地下工程中自由场地的动力分析及影响因素对比

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

地下结构地震反应分析需要考虑结构-地基的动力相互作用,其中场地土层的自由场反应分析是确定地震输入的关键。通过对竖向纵波与水平向横波联合作用下土层自由场反应的研究,发现与仅考虑水平向横波相比,联合考虑水平与竖向输入波时地表土体的短周期谐波含量更丰富,地表水平向响应增大。因此,对于抗震安全等级要求较高的复杂地下结构,其地震输入应考虑竖向纵波的影响。此外,分析了水位变化及加载强度对土层动力反应的影响。分析显示:随输入地震波强度的增大,土层的应力状态改变,地表放大系数沿竖向逐渐增大,而在水平向逐步降低;由于水位变化,地表振动的频率响应曲线发生了改变,水位下降引起地表水平向振动减小,而竖向地震响应增大。

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.