交通动载下饱和软黏土累计变形的不排水循环扭剪试验

饱和软土地基上长期往复交通动载将诱发显著的路基运营沉降。交通移动荷载下场地应力路径下呈现心形旋转路径,然而现有路基长期运营沉降分析是建立在循环三轴试验基础上,忽略交通移动荷载下地基土单元主应力轴的旋转效应。利用动态空心圆柱扭剪仪模拟心形循环加载路径,并在动态空心圆柱仪上模拟了传统的循环三轴加载路径。对饱和软黏土开展试验研究,对比观察一系列心形循环加载和三轴循环的不排水累计变形行为。试验研究发现,心形循环加载将诱发更大的累计轴向应变和累计孔压,伴随着初始动应力比的增加,二者差异性更为明显。循环加载过程有效动应力比随着循环次数而增大,循环增量破坏时对应的有效动应力比近似为常数。大数目循环加载下,传统塑性安定行为可渐变为循环塑性蠕变状态。

Undrained cyclic torsion shear tests on permanent deformation responses of soft saturated clay to traffic loadings

Long-term traffic loadings usually induce huge settlement in the pavement subgrade underlying soft subsoil. Traffic moving loading essentially produces a heart-shaped stress path in the deviatoric stress space. The conventional cyclic apparatus is usually used to simulate the permanent settlement of pavement, neglecting the rotation of the principal stress axis caused by the traffic loading. In order to achieve the heart-shaped stress path, a series of undrained cyclic torsion shear tests are performed on DHCA. In comparison, a series of undrained cyclic triaxial shear tests are also done on DHCA. The saturated soft clay is selected to be tested by applying two types of cyclic loadings, i.e., cyclic heart-shape shear and cyclic triaxial shear. The two types of cyclic shear loadings produce much different permanent undrained responses under the same initial confining and dynamic stress levels. The difference of permanent behaviors becomes more significant with the increase of the dynamic stress level. Compared with the conventional undrained cyclic triaxial shear, the undrained cyclic torsion shear generates larger permanent axial strain and higher accumulated pore pressure. In addition, the effective dynamic stress ratio increases during cyclic torsion shearing and tends to become constant at the occurrence of cyclic incremental collapse. The classical plastic shakedown response observed during a low number of loading cycles gradually becomes an intermediate response of cyclic plastic creep after a high number of loading cycles.