Discontinuous Deformation Analysis On Stability Of Jointed Rockmass Based On Pre-stressed Rockbolt Model

Rockmass in deep engineering can be regarded as block system composed of many joint surfaces and its cutting rockmass. An unloading process related to underground excavations could give rise to structurally controlled instability due to the falling, or sliding, of rock blocks from the excavation perimeter. In these conditions, the design of the underground excavation should be able to define stabilization measures that prevent both major and local rock block instabilities.The main research objects in this paper were jointed rockmass and rockbolt. For the rockbolt, it was assumed that the reinforcement affected on joint surfaces only and the influence inside the blocks was neglected. Based on this assumption, the rockbolt reinforcement model in jointed rockmass was established, and verified through an experiment, then a numerical model compiled by FISH code in 3DEC was proposed; For the jointed rockmass, it can be regarded as deformable block which was explained by the block theory. The criterion of instability of surrounding rockmass was put forward which synthesised the structural instability and strength instability of rockmass, and the safety factor which could express the safety of surrounding rock was calculated.First of all, in order to predict the contribution of rockbolts to the shear strength of a rock joint, a new calculation model is presented in this paper based on the analysis of the interaction between rock and rockbolts in an underground excavation where potentially sliding rock blocks are present. The main characteristics of this model are the accounts for the interaction of the axial and the shear forces mobilised in the rockbolt, as well as the large plastic displacements of the rockbolt occurring during the loading process. The assumed interaction scheme is activated by a small displacement, while this displacement does not induce a destressing of the rock block. According to the state of rock in the borehole interacting with unilateral bolt, such a model introduces a variable which divides the active length, the short length of reinforcement which spans the discontinuity and changes orientation during shear displacement, into the elastic deformation zone and the failure zone. Based on analysing the failure mode of rockbolts, the maximum rockbolt contribution is obtained, meanwhile, the complete curve of the rockbolt contribution as a function of the displacement along the joint can be computed.The shear tests of bolted jointed rockmass were carried on in the laboratory to further study the mechanical behaviors of rockbolt in the jointed rockmass, and to verify the rockbolt model. In the experiment, rockmasses of different strength, different anchor mode and different prestress were simulated, meanwhile, the deformation characteristic of rockbolts was monitored to compare the shear deformation regularity before and after bolting and to analysis the influence of pretension, rock strength, the mode of anchorage on shear capacity of jointed rockmass. In the process of test, the strain of axial force bolt was monitored through the strain gauge in order to study the axial deformation mechanism and characteristic of rockbolt. After shearing test, the failure rockbolt was taken out to statistic the relationship between the deformation length and other parameters.The rockbolt model compiled by the fish language in the three-dimensional discrete element software-3DEC was estalished and was embedded into 3DEC successfully. In order to verify the model, a simple shear test was simulated with 3DEC, whose result shows that the model can achieved satisfactory effect on simulating the prestressed, end-anchored, and full length anchoring rockbolt. At last, the sensitivity analysis and acting law of the shearing parameters was discussed.Secondly, taking jointed rockmass as the research object, the stability methods of block theory and strength criterion were considered comprehensive based on the judging method of keyblocks, and the instability criterion of surrounding rockmass was put forward:Instability of block sliding along the joint surface in surrounding rock occurred prior to all others; when the block was determined to be stable, shear failure in block may happen, and the rockmass should be checked with strength criterion.According to the characteristics of underground engineering, joint network was generated using Monte-Carlo method. Then, taking the influence of in-situ stress into consideration the interaction between blocks was analyzed using vector method to find the keyblocks, and its blocky safety factor, the designed anchor prestress value and the safety factor expressing the state of surrounding rock were calculated thougth the procedures compiled by FISH code for the safety evaluation of the project in deep underground.Finally, taking the fault fracture zone in deep roadway of GuQiao coal mine as the research object, a large-scale underground engineering numerical model was established, then the stability of bolting reinforcement roadway engineering was evaluated, and in this foundation, the rock bolting reinforcing scheme was optimized adjustment, and the numerical simulation results showed that the support could meet requierment.