细粒土黏性对其动力变形特性及液化势影响研究

Fine-grained soils are widespread in nature, and which happened liquefaction or seismic settlement in history earthquakes has become increasingly serious. However, there are still many uncertainties about the damages mechanisms and prediction methods. In this proposal, the relationship of fine-grained soils plasticity and skeleton strength with clay content will be specially analyzed. Using triaxial compression tests, a new fine-grained soils samples preparation criterion will be established, which about a constant skeleton void ratio for samples with very little clay content and a constant void ratio for samples with higher clay content. And combining with theoretical analysis, dry density of fine-grained soil samples with different clay contents will be obtained. Using cyclic triaxial liquefaction tests, clay content effect on cyclic resistance strength will be obtained. And clay content effect on “cyclic soften coefficient”, which is defined as the ratio of cyclic shear strength to static shear strength, will be obtained too. By comparing modulus degradation, “cyclic soften coefficient”, pore-pressure buildup and dynamic strain of saturated fine-grained soils under cyclic loading, the characteristics and mechanisms of silts liquefaction and silty clay seismic settlement will be revealed, and the mechanism difference between these two disasters will be discovered. The expected results will provide scientific references for prediction methods of these two seismic disasters individually.

细粒土在自然界中广泛分布，并且在历次地震中其表现出的液化或震陷灾害已日益突出，而关于细粒土的震害机理及预测方法还有很多不确定性。本项申请拟以细粒土黏性及骨架强度与黏粒含量关系为出发点，以改进重塑细粒土制样标准为突破点，基于三轴压缩试验结果并结合理论分析，获取多组黏粒含量细粒土制样干密度。通过动三轴液化试验，建立细粒土抗液化强度与黏粒含量关系。通过对比动、静三轴试验结果，获取代表动抗剪强度与静抗剪强度之比的“地震软化系数”与黏粒含量的关系。通过比较粉土与粉质黏土在循环应力作用下的模量软化、“地震软化系数”、以及孔压增长和动应变发展规律，同时考虑上述特征随土体黏性增加而可能存在的渐变或突变特性，分析饱和细粒土液化特性及机理与土体黏性关系，初步揭示软黏土震陷机理及其与粉土液化的本质区别。预期结果，将为细粒土液化与震陷灾害的防治提供科学依据。

含细粒砂性土相对于纯净砂在自然界中分布更为广泛，但是对于其液化判别，一直都是作为纯净砂液化判别的附属成果，没有得到足够的重视。世界范围内大量细粒含量对砂土液化势影响的室内试验研究结果之间充满了矛盾与困惑，并且对改进液化判别方法没有实质意义。本项目利用室内三轴试验，分别以等孔隙比和等骨架孔隙比原则制样，对配以不同黏粒含量细粒土，测试土体抗剪强度指标，研究重塑细粒土黏性可靠指标，细粒土黏性及骨架强度与黏粒含量的关系。通过分析三轴试验结果，并结合历史文献中细粒含量对砂土液化势影响试验研究结果，从砂土配粉粒最大、最小孔隙比试验结果入手，分析得出以等孔隙比（或等干密度）制样，动三轴液化试验结果反应更多的是相对密度的变化；等骨架孔隙比的概念有问题，即使很小的黏粒含量，也使试样强度及黏聚力指标产生较大的提升，说明黏粒对骨架强度有贡献，并且，最小孔隙比随细粒含量下降斜率较大，表明细粒更容易进入粗粒孔隙中，而最大孔隙比试验，细粒更容易存在于粗粒接触点之间，由于液化问题一般都针对较松散土体，所以等骨架孔隙比意义不大。最后，本项目对比了基于SPT试验的中国规范方法与NCEER推荐方法，发现两个方法定义纯净砂的矛盾，建议结合细粒含量和黏粒含量判别液化，并以此改进了方法。以历史地震数据检验了基于剪切波速试验的NCEER推荐方法，发现剪切波速方法对含细粒土体过于保守，并且细粒含量越高，对非液化点越不可靠。剪切波速试验为小应变无损测试，对含细粒土土颗粒间胶结力较为敏感，而液化状态为大应变破坏阶段，土体胶结力基本丧失，因此对胶结较强的粉砂、粉土，剪切波速指标与液化难以建立唯一联系。另外，中国规范剪切波速液化判别方法存在误导，由于判别式已经用黏粒含量修正，因此，Vs0经验系数应统一只取砂土数值。