液化场地自由场体系的数值分析及振动台试验验证

NUMERICAL ANALYSIS OF FREE FIELD SYSTEM IN LIQUEFIABLE SITE AND VALIDATION OF SHAKING TABLE TESTS

应用有限差分法程序FLAC3D对液化场地自由场体系的地震响应进行三维完全非线性分析。与通常采用的等效线性法相比,完全非线性分析方法由于采用了弹塑性模型,可以自动计算永久变形;采用合理的塑性方程,使得塑性应变增量与应力相联系;结合孔隙水压力模型可以模拟地震作用引起的饱和砂土液化问题。计算模型考虑了土体非线性、孔隙水压力的变化以及模型边界条件的处理等。通过与液化场地自由场体系振动台试验结果的对比分析,方法的有效性得到验证。结果表明,砂土对地震动可起滤波和隔震作用;当加速度峰值到达时砂土层中的孔隙水压力比存在负值;震后土中孔隙水压力不一定随振动的停止而立即开始消散,短期内也可能继续增长。

3D fully nonlinear analysis of seismic response of free field in liquefiable site is carried out using the general finite difference program FLAC^3D.Compared with the equivalent-linear method,the permanent and irreversible displacements can be modeled automatically in the fully nonlinear method.A proper plastic formulation is used in the soil nonlinear constitutive relation,whereby plastic strain increments are related to stresses.Combined with the pore water pressure model,soil liquefaction under the seismic excitation can be modeled.Soil nonlinearity,pore water pressure build-up process and boundary condition of the model are taken into account in the calculation model.A comparison between the results from the finite difference analysis and those from the shaking table tests is made to validate the effectiveness of the computational model.Numerical results demonstrate that the liquefied soil can filter and isolate vibration.The phenomena of transient minus pore pressure occurr in the test.Pore water pressure does not always dissipate in short time immediately after the excitations,but it may keep on increasing.