Asymmetric Failure Mechanism And Regulation System Of Gob-side Entry Roof With Fully-mechanized Caving Mining And A Loose And Weak Coal Pillar

In recent years,with the improvement of the fully-mechanized caving mining technology and the mining equipment enlargement and automation,the large-scale and intensive fully-mechanized caving mining has been the important development direction of thickness and super-thick coal seam mining with high productivity and effiency and safety in China.The large-scale fully-mechanized caving mining has facilitated the high-efficiency mining of thick coal seam resource,however,the matched gateroads have to encounter a series of surrounding rocks control difficulties such as large cross-section,intensive mining-induced influence,and soft and weakness coal roof.Especially for recent years,to respond to national appeal of building resource-saving coal mines,large cross-section gob-side entries with a narrow coal pillar are growing popularity.It was found in large numbers of field engineering practice that gob-side entry roof exhibits asymmetric subsidence failure in plumb direction and squeezing failure in horizontal direction,and in the conditions of large-scale fully-mechanized caving mining or narrow coal pillar,the asymmetry of the deoformation and failure tends to obvious,and even causes roof caving and support system failure.The results are roof strata instability and gateroad with lower security.The traditional failure mechanism,corresponding control theory and technique of gob-side enty roof with fully-mechanized mining caving cannot resolve this kind of surrounding rocks instability,thus it is of immediate necessarity and urgency to develop in-depth and systematical research pertinently.Based on the asymmetric difference of the disturbance degree of mining behaviors and the property and structure of surrounding rocks,the asymmetric strata behaviors of the coal-rock roof and support mass are studied,which correlations with influence factors are researched,and as a result,the importance rank of different factors and key factors are obtained.The instability principle and criterion of coal-rock roof mass are researched,and deviatoric stress field evolution law of coal-rock roof mass is studied,the asymmetric roof failure mechanism and control direction are clarified.The new cable truss structure which can effectively control the asymmetric roof deformation failure is designed,and its function principle to control the mining pressure is revealed.The asymmetric regulation relationships and index system about the roof and cable truss are studied,so the control system of new cable truss support to the gob-side entry roof with fully-mechanized caving mining is set up.The following conclusions are drawn:(1)Deformation and failure characteristics of gob-side entry roof were investigated in the field.The investigation results showed that the gob-side entry roof exhibits asymmetric strata behaviors by the centreline of the gateroads in plumb and horizontal direction.In the plumb direction,roof on coal pillar side occurs serious subsidenece and even roof caving in some regions,and there exist slide,dislocation,impaction and step convergence phenomenons between immediate roof and coal pillar;in the horizontal direction,there exists intensive horizontal movement and its caused obvious squeezing crush belt in roof strata,which contribute to the failure of W-type steel band,steel ladder beam and steel meshes.The deformation and failure took place in the roof and its roof corner on the coal pillar side and the middle and upper part of the coal pillar,which accounts for 25.8%,45.2%,and 22.6%,respectively.(2)The borehole observation method was adopted to capture the fractures development situation in the roof strata.The observation results indicated that the fractures in the roof on the virgin coal side are mainly transverse fissures,separation and dislocation developed in the shallow roof,while the fractures in the roof on the coal pillar side are mainly transverse fissures and separation developed in the shallow roof(0~3.0 m away from the roof surface),and the vertical fissures and its developed broken belts in the deep roof(4.9~12.3 m away from the roof surface).According to the length of borehole,angle of borehole and the range of the fractures zone,it was deduced that the break line of the main roof is about 5.496~6.847 m away from the gob edge.(3)To better understand the physical and mechanical properties of the rock mass surrounding the entry,laboratory tests were carried out on the coal and rock samples collected from the 20103 headgate.The 2# coal seam with a uniaxial compressive strength of 13.89 MPa,very weak coal,exhibites plastic characteristics.The immediate roof is stable sandy mudstone with a higher compressive,shear and tensile strength.The main roof is siltstone with a uniaxial compressive strength of 142.34 MPa.The immediate floor is mudstone with a uniaxial compressive strength of 44.64 MPa and has obvious water-swelling characteristics.(4)The structure characteristics of main roof above the gob-side entry and its correlations with roof asymmetric failure were studied by analytical method.The geometric size and break location of lateral key block were estimated,and thus the broken structure model of lateral key block was established,and then the expression of bending moment and deflection of the gob-side entry roof was resolved.The bending moment and deflection exhibit asymmetric behaviors by the centreline of the gateroads,and the maximum bending moment and deflection occur in the roof about 1.5m away from the coal pillar side.The roof strata in this zone should be paied more attention during support scheme design.