多维地震作用下含细颗粒饱和砂土液化特性及其微观机理研究

According to the insufficiency of current study on the liquefaction behavior of natural sand during earthquakes, this proposal will investigate the liquefaction behavior of saturated sand with fines subjected to multi-directional seismic loading using cyclic triaxial device with shear wave measurement (bender element), cyclic simple shear device and dynamic hollow cylinder apparatus. The test results will clarify the effects of multi-directional property of seismic loading and the fines content on the liquefaction behavior of sand. The current liquefaction potential evaluation method will be modified to account for the effects of multi-directional property of seismic loading and the fines content of sand, which will definitely improve the reliability of liquefaction potential evaluation in practice. The evolutions of the soil microstructure with fines content will be investigated by the Micro CT. Special attentions will be paid on the effects of characteristics of sand and silt particles (e.g. size, particle size distribution and shape) and the plasticity of clay particles on the soil microstructure. The soil microstructure will be indexed by the shear wave velocity measured by the bender element tests, and attempts are made to link the soil liquefaction potential with shear wave velocity. Discrete element method will be also used to simulate the laboratory cyclic triaxial, cyclic simple shear and dynamic hollow cylinder tests and the evolution of microstructure of the soil will be analyzed during the liquefaction process in order to reveal the liquefaction mechanism. The research outputs will not only improve the understanding of liquefaction behavior and its mechanism, but also provide technical supports for reliable evaluation of liquefaction potential in practice and proposing effective countermeasures of liquefaction if necessary.

针对现有天然砂土液化特性研究存在的主要不足，本项目采用动三轴（带弯曲元波速测试）、多向动单剪仪和动态空心圆柱仪研究多维地震作用下含细颗粒饱和砂土的液化特性，修正现有的砂土液化判别方法来考虑多维地震作用和细颗粒含量的影响，提高实际工程场地液化判别的准确性；通过显微CT研究不同细颗粒含量下土体微观结构的变化，重点关注砂粒、粉粒的颗粒特性（如颗粒大小、级配、形状）和黏粒塑性对土体微观结构的影响；利用弯曲元测定剪切波速来表征土体的微观结构，并建立剪切波速和抗液化强度的关系；采用离散元方法研究不同细颗粒含量下砂土试样的微观结构、剪切波速及其在振动液化过程中的演化，建立土体微观结构、剪切波速与液化特性的内在联系。研究成果不仅可以丰富和完善砂土液化等土动力学理论，揭示含细颗粒砂土的液化机理，也为准确评价实际场地地震液化势和抗液化处理提供科学依据和技术支撑，因而本项目具有重要的理论和实际工程价值。

饱和砂土的地震液化是最常见的地震灾害之一，是导致大面积地基失稳与沉陷、建筑物或构筑物坍塌和城市生命线工程破坏的主要原因，造成极大人员和财产损失。本项目针对现有天然砂土液化特性研究存在的主要不足，采用共振柱和弯曲元波速系统测定了含细颗粒饱和砂土的剪切波速和动力特性，研究表明细颗粒含量对砂土剪切波速有显著影响，土体剪切波速和等效骨架孔隙比存在统一的关系，并可以用应力归一化的剪切波速来定量表征土体的微观结构。通过动三轴试验研究了不同细颗粒含量、密实度、级配条件下饱和砂土的抗液化强度，试验结果表明：相同孔隙比条件下，类砂土的抗液化强度随着细颗粒含量的增加而降低，而不同细颗粒含量砂土的抗液化强度与等效骨架孔隙比也存在统一的关系，即等效骨架孔隙比是含细颗粒砂土抗液化强度的有效量化指标。开展了不同黏粒含量饱和砂土的离心振动台模型试验，再现了实际地震中天然饱和砂土场地地震液化现象，研究了地震下饱和砂土自由场超孔隙水压力、加速度、地表沉降的发展。研究结果表明：含黏粒饱和砂土在震动期间的沉降显著大于洁净砂，砂土抗液化强度随黏粒含量由0%增至20%呈先增加再减小规律。建立了基于大量室内试验结果的含细颗粒砂土抗液化强度数据库，并提出了基于等效骨架孔隙比的含细颗粒砂土抗液化强度的预测公式。采用离散元数值方法研究了不同细颗粒含量下砂土试样的微观结构、剪切波速、单调剪切特性及振动液化特性，研究阐明了试样微观结构在振动液化过程中的演化，确定了液化和非液化的冗余度指标界限值，揭示了含细颗粒砂土抗液化强度主要由土体初始接触力学配位数决定，建立了基于微观状态参数的抗液化强度理论。研究成果不仅丰富和完善了砂土液化等土动力学理论，也揭示了含细颗粒砂土的液化机理和其抗液化强度的决定指标，为准确评价实际场地地震液化势和抗液化处理提供了科学依据和技术支撑。