# Experimental Assessment of the Effect of Vertical Earthquake Motion on Underground Metro Station
## 1 Introduction
Numerical results show that under vertical seismic excitation the tunnel experienced greater vertical stress from time to time and vertical compressive deformation, and sometimes it completely separated from the above soil layer. The effect of the vertical shaking during an earthquake has been experimentally investigated previously through just a few shaking table tests. In terms of the experimental study on the tunnels, [PERSON] et al. [18] investigated the mechanism and effect of the seismic measures of the mountain tunnel through the three-dimensional shaking table tests, where the peak ground acceleration (PGA) of the vertical input motion was taken to be 2/3 of the value corresponding to the horizontal input motion. Experimental results show that with 63% probability of exceedance in 50 years, including vertical components of the earthquakes increased the peak dynamic soil pressures on the tunnel sidewall, while the opposite was true for 1% probability of exceedance in 50 years. In both cases there was no significant change in the peak dynamic strain of the inverted arch on two sides of the flexible joints. [PERSON] et al. [19] also adopted the above-mentioned definition of the horizontal and vertical strategies and investigated experimentally two shallow-bias tunnels with a small clear distance subjected to the horizontal-vertical earthquake excitations with different levels. Results show that the acceleration amplification factors of the vertical-direction were generally larger (1.02-3.94 times) that that of the horizontal-direction and the earthquake intensity presented different influence on the acceleration responses in the horizontal and vertical directions. In addition, [PERSON] et al. [20] investigated experimentally the dynamic responses of a tunnel subjected to near-fault pulse-like earthquake motions, where both vertical excitations and transverse-longitudinal-vertical excitations were included. With respect to the underground metro stations, for example, [PERSON] et al. [21] studied the vibration behavior of a framed metro station under horizontal and horizontal-vertical seismic excitation using a shaking table. Experimental results show that for the lower shaking intensity the structural relative displacements under the horizontal seismic excitation were lower than those under the horizontal-vertical seismic excitation, while the opposite was true for the greater shaking intensity. It is believed that in the latter case, the vertical seismic excitation increased soil densification and ground subsidence, which in turn imposed greater constraints on the station. [PERSON] et al. [22] investigated the effect of the vertical ground motion on the dynamic response characteristics of the central columns in a six-story metro station and the test results revealed that with the increasing ratio of the vertical/horizontal acceleration amplitude, the central columns would undertake much more vertical dynamic axial forces compared to side walls. [PERSON] et al. [23] experimentally investigated the dynamic behaviors of a single-story subway station embedded in the dry sand excited by the vertical sinusoidal and vertical random waves, respectively. They found that in both cases there was difference in the acceleration amplitude of the ground between the free-field model and soil-station model. Some attempts were also made to explore the relationship between seismic soil pressures and shear strains of the surrounding ground based on the regression analysis of the experimental data. Numerical and analytical studies of the effect of RVH on underground tunnels dominate most of the previous work. Extremely limited studies have been dedicated to study the influence of the RVH or incident angle of earthquake waves on the seismic responses of the underground metro station, especially for the experimental studies.