可液化场地碎石桩复合地基地震动力响应分析

采用砂土液化大变形弹塑性本构模型分析可液化砂土，采用模量随应力与应变变化的等效非线性模型增量形式分析碎石桩，应用FLAC3D有限差分软件对地震动力作用下可液化场地碎石桩复合地基进行三维动力响应分析。模拟分析了在地震作用下碎石桩刚度效应和排水效应对加固处理可液化场地的抗液化效果，从初始小变形到液化后大变形的变形发展，超静孔压累积与消散，及桩与土的变形与应力分配变化等。结果表明，所用模型与方法可合理描述可液化场地碎石桩复合地基在地震作用下场地的动力响应特性和抗液化效果；在地震作用下可液化场地中桩周土体与碎石桩体的竖向应力与水平向剪切应力向碎石桩体集中，竖向有效应力比可降至约1/6～1/3；桩周土体与桩体为非协调变形，剪应变比可达7～10；碎石桩抗液化影响范围约为2.5～3倍桩径，对超过3.5倍桩径范围影响较小；碎石桩与砂土渗透系数比大于100时对降低砂土中超静孔隙水压影响明显；碎石桩对场地的加密效应可显著降低超静孔隙水压力，而碎石桩刚度则对超静孔隙水压力变动影响较小，但有助于减低地面加速度响应峰值。

Analysis on the seismic response of stone columns composite foundation in liquefiable soils

With the application of a plasticity model for large post-liquefaction deformation of sand to model the liquefiable soil and an equivalent nonlinear incremental model to model stone columns (SC), three-dimensional dynamic responses of stone columns composite foundation in liquefiable soil are numerically investigated using finite difference code FLAC3D. The analysis investigates the effect of the SC’s high stiffness and improved drainage on soil liquefaction mitigation, the excess pore water pressure (EPWP) build-up and dissipation, the deformation process of the liquefiable soil from small to large deformation in the pre- and post-liquefaction regimes, and the variation of stress distribution between SC and surrounding soils. The results show that the model and the program can reasonably reproduce the seismic response of stone columns composite foundation in liquefiable soils and its effect of liquefaction mitigation. The vertical stress and horizontal shear stress gradually concentrate to SC during earthquake shaking and vertical effective stress ratio may decrease to 1/6-1/3, the deformation in soil and SC is incompatible and the ratio of shear strain in the soil and SC may reach 7-10. A ratio of SC permeability to soil permeability larger than 100 significantly decreases the EPWP, while the stiffness of SC slightly decreases EPWP but helps reduce the surface peak acceleration.