LAUSR

Abstract Rock bolts are widely employed as an effective and efficient reinforcement method in tunnel engineering. Owing to the difficulty in considering the rock breakage due to tunneling, and lack of the ability to reveal the microscopic mechanism of the interaction of bolt and rock mass properly by the conventional numerical methods, the reinforcement mechanism of bolts is not very clear. A new efficient hybrid DEM-Analytical model is proposed in this study for analysis of the deeply buried tunnels, where the DEM and analytical solutions are respectively employed to model the nearby and far fields of tunnel opening. The accuracy and applicability of the model are tested by comparing the hybrid model with analytical and full DEM model. The largest deviations of stresses and displacements for elastic problem are within 7–8% from the analytical solution, and the bond breakage regions obtained by hybrid model and full DEM model are about the same on average when the rock breakage is considered. The computational time of hybrid model is only about one-third of that of the full DEM model in this case. In addition, the variation of bolt axial forces obtained by the hybrid model is in good qualitative agreement with field measurements. The reinforcement mechanism of fully grouted passive rock bolting and the effect of supporting parameters are analyzed by the proposed hybrid model. Some useful numerical results are presented. It is found that rock bolts prevent the breakage of particle bonds immediately around them, and two transmission patterns of bond forces are observed in the nearby areas of rock bolts. Increasing the number of bolts contributes to significantly reduce the rock breakage region and bond breakage number. A threefold reduction of the number of broken bonds occurs passing from 4 to 12 rock bolts. The reinforcement of rock bolts is not obvious when the length is less than 0.5 r 0 , however, the increase of bolt length is not efficient in preventing rock breakage when the length exceeds 1.0 r 0 - 1.2 r 0 . Therefore, a reasonable efficient bolt length is suggested under the specific conditions of this study.