Effects of strength and weight of pre-ground improvement on seismic behavior of shallow overburden tunnel.

• Effect of strength of improved ground differs in Level 1 and Level 2 earthquakes. • Expansion of plasticized improved ground affects seismic tunnel behavior in Level 2 earthquake. • Optimum improvement in strength differs depending on ground-improvement pattern. • Weight of improved ground has only small effect on seismic tunnel behavior. Pre-ground improvement is an auxiliary method for shallow overburden tunnels in which the tunnel excavation is performed after improving the ground on top of the tunnel with/without sides by the replacement method and/or the cement mixing method. Although it has been confirmed in previous studies that pre-ground improvement enhances the tunnel face stability and reduces the settlement of the ground surface during the tunnel excavation, the seismic behavior of tunnels with pre-ground improvement has not been fully investigated. Therefore, the authors conducted a shaking table experiment under the centrifugal acceleration of 50 g in order to understand the effect of the ground-improvement pattern on the seismic behavior of a shallow overburden tunnel. Furthermore, the authors performed a two-dimensional elasto-plastic finite element analysis of the centrifuge model experiment to verify the applicability of the numerical method and to examine the influence of the stress release associated with the tunnel excavation, which cannot be considered in the centrifuge model experiment. In this study, the effects of the strength and weight of the improved ground on the seismic behavior of a shallow overburden tunnel are investigated using the same numerical method. The results confirm that, for a Level 1 earthquake, the increments in bending moment are almost the same regardless of the strength of the improved ground in both ground-improvement patterns. On the other hand, for a Level 2 earthquake, the seismic behavior of a tunnel has different tendencies depending on the ground-improvement pattern because of the expansion of the plastic area in the improved ground and the difference in stiffness between the improved ground and the surrounding ground. Therefore, the optimum strength of the improved ground differs depending on the ground-improvement pattern. It is also found that, although the translational motion of the tunnel with the improved ground is slightly greater when the weight of the improved ground is larger, the increase in weight due to the ground improvement has only a small effect on the seismic behavior of the tunnel because the behavior of the tunnel during an earthquake is dominated by the surrounding ground. [ABSTRACT FROM AUTHOR]