Quantitative model of full section anchoring in thick soft rock tunnel

The pressure arching effect of rock bolt on weakly cemented soft rock should be strengthened in China west- ern mining area instead of suspending action,so it is better to increase the supporting density rather than increase the length of rock bolt. Based on the idea above,a mechanical model of full section anchoring in thick weakly cemented soft rock tunnel was established. Considering the stress release of surrounding rock,the model was decomposed into two sub-models by employing the superposition principle:one is the initial state model before anchoring,and the other is the enhance model after anchoring. Then,the analytical solutions of mechanical responses of surrounding rock in the two models were derived based on Lame’s solution and bolt model,and the solutions of the original model were estab- lished. The analytical results show that the radial stress in anchorage zone is greatly improved,which is beneficial to construct the load-bearing arch. However,the tangential stress remains stable. In the non-anchorage zone,the stress so- lutions are the same with Lame’s solution without rock bolt. Displacement in surrounding rock decreases significantly especially in the anchorage zone. According to the above conclusions,three quantitative indexes to quantify the anchoring effect were defined by the relative variation of responses in surrounding rock before and after bolting,and then the effect of elastic modulus of rock mass and pre-stress in rock bolt were discussed in detail. Results show that the me- chanical responses in soft rock are changed more significantly than those in hard rock. Finally,the functional relation- ship between the support parameter and elastic-plastic state of surrounding rock was determined and the critical sup- port values to maintain the rock mass with different strengths in elastic stage were calculated.