Seismic Dynamic Analysis of Free Fields and Comparisons of Influence Factors in Underground Engineering

Seismic response analysis on underground engineering must consider the dynamic interaction between structures and foundation, in which the free field response analysis of soil strata is an essential issue to determine the seismic input. Studying free field responses of soil strata subjected to vertical compression waves combined with horizontal shear waves, it was found that compared with under the action of horizontal shear wave only, the composition of short period harmonic waves in the upper soil stratum was more abundant under the action of the horizontal shear wave combined with vertical compression wave, and the corresponding horizontal response of the earth’s surface in free fields increased. Therefore, it was necessary to consider the influence of the vertical compression wave on field responses to ensure the safety of underground engineering with high seismic safety level requirements. Moreover, the effects of groundwater level variation and loading intensity on dynamic responses of soil strata were also analyzed. The analysis showed that with the increase of earthquake strength, the stress state of soil strata changed, the amplification factor in the earth surface gradually increased along the vertical direction and decreased along the horizontal direction. As the fluctuation of groundwater level, the frequency response curve of the surface changed steadily, the drop of groundwater level induced the decrease of horizontal vibration and the increase of vertical vibration in the earth surface.