ANALYTICAL APPROACH TO THE VISCOELASTO-PLASTIC MECHANICAL RESPONSE OF DEEP SOFT ROCK TUNNEL CONSIDRING THE ROCKBOLT REINFORCEMENT EFFECT

The surrounding rock of deep soft rock tunnel shows significant plastic softening and dilatancy characteristics. The ignorance of the coupling influences of these two characteristics in current studies leads to the inaccurate prediction results of tunnel deformation. In order to solve the problem, a viscoelasto-plastic calculation model of deep soft rock tunnel is established, based on the Kelvin-Voigt model and Mohr-Coulomb strength criterion. In this model, the plastic softening and dilatancy of surrounding rock are considered and the restriction effect of tunnel face is introduced as well. Furthermore, a mechanical model of rockbolt-reinforced surrounding rock is established by using the equivalent stiffness method, which takes the rockbolt reinforcement into account. Considering the relationship between plastic radius of surrounding rock and bolt length, analytical solutions for tunnel responses under the six conditions are provided. In addition, the reliability and effectiveness of the theoretical derivation are well validated by comparing numerical and analytical results. Finally, the reinforcement effect of rockbolt is discussed based on the analytical solution and a comprehensive parametric investigation is carried out, including the installation time and stiffness of rockbolt and tunnel excavation rate. Results indicate that without considering the rockbolt reinforcement effect, the excavation rate only poses the influence on the development law of early deformation of tunnel, and its influence on the final tunnel displacement can be ignored. The tunnel deformation will be greatly underestimated without considering the plastic deformation of surrounding rock, resulting in a large gap between the predicted and actual results. When the excavation rate is large, the rockbolt should be installed as earlier as possible, to ensure that the rockbolt can play an effective role inmiting surrounding rock deformation. There exists a sub-linear relationship between bolt stiffness and tunnel displacement, and the increase of rockbolt stiffness can prolong the time required for surrounding rock entering plastic deformation. The results of this paper can provide useful guidance for the similar projects.