粉质黏土变形特性的冻融循环效应及其双屈服面本构模型

Effect of freeze-thaw cycles on deformation characteristics of a silty clay and its constitutive model with double yield surfaces

冻融循环是寒区最为常见的物理风化过程之一,会显著影响土体的变形特性。基于椭圆-抛物双屈服面本构模型,建立了考虑冻融循环影响的修正模型。以青藏高原粉质黏土为对象,进行一系列的冻融循环试验和三轴固结排水试验。结果表明,试样表现为应变硬化型应力-应变关系和剪缩特征,随着冻融