脉冲地震动中竖向加速度对场地液化的影响

EFFECT OF VERTICAL ACCELERATION ON SITE LIQUEFACTION IN PULSE GROUND MOTION

为研究近断层脉冲地震动中竖向加速度对砂土场地液化的影响,基于有限元平台Open Sees开发的边界面塑性本构模型,建立了动单剪单元试验模型和饱和砂土三维有限元模型。选取台湾Chi-Chi地震中10条具有速度脉冲特性的地震波,对比分析了水平双向脉冲波与三向脉冲波作用下土柱竖向位移、循环应力比、孔压比及等效循环周数的差异性,继而明确了脉冲地震动中竖向加速度对砂土液化的影响规律。研究表明,三向脉冲地震波中竖向加速度分量对场地永久位移值影响较小,但使永久位移的发展持时明显增大;土柱循环应力比受竖向地震动影响较小,因此分析脉冲地震动对场地剪切特性的影响时,可将三向脉冲地震动简化为水平双向地震动;考虑竖向地震动的三向脉冲地震波引起的孔压比变化幅度较大,孔压消散时间较长;三向脉冲地震波对应的等效循环周数较大,地震动发展持时长,可认为竖向加速度对场地液化有促进作用。

This paper is based on the boundary surface plastic constitutive model developed with finite element platform OpenSees.It established a dynamic simple shear test model and a three-dimensional finite element model of saturated sand.The target was to explore the effect of vertical component of pulse-like ground motion on the site liquefaction.Therefore,10 sets of seismic waves with pulse-like characteristics were selected from the Chi-Chi earthquake in Taiwan as inputting load.The simulated vertical displacement,cyclic stress ratio(CSR),pore pressure ratio and equivalent cycle number of the soil column were compared and analyzed under the ground motion with or without the vertical component.Hence the differences clearly defined the effect of vertical acceleration of pulse-like ground motion on the liquefaction of sand.The results show that the vertical acceleration component of tri-directional pulse-like seismic wave has little influence on the final site settlement but significantly increased the duration.As the CSR of soil column is less affected by vertical ground motion,the tri-directional ground motion can be simplified as horizontal bi￣directional ground motion when analyzing the shearing properties of the site.The divergence of pore pressure ratio caused by the tri-directional pulse-like seismic wave is larger,and meanwhile,it spends more time to dissipate the pore pressure.Besides,tri-directional pulse-like seismic waves generate a larger equivalent cycle number and increase the duration of ground motion.As a result,the vertical acceleration have more tendency to induce site liquefaction.