Investigation of the dynamic response of rock slopes at the entrance and exit section of a tunnel based on modal analysis

Abstract

In the construction and operation of tunnel engineering in high intensity areas, the slope at a tunnel’s entrance and exit section has become a section prone to problems because of seismic load and tunnel excavation disturbance. To study the dynamic response characteristics of rock slopes at the tunnel entrance and exit section, considering the strong earthquake area in western China as the research area, a three-dimensional finite element (FE) model was established and the FE modal analysis of the slope at the tunnel entrance and exit section was conducted. The dynamic response characteristics of the slope body were investigated based on a frequency domain by analyzing the natural frequencies and corresponding vibration modes of different models. The results show that the tunnel excavation affects the slope’s dynamic response characteristics and natural frequency but has little effect on its vibration mode. The natural frequency ratio of slopes without and with a tunnel is 1.1:1.35 as a whole, under the same conditions. The slope gradient has a considerable effect on the natural frequency of the slope at the tunnel entrance and exit section. The natural frequency of the slope gradually increases with the gradient, and the changing trend is more obvious with an increase in the vibration mode order. The low-order vibration mode of the slope at the entrance of the tunnel is relatively simple, primarily demonstrating bending and torsional deformation. The vibration mode of the slopes with and without a tunnel is the same when the slope gradient is the same. The vibration modes of the two tunnel slope models with different gradients are also the same. This study investigated the dynamic response characteristics of the slope and tunnel structure based on the analysis of the inherent characteristics of the slopes at the tunnel entrance and exit section, which can provide a reference for the seismic design of the tunnel entrance section.