超大次循环荷载下超固结黏土的长期不排水力学特性研究

风、波浪等荷载具有超大循环次数的特征,极大影响黏土的长期不排水动力特性,是黏土地基基础设计的关键问题之一。针对现有动力试验荷载循环次数不足的现状,以超固结度为4的重塑土试样为研究对象,开展了一系列不排水三轴动力试验,研究了在超大次(比如超过10~6次)循环加载情况下黏土的相关特性。试验结果表明,在对称循环加载作用下,饱和黏土存在循环应力比门槛值为0.44~0.48。在循环加载初期,试样的轴应变及孔压的变化与前人对黏土动力试验研究结果一致。然而,随着循环加载次数的增加,动轴向应变减小,累积轴向应变增加;尤其是加载过程中由正孔压降低到负孔压。针对这一新的发现,增加了一个超固结土的蠕变试验结果进行了分析解释,为研究超大次循环荷载下超固结黏土的长期不排水力学特性的机理提供了新的思路。

Long-term undrained mechanical behavior of overconsolidated clay under cyclic loading with a large number of cycles

The wave and wind loadings applyed on structure foundations with a large number of cycles affect the long-term undrained mechanical behavior of clay, which is one of key issues for the foundation design. Qwing to lack of cyclic tests with a large number of cycles up to now, laboratory tests on over consolidated clay are perfomed to study the cyclic behavior with a large number of cycles (e.g., more than one million). Based on the results of reconstituted specimens with OCR=4, the possible existence of the threshold stress ratio under the symmetric cyclic loading is confirmed. The value of the threshold stress ratio is 0.44~0.48. At the early stage of cyclic loading, the evolutions of axial strain and pore pressure are similar to the results of the conventional cyclic tests. However, as the number of cycles increases, the cyclic axial strain decreases, whereas the permanent axial strain increases and the pore pressure decreases from a positive to negative value. These new phenomena are analyzed by conducting additional creep tests on the overconsolidated clay, which throws new light on studying the mechanism governing the long-term undrained mechanical behavior of overconsolidated clay under cyclic loading with a large number of cycles.