Study On In-situ Pullout Test And Equivalent Pretensioned Stress Mechanism Of Rib Bolt

Coal bolt supporting technology is far behind the roof supporting problem,based on the field test,theoretical analysis and numerical simulation,the in-situ drawing experiment of coal bolts,the evolution of mechanical properties of coal anchor system and the equivalent preload and control of coal bolts are studied,and the engineering verification has been carried out.The main conclusions are as follows:(1)The transformation of the drawing load of the bolt can be divided into four stages: linear elasticity rising stage,nonlinear rising stage,constant resistance stage and fall failure stage.Accordingly,the damaging process of the can be divided into three stages: elasticity stage,shear failure stage and slipping stage.The loss of anchorage force and incremental dynamic equilibrium during shear failure are the cause of constant resistance.(2)Compared with the soft coal condition,under the hard coal condition in Youzhong coal mine,the P-S curve trend is more turbulent,and the load shock is obvious while the resistance rate is significantly reduced.In this condition anchorage force shock caused by coal fissures become more obvious and the anchoring section show the fragments bearing failure.(3)Based on the trilinear shear-slip constitutive model,the anchored state has been divided into three stages: elastic stage,plastic slip stage and debonding stage.Moreover and the analytic solution of the distribution of anchors force in elastic state and plastic state has been deduced.With the increase of shear characteristic value 1 1?u,the transfer performance of bolt axial force and shear stress gradually decrease,and the increase of the anchor hole diameter will lead to the transitivity decline of the bolt axial force and shear stress.(4)Shear slip failure is more likely to happen in the interface of coal and rock mass,which may cause the net pocket damage in the upper and lower corner of the rib.The internal shear failure of coal mass is the essential cause of its large bulging.Bolt supporting applies preload to coal mass which transfer its stress state from the two direction to the three direction improving the mechanical properties of coal mass.And bolt supporting will also make the Mohr stress circle away from the strength envelope reducing the width of the plastic zone and improving the stability of the rib.The diffusion effect of bolt pretightening force is closely related to the anchor length,the array space between bolts and the pretightening force value,and the key is to form the effective compressive stress connection area between the anchor bolts(5)The optimized supporting scheme of roadway has been proposed and implemented based on Youzhong coal mine condition.After the installation of the bolt,the supporting system maintained a good working resistance,working resistance had remained stable around 30 k N.From the observation and monitoring of mine pressure,the cumulative displacement of the roof and floor of the roadway was 23 mm,and the cumulative displacement of the two coalside was 53.5mm,which could meet the actual needs of production.