前期动载对低塑性粉土静态和动态强度的影响

低塑性粉土广泛存在于世界范围内，在地震中容易产生液化现象，然而一些基础设施破坏不仅见于地震中也发生在地震后，这就决定了研究低塑性粉土震后行为的必要性。以美国中部密西西比河沿岸低塑性粉土为试验材料，研究动载对低塑性粉土静态和动态强度的影响。在动三轴仪上对试样施加动载引起超孔隙水压力，排水重固结后，分别对2组震动后试样进行静态和动态三轴强度试验。试验结果表明，当液化水平小于0.70时，前期动载对粉土的不排水剪切强度影响不大；相反地，只有当液化水平大于0.70，密西西比河沿岸粉土的震后重固结体积应变和不排水剪切强度才伴随着液化水平的提高显著增加，但相对于砂土而言，重固结体积应变在较低的液化水平时即有明显增加。与前期动载对不排水剪切强度的影响不同，当动载所引起的液化水平为0.35或轴向应变为0.2%时，抗液化强度达到最大值，若液化水平大于0.35，抗液化强度伴随液化水平提高而降低。如果前期荷载引起较大的压应变，在重固结后第二次动载循环中，轴向压力相比轴向拉力引起较小的超孔隙水压力。

EFFECT OF PREVIOUS DYNAMIC LOADING ON STATIC AND DYNAMIC STRENGTHS OF LOW-PLASTICITY SILT

Low-plasticity silt widely spreads all over the world， and its liquefaction easily develops in earthquake events. Some failure of civil infrastructures happened not only during but also after earthquakes，indicating that it is necessary to study postcyclic behavior of low-plasticity silt. The Mississippi River Valley silt in central America was selected as testing material. Excess pore water pressure was produced by cyclic loading in cyclic triaxial testing system and then dissipated for reconsolidation. Monotonic and cyclic triaxial tests were respectively conducted on two series of specimens experiencing the previous dynamic shearing. The testing results show that the dynamic shearing with a liquefaction level larger than 0.70 produced significant increases in the volumetric strain due to reconsolidation and undrained shear strength. Compared to sand，the Mississippi River Valley silt has an obvious increase on undrained shear strength at lower liquefaction level. The previous dynamic shearing showed a different effect on liquefaction resistance with on undrained shear strength. With a liquefaction level of 0.35 or a cyclic axial strain of 0.2%，the liquefaction resistance reached the peak and then decreased with an increase in liquefaction level larger than 0.35. With a large compression strain due to previous dynamic loading，the excess pore pressure developed less in the compression than in the extension during the second cyclic loading after reconsolidation.