Research On The Method To Determine The Reasonable Pre-stress Of The Pre-stressed Rock Bolt

Rock bolt supporting as a kind of effective reinforcement way already widely used in geotechnical and mining engineering. The rock bolt that was applied pre-tension load by pallet on the one hand could actively restrain early deformation of roadway and reduce the magnitude of strength decline, on the other hand improve the bolt's capacity to constrain the surrounding rock and improve the anchoring strength. The reasonable pre-stress could increase the rock's self-supporting capacity and ensure the whole system of bolts give full play to support efficiency. Therefore the determination of the reasonable pre-stress is greatly significant for anchor design in projects domain. Based on the deformation of surrounding rock, combined with the elastic theory and visco-elastic theory, the elastic and visco-elastic model of the interaction of surrounding rock and bolt was set up, the pre-stressed rock bolt's stress distribution function was carried out and the method to determine the reasonable pre-stress of rock bolt was putted forward. Results obtained are as follow:(1) The main function of pre-stressed rock bolt support is to provide high pre-stressed early for surround rock, which can limits the further distortion of surrounding rock. Pre-stressed rock bolt support can form a combined beam structure, reduce roof tensile stress area and roof distortion, ameliorate surrounding rock stress distribution and enhance the stability of entry.(2) On the assumption of the relation is a linear function between the small section of bolt and the country rock mass, the bolt's load transfer differential equation was established according to the stress equilibrium in a small section of a bolt and the rock mass deformation before and after bolt assembling under the interaction between country rock mass and bolt. And combining the radial deformation of roadway rock mass, the function of the shear stress and axial force were established with the elastic and visco-elastic rock on the base of the pallet. And then the main factors which influence the distribution of bolt stress were analyzed exactly.(3) Based on the deformation of surrounding rock, combining with the mechanics of the pull-out test, the pre-stressed rock bolt's shear stress distribution function and axial force function under the pre-stress was carried out and the method to determine the reasonable pre-stress of rock bolt was putted forward. And combining a example to determine the size of the reasonable pre-stress. Though analyzing the distribution of bolt stress, it was gotten that the reasonable pre-stress could improve the bolt's stress state, decrease the bolt's stress concentration, give full play of the bolt's support performance and improve the anchorage strength.(4) The research results on the above were applied in the bolt support design of Dongjiahe mine coal. The size of the reasonable pre-stress was determined by calculating, and then the field trial was done. By comparing the supporting effect in the testing roadway under the condition of the different pre-stress, it was obtained that the deformation and separation of roadway roof was decreased significantly. It was demonstrated that the pre-stress designed was reasonable.