Shaking table test for near-valley subway station - Part I: Seismic site effects

Abstract

As a typical local irregular topography, valley would have a significant influence on the seismic distribution around the local site as well as on seismic responses of near-valley underground structures, which has been evidenced by ground motion observation and field seismic records. However, current knowledge on the seismic site effects of the valley is limited to analytical solutions or numerical methods, and lack of experimental data from physical models. In this paper, a physical model test method based on 1g shaking table for arc-shaped valley topography is proposed, and a series of shaking table tests are carried out to investigate the seismic site effects induced by the valley topography. Details of experimental setup include not only the dynamic similitude design, the laminar model container and the model soil, but also the design and fabrication of arc-shaped valley topography with three kinds of depth-to-width ratios, i.e. 1/3, 1/4, and 1/8, as well as procedure for simulation of seismic excitations considering harmonic waves and actual seismic records. The free-field test case without valley site is also involved and taken as a benchmark comparison to better understand the seismic site effects of valley terrain. Acceleration sensors are placed and embedded in the model soil with different buried depths and distances to the local valley site in order to record the overall free-field seismic responses as well as the local topography effects. Results show that the dynamic response on the surface of model soil exhibits strong spatial variability due to the impact of the local valley topography. Results also indicate the ground acceleration increases within the local site, and the amplification effect is most pronounced at the valley top. The influence of the valley on the acceleration response of the ground is more significant when the predominant frequency of the seismic motions is higher and closer to the natural frequency of the site under different seismic motions. Furthermore, when the depth-to-width ratio of the valley increases, the acceleration amplitude at the valley top gradually increases, and the predominant frequency of Fourier spectrum at the valley top is also amplified. Notably, the Arias intensity amplification factor of the surrounding strata increases significantly due to the local valley topography, and the Arias intensity is also increased with the depth-to-width ratio of the valley.