循环荷载下考虑主应力旋转的软土力学响应研究

目前用于轨道路堤分析计算的参数多是通过三轴试验获得，而列车荷载经过时土单元应力路径与循环三轴试验加载路径的显著差异可能导致预测失真而引发工程问题。针对某城市轨道进行了三维计算，并分析了移动列车荷载下土单元应力幅值、循环周数、主应力轴旋转的分布规律。在该基础上，采用空心圆柱循环扭剪试验对该复杂应力路径进行了模拟，研究了饱和软黏土的孔压及变形累积特性。结果表明，列车荷载经过时地基土单元大主应力将在(－90°, 90°)内发生旋转；该主应力轴旋转将显著促进软黏土孔压和应变的累积，竖向应力幅值为15 kPa时，循环扭剪试验产生的孔压值比循环三轴试验高77%，累积应变增大了近50%；随着循环应力水平提高，二者累积孔压及累积应变的差值还会进一步增大，甚至出现循环三轴下仅产生较小应变而循环扭剪下已破坏的本质性差异。

Research on mechanical response of soft clay under cyclic loading involving principal stress rotation

Parameter values adopted in railway engineering analysis at present are generally obtained through triaxial tests, whereas the remarkable difference between the actual stress path during train passage and the loading path in cyclic triaxial tests may lead to inaccurate predictions and even engineering problems. Published researches show that the actual stress path probably contains principal stress rotation(PSR), which has a sisgnificant influence on the mechanical response of soil. However, the conventional approach, i.e. cyclic triaxial test is not efficient in simulating this complicated stress path involving PSR. Instead, hollow cylinder test with cyclic shear stress has been proved to be an excellent selection for PSR study. In order to obtain reliable stress path for guiding hollow cylinder tests, three-dimensional calculation for a certain urban railway is carried out; and the distribution regularity of cyclic stress amplitudes, cycle numbers and PSR under train loads is analyzed. Furthermore, a series of cyclic torsional shear tests on hollow cylinder samples is employed to simulate the complex stress path and to research the deformation and pore pressure accumulation properties of saturated soft clay. Considering stress in foundation soil decreasing along with the depth increasing, various cyclic stress amplitudes corresponding to different depths are adopted in this experiment study. Three-dimensional calculation shows that the major principal stress axis will rotate from -90° to 90° during the train loading passage as well as the number and amplitude of stress waves reduce gradually with the increase of depth. It is also shown that both PSR and stress are quite sensitive to train loads as the upper soil element can identify a single wheel load while the lower soil element can just identify a bogie and even the whole train load. And through the cyclic torsional shear tests, it’s obvious that the rotation of principal stress can promote the accumulation of pore pressure and strain in soft soil foundations significantly; and in the case when vertical cyclic stress amplitude equals to 15 kPa, the pore pressure and strain in cyclic torsional shear test are 77% and nearly 50% higher than that in cyclic triaxial test respectively. What’s more, as the cyclic stress amplitude increases, the gap of cumulative pore pressure and strain between that two types of tests will enlarge further, which may even result in essential difference that samples in triaxial tests show only small strain whereas failure in torsional shear tests.