree of the "composite bearing body" can be determined by the parallel and rotational subsidence of the roadway roof.Based on this,the instability criterion of the "composite bearing structure" is put forward.(5)After analyzing the stress characteristics of the "roof-cutting and pressure-relief structure",a mechanical model of the "cutting short-arm beam" structure,which is composed of the "roof-cutting and pressure-relief structure",is established.Meanwhile,the deformation and stress characteristics of the "cutting short-arm beam" are analyzed,and the mechanical conditions to maintain the stability of this special beam are obtained.At the same time,an mechanical model for gangues in the goaf is established based on the theory of retaining wall,and the lateral pressure of the gangue preventing structure is studied.The study shows that the lateral pressure is not large as a whole because the gob-side entry is located in the pressure relief zone,and the main pressure is from the fault zone.(6)Based on the mechanical model of the roof strata,the mechanism of active pressure relief for roof pre-spliting is analyzed,the internal relationship between the support and the self-supporting capacity of surrounding rocks is clarified,and the formation mechanism of the co-bearing system composed of supporting and surrounding rocks is revealed.The results show that after the pre-spliting of the roof,the fractured rock in the goaf will lose contact with the residual boundary.On the one hand,the vertical shear force will not be transmitted to the roof of the roadway,and on the other hand,the ultimate bending moment will be weakened.The final result is that the loading that constitutes the loading-bearing balance system is greatly decreased,thus reducing the surrounding rock pressure of the roadway.On this basis,key parameters of roof pre-splitting are studied through theoretical model,and some critical technologies of roof pre-splitting are summarized.(7)According to the movement characteristics of roof in the process of roadway formation,the generation mechanism of dynamic load on surrounding rocks is studied.The high constant resistance and absorbable pressure anchorage technology,and the auxiliary support technique against dynamic load are put forward according to the previous research.It is proposed that the surrounding rock support of the roadway be divided into three zones,i.e.the advance influence supporting zone,the dynamic pressure auxiliary bearing zone and the roadway stability zone.(8)After analyzing the movement characteristics of gangues in the goaf,it is proposed that the movement process of gangues can be divided into two main stages:the fast collapse stage and the slow compaction stage.The mechanical characteristics of the gangue retaining structures in different stages are analyzed,and the interaction mechanism between the gangues and the retaining structures is clarified.Based on these studies,a control ideal,combining static and dynamic in the horizontal direction and extensible at the vertical direction to relief pressure,was put forward for the retaining structures.As a result,a gangue control system has been formed,and three types of gangue retaining structures are proposed for difrerent mining heights.Field tests are carried out for different structures,and the results show that good control effects are achieved.In view of the above research contents and results,an industrial test for NMRC is carried out in Ruineng coal mine.The test results show that the effect of roadway formation is well,and great economic and social benefits have been achieved,which provides an important reference for the popularization and application of this new technology.