循环动载对低塑性粉土剪切行为的影响

Low-plasticity silt widely spreads in China, and its liquefaction was found in many earthquake events. The cyclic loading due to earthquake produces excess pore water pressure, and with its dissipation the silty ground is reconsolidated. The shear behavior after reconsolidation is influenced by the previous cyclic loading.This program will study the effect of cyclic loading on shearing behavior of low-plasticity silt, which will be loess, a typical low-plasticity silt in our country. This program will start from the study of monotonic shear behavior and investigate the effect of consolidation pressure and overconsolidation ratio on stress-strain behavior to check whether the stress-strain behavior can be normalized or not. On the base of the montonic shear behavior, the effect of previous cyclic loading on it will be studied. Excess pore pressure will be produced in silt specimens by cyclic loading in cyclic triaxial system. The postcyclic specimens will be sheared after different levels of reconsolidation to obtain the variation of shear strength and stiffness with reconsolidation level and the effect of apparent overconsolidation on postcyclic behavior.Additionally, the different excess pore pressure ratios will be induced by different levels of cyclic loading. The microscale structure of postcyclic specimens and monotonic shear behavior after reconsolidation and no reconsolidation will be investigated. The effect of plasticity, consolidation pressure, and overconsolidation ratio on the change of monotonic shear behavior of low-plasticity silt due to previous cyclic loading will be also studied.This study will advance the understanding of postcyclic behavior of low-plasticity silt. The findings in this project will provide a reference to avoid failures of civil infrastructure.

低塑性粉土广泛分布在我国，它的可液化性见于很多地震。地震动载引起超孔隙水压力，伴随它的消散粉土地层发生重固结，震后剪切行为受到前期动载的影响。本项目将以国内典型低塑性粉土- - 黄土为试验材料，研究循环动载对低塑性粉土剪切行为的影响。本项目从研究静态剪切行为出发，分析固结压力和固结比对应力应变行为的影响，调查是否具有正规化行为。在得知静态剪切行为的基础上，研究前期动载对低塑性粉土剪切行为的影响。采用动三轴仪对试样施加动载引起超孔隙水压力，待土样液化后进行不同程度的重固结并进行不排水静态剪切，研究剪切强度和刚度随重固结程度的变化规律。另外，改变动载强度，观测试样微观结构和研究重固结和非重固结两种情况后剪切行为随超孔隙水压力比的变化。研究塑性、固结比和固结压力对低塑性粉土剪切行为由于前期动载变化的影响。本项目将提升对低塑性粉土震后力学行为的认识，为预防基础设施受震破坏提供依据。

低塑性粉土广泛存在于世界范围内，尤其是在中国和美国等大陆国家。作为一种典型的低塑性粉土，黄土分布面积占全球陆地面积的9.3%。长期以来，由于低塑性粉土呈现砂土和黏土的双重力学行为，研究低塑性粉土力学行为存在着较大的困难。为了研究低塑性粉土震后力学及其影响因素，对塑性指数为5.8的低塑性粉土开展大量室内动三轴和静三轴试验，结合理论分析，形成以下主要研究成果：（1）该粉土呈现准稳态特性，当添加膨润土使塑性指数≥6.2，准稳态特性逐渐消散，试样呈现塑性行为，确定类粘土和中间性态土的临界塑性指数为6。（2）震后不排水抗剪强度和刚度随着重固结度的增加而提高，低塑性粉土呈现类超固结特性。（3）超孔隙水压力大于0.70是震后重固结试样不排水抗剪强度显著增加和非重固结试样屈服剪切强度和初始刚度显著降低的前提条件，低塑性粉土比干净砂和粉砂出现重固结体积应变显著增加所需的动载超孔隙水压力比要低。（4）同先前未受循环荷载试样不同，超固结比对震后低塑性粉土初期剪切阶段有效应力路径影响小；随着OCR升高，重固结过程更加类似于回弹行为；提出了震后不排水抗剪强度计算公式，认为动载液化可改变低塑性粉土试样临界状态曲线；当塑性指数小于6时，震后试样相对于未受震试样不排水抗剪强度和刚度显著变化，否则变化不明显。（5）当先前动载残余轴向应变为0.2%或经历大于4次液化动载液化后，该粉土抗液化强度得到明显提高。（6）基于γcyc-p’空间第一象限内两条包络线交点处确定的屈服循环轴向应变与循环轴向应变阈值没有显著差异。研究成果对于提升低塑性粉土震后力学行为认识具有重要的科学价值，对指导地层抗震设计和处理具有较好的借鉴意义。