Earthquake Mitigation Measures for Tunnels Considering Loosen-zone in Surrounding Rock

To study the effectiveness and mechanism of mitigation measures for tunnel structures considering loosen-zones under earthquake, a finite element analysis model considering both loosen-zone and mitigation measures was established. First, two kinds of mitigation measures, namely buffer layer and grouting, were set up, respectively. Then, a model considering the combined measures of buffer layer and grouting was also established. The impact on seismic responses of the tunnel structure considering these measures was analyzed. Results of the finite element model established were compared with results from a former shaking table test of a model with a buffer layer around the tunnel structure, and the effectiveness of the mitigation measures and the validity of the finite element model were verified. Furthermore, the finite element model was used to analyze stress, strain, plastic zone of the surrounding rock and damages of lining under the two kinds of mitigation measures including the buffer layer and the grouting. Results show that compared with the structure with no mitigation measures, the principal stress variation of the structure is small when the buffer layer is installed. At the same time, this buffer method not only increases the circumferential deformation of the whole lining, but also reduces the plastic strain of the surrounding rock. The tensile damage of the lining increases, too. By applying the grouting, the main stress response throughout the lining is reduced, and also the strain response of the lining is reduced. So that the adverse effect of the loosen zone on the tunnel deformation is weakened. Under the combined seismic mitigation measures with both buffer layers and grouting, the principal stresses and strains of the lining are significantly reduced, and the seismic performance of the lining is significantly improved. Therefore, the comprehensive mitigation measure combining the buffer layer and grouting reinforcement is more suitable for tunnels while considering the loose zone, which is also easy to be applied in project construction.