Deformation Instability Mechanism And Supporting Technology Of Large-section Coal Roadway With Layered Roof In Zhaozhuang Coal Mine

The stability control of roadway roof is the basis of realizing safe and efficient production of coal mine.With the increase of mining scale and intensity,mine develop to large-scale,intensification and mechanization,the roadways with large section become inevitable trend in coal industry.For large section roadway in coal seam,because the strength of coal seam is low and roof has the typical layered characteristics,when roof bedding,fracture development,the thickness of stratum is small,degree of interlayer bonding is low and coal integrity is poor,the problem of roadway roof falling is serious,make the roof maintain of roadway becomes difficult.In this paper,the main roadway of Zhaozhuang Mine in coal seam is taken as the engineering background.Comprehensive use of field research,theoretical analysis,numerical simulation and field engineering tests and other methods,the deformation and failure law of weak bond,thin layer and multi-layer soft sandwich roof are systematically studied.The failure mechanism of the roof and the fracture reasons of cable bolt is revealed under the typical conditions.Based on the above research results,the multi-level roof control technology with long rock blot,short cable bolt,long cable bolt is proposed to ensure the continuity of roof rock beam and inhibit the damage of roof corner.The main conclusions are as follows:The typical deformation failure character of large section roadway is: roof sink seriously,roof falling frequently;coal rib falling easily,roadway section shrinkage large;cable bolt broken frequently,supporting structure broken seriously;high roadway repair rate and maintain cost;roadway floor heave.Large-scale coal roadway roof bedding,cracks development,low degree of inter-layer bonding,its poor stability,roof jumping,non-continuous destruction of the phenomenon more obvious,separation is observably,inner has multilayer shale interlayer or seam.Roof mudstone have a high clay mineral content,fragmentation and soften easily with water;coal rib is soft and the broken scope is large.Roadway roof has typical layer character,mechanic characteristics of rock mass is restricted by lithology and structural surface.Be basis on roof rock beam theory,according to the character of layer roof,consider the influence of bedding surface and soft rock strata,illuminate the form and development of layer roof separation;roof separation has three types: succession separation,divided separation and non-separation;and point out that use the reasonable supporting to prevent inter lamination shear action,can restrain roof separation and decrease roof deflection.According to the degree of development of roof bedding,the layered roof divided into two types,the gravity driven instability and delamination buckling instability mechanism of large section roadway with layered roof were revealed.Point out the layered roof separation and bend lead to shear stress concentration in roadway corner easily and emerge shearing zones,with the increase of roof bending,shearing zones scaling up to a certain angle until to the hard rocks,the load that cable bolt bears is increased and broken,the strength of supporting is not enough,induce roof falling.Coal rib as a carrier of roadway roof beam,the different broken degrees and ranges would provide different supporting ways.Reasonably increase the strength of coal rib,decrease the scope of plastic zone,provide a reliable support to roof,restrain the harmful transformation of roof,helpful to maintain the stability of coal rib.For large span roof and higher soft coal rib roadway should insist the principle of coordinated control,to ensure a longtime stability of surrounding rocks.Soft interlayer lead to the roof broken easily,and a big scope of separation;the distance of soft interlayer in roof is longer,the influence to the roof is smaller,but roof can wear layer destruction,when soft interlayer in the range of affection,interlayer would break unless the hard rock between them.The number of soft interlayer increased,the number of layered roofs cut into layers is larger,resistance capacity to deformation is lower,the scope of roof broken is increased,these increase the risk and height of roof falling.Crack water in roadway roof and air humidity seasonal cycle,aggravate the roof weathering,lead to the cable bolt plate empty,prestress loss,aggravate the roof sinking,cable bolt fractured,the strength of supporting decreased and causing roof falling.This type of roof must spray layer of concrete at once when exhuming,airtight rock and separate air and water,prevent rocks weathering,deliquescence spalling,decrease the strength of rocks.Cable bolt loss prestress and fractured lead to the working state changes,which is the major factor to roof falling,it is the key to make the mechanical property of supporting structure and large deformation of rocks matched,when control the large deformation roadway.The main failure mode of cable bolt is fracture,the location of cable broken mainly happens in the cable bolt plate orifices and range of 2m to roof,the main reason of fracture in the plate orifices is roof heterogeneous deformation and make plate overturn,lead to anchor bending and shearing.The fracture of anchor tensile test are cup-cone shape mostly,have necking obviously,fracture abreast basically,the location of fracture is concentrate,which is the typical ductile fracture;axial load capacity of rock bolt below cable bolt generally,but it has higher ductility,the ability to adapt large deformation is strong.The shear mechanical properties of different types of cable bolt and rock bolt were analyzed.The force of the cable bolt loaded in shear test is more complicated,the steel wire fracture is mainly oblique cut,and it contains the necked tensile fracture,the spiral twist fracture,and the tension-shear fracture.The displacement of the contact surface of the mortar block during the shearing when the cable bolt fractured is influenced by the type of the cable bolt,the prestress,the anchoring method and the strength of the mortar block.The shear stiffness of supporting system with the fully encapsulated cable bolt is better than that of no encapsulated cable bolt.Supporting system of rock bolt has higher initial shearing rigidity,make for increasing the shearing impedance of roof layer,restrain separation of roof.Using FLAC3 D software and modified pile elements simulating cable bolt(which can bear bending and apply prestress)analysis the interaction relationship between layered roof and cable bolt,different location of anchor has different stress state in roof;the more anchor closed to rib,layer shearing is bigger,axial tensile force is smaller;the more anchor closed to the central,axial tensile force is bigger,layer shearing is smaller;cable bolt fracture is the result of roof layer shearing diastrophism and vertical sink,the roof cable bolt closed to rib is shearing fracture and closed to central is stretch fracture mainly.Long time served anchor is effected by localized corrosion,generate stress corrosion and corrosion fatigue fracture easily.According to different broken reasons,relevant control measures were put forward.Analysis the main problems of roadway supporting at present: subjective assume is strong,not clear about roof instability mechanism,supporting measures pertinency is poor;inadequate understanding about roadway geological conditions and stress conditon,the supporting measure is same,lead to the area supporting strength inadequacy,roof falling has high safety risk and excessive waste of supporting material;inadequate attention to mine pressure monitoring,solve local distortion not in time;coordinate support theory of cable bolts and rock bolts lack deep and systematic study,cable bolts and rock bolt can not bear together;lack of the consideration of roadway service time,insufficient consideration of strength of rock degradation and supporting structure mechanical property degradation to influence of time.And put forward the control thought of large section roadway with layered roof.Analysis the effect law of anchor prestress,length of anchor to surrounding rock control effect,prestress has decisive effect to active supporting effect;the maximum bolting force of anchor exponential increase with the increase of length of anchor;increase anchor length reasonably is good for controlling separation of lower roof;increasing the length of cable bolt is not good for controlling central rock,properly reduce the length of cable bolt is good for prestress overlaying and spreading in rock,and increase the control effect.According to the roof falling risk and bearing capacity in different areas of large section coal roadway,divided into instable layer,metastable layer and stable layer,and as the corn according to the different area rock deformation behavior,put forward the multilevel supporting technology to ensure roof beam continuity and restrain roof corner fracture.Multilevel supporting technology is basis on the supporting system of combination of long and short,weak and strong,sparse and dense.Select the typical roadway,according to the engineering geological conditions to choose 3.2m long rock bolt,5.4m short cable bolt,7.4m long cable bolt to support roof,form successive prestress bearing structure and make plates,nuts and anchors matched with them,strengthen the control of coal rib,spray layer of concrete timely after supporting to prevent strength of rock degradation.Field monitoring shows the new supporting design realize the longtime stability of surrounding rock and obtain good supporting effect.