(5)The numerical modelling software UDEC was implemented to study the relationships between gob-side entry roof movement and surrounding rocks stability,and quantitative relevance between break location of main roof and roof asymmetric strata behaviors was also researched.In the shallow range of 0~2.0m away from the roof surface,the subsidence of roof on the coal pillar side was obvious larger than that on the virgin coal side;in the deep range of 2.0~5.0m away from the roof surface,the roof subsidence increases linearly from virgin coal side to coal pillar side.In the shallow range of 0~1.5m away from the roof surface,squeezing movement from two side to central line occurred in the roof strata,and 0 horizontal displacement part moves 0.9m from entry central line to virgin coal side;in deep range of 1.5~6.0m away from the roof surface,squeezing movement from coal pillar side to virgin coal side occurred in the roof strata.With the distance between break line of main roof and entry central line decreasing or the subsidence of main roof increasing,more vertical loads are exerted to coal pillar,and thus its bearing capacity and supporting function to roof strata are obvious less than that of virgin coal rib,which aggravates the asymmetry of surrounding rocks structure and roof failure.(6)The numerical modelling software FLAC was employed to study the evolving process of distortion energy in roof strata at different stages of fully-mechanized mining caving.The modelling results indicated that the second deviatoric stress tensor invariants exhibits the bimodal distribution pattern in the shallow range of 0~3.5m away from the roof surface,while unimodal distribution pattern in deep range of 3.5~11.5m away from the roof surface.During the retreat period of current panel,due to the broken movement of the overlying strata,the bearing capacity of coal pillar decreases,resulting in its overlying strata storage capacity of distortion energy decreasing,which causes the peak value of the second deviatoric stress tensor to shift to the roof strata above the virgin coal.With the decreasing of coal pillar width or strength,the bearing capacity of coal pillar rib decreases with the compression deformation,resulting in the decreasing of the bearing capacity of its overlying strata and deviatoric stress moving to the roof strata above the virgin coal,and thus causing the asymmetric deformation of the roof strata on the coal pillar side in the plump and horizontal direction.(7)The main factors for the asymmetry of property and structure of surrounding rock and stress distribution are roation and sink of the main roof,soft and weak coal pillar rib,large cross-section and unreasonable support pattern.The evolution process of asymmetric faiure of gob-side entry roof can be described as follows: the retreat of neighbor panel causes main roof strata broken,rotation and sink movement and coal and rock masses surrounding the entry damage;the excavation action facilitates the asymmetric distribution of the property and structure of surrounding rocks and roof stress,resulting in the coal and rock masses on coal pillar side deformation and even broken,dislocation,compaction,and step convergence.Consequently,support structure suffers the nonuniform loads,and then rock masses on virgin coal side occur displacement and deformation,resulting in surrounding rock deformation and support damage on a large scale.During the retreat of current panel,the overlying strata structure is activated again,resulting in the asymmetric deformation and failure more serious.(8)Based on the asymmetric failure mechanism of gob-side entry roof,a regulation system for gob-side entry,with a core of asymmetric cable-beam structure,was developed.The regulation system consists of high-strength bolts,prestress cable truss,and asymmetric cable-beam structure.This system can not only control large-scale plastic failure and pose high shear-resistance,but also response to asymmetric failure of roof coal and rock mass positively and control it effectively.The asymmetric cable-beam structure linked by high-strength steel ladder beam and 16# steel channel is of pressure-bearing and load-shedding,span-reducing and tensile-resistence,asymmetric control,and adapt to horizontal movement.(9)In consideration of the geological and production conditions of 20103 headgate,support parameters design was carried out,and an optimal control idea was put forward.The detailed support step was represented as follows:(1)high-strength bolts was used for roof support to reduce the roof subsidence and ensure roof integrity and safety;(2)high-strength bolts was used for coal pillar ribs support to improve its bearing capacity and reduce coal pillar compression deformation and decrease the asymmetry of the roof failure.(3)Asymmetric cable-beam support was used to improve the bearing capacity of the roof on coal pillar side and restrain its asymmetric subsidence and horizontal squeezing failure.(4)Single cable was used to improve the stability of rock masses in the corner of roof strata to prevent its caving.(5)Pre-reforcement measures were used to support roof and coal pillar in the range of mining influence,and thus improve the stability of the entry.The field engineering practice of 20103 headgate demonstrated that with the effect of asymmetric cable-beam support system,the headgate was in a good working model without roof caving phenomenon,and the roof asymmetric failure was controlled effectively.And the cross-section can meet the requirements for ventilation,transportation and walking.