Research On Bearing Mechanism Of Coal-rock Combined Anchor Suppor

Coal-rock roadways are widely distributed in Guizhou Province.Due to the low mechanical strength of surrounding rock of such roadways and the large difference in the bearing capacity of coal and rock,as well as the existence of the structural effect of coal-rock interface,it is not uncommon to see the failure of bolts which cross the coalrock interface.This paper took the coal-rock composite anchor body as the research object,comprehensively adopted the research methods of field investigation,indoor pull-out test,theoretical analysis and numerical simulation,and carried out a systematic study around the influence mechanism of the anchor bearing performance of the coalrock composite anchor body in coal-rock roadway,and drew the following main conclusions:(1)Based on the field investigation and detection,the deformation of surrounding rock and the failure characteristics of the support body in the coal-rock roadway are analyzed,and it is concluded that the main characteristics of the deformation are the failure of the surrounding rock,the failure of the support body bolts in the coal-rock roadway,the asymmetric deformation of the two sides,the bulge at the coal-rock interface due to slip and dislocation,and the failure of the bolts crossing the coal-rock interface.The bearing capacity(ultimate anchoring force)of the bolt crossing the coalrock interface is slightly higher than that of the bolt in the coal body and lower than that of the bolt in the rock body,but the measured working load is greater than that of the bolt in the coal body and the rock body.Preliminary analysis shows that the shear action of the interface structure of the coal-rock interface and the different bearing capacity caused by the strength difference of the coal and rock are the main reasons for the failure of the bolt.(2)The load-bearing characteristics of the bolt on coal-rock composite anchor under different factors are revealed: under different factors,the pull-out loaddisplacement curve of the bolt on coal-rock composite anchor body can be divided into five stages which are Chemical debonding stage of anchoring agent,Development stage of fractures on coal and rock,Shear friction stage,Mechanical interlocking stage,friction stage of coal and rock.There is a critical pull-out rate,and the condition of low pull-out rate can greatly improve the bearing capacity of the anchorage system which crosses the coal-rock interface,but when the pull-out rate exceeds a certain threshold,the bearing capacity of the anchorage system will be reduced.The increase of coal body height can weaken the bearing capacity of coal-rock composite anchor.The increase of dip angle of the coal-rock interface can weaken the bearing capacity of the bolt on coalrock composite anchor,especially when the dip angle of the coal-rock interface is greater than 45°.(3)There is a phenomenon of time differential development of the destruction of pulling-out the bolt on the coal-rock composite anchor body.The destruction of rock lags behind the failure of coal.The pull-out rate has no obvious effect on the time differential development of the destruction between coal and rock.The increase of coal body height will lead to the reduction of the difference between the coordinated deformation and failure of coal and rock,thus shortening the time difference between the time differential development of the destruction of coal and rock.The increase of the dip angle of the coal-rock interface can reduce the time difference of the time differential development of the destruction of the coal-rock composite anchor.(4)Based on the pull-out mechanics model of coal-rock composite anchor,the maximum pull-out load formula was deduced.Based on the nonlinear shear slip model of the anchorage interface,the mechanical transfer model of the pull-out load of the bolt on coal-rock composite anchor is constructed,and the basic equation of the load transfer of the bolt in each layer of the coal-rock composite anchor body was derived.The pull-out load-displacement curve of the pull-out load of the bolt on coal-rock composite anchor under different conditions of the coal-rock height ratio can be calculated.By decomposing the stress field of the coal-rock composite anchor body element with the dip angle of the coal-rock interface,the final anchoring force formula of the bolt was deduced.(5)Based on the numerical simulation analysis,the time differential development of the destruction and debonding mechanism of the bolt on coal-rock composite anchor body under different factors was clarified: from the pull-out end to the anchor bottom gradually develop,and its failure and debonding process can be divided into three stages: the stage which "coal-rock anchor" joint action,the stage which is the "rock anchor" action and the stage which "coal rock anchor" residual action.(6)A technical path for optimizing support in coal-rock roadways was proposed,which focuses on "strengthening" support at the coal-rock interface and increasing the installation angle between the bolt which passing through the coal-rock interface and the coal-rock interface.Taking the 150913 roadway of a certain mine in Guizhou Province as an example,numerical simulation verification analysis was conducted on the support effect before and after optimization.The results showed that compared with the original support plan,the optimized support effect improved significantly.It is beneficial for controlling the deformation of the surrounding rock of this type of roadway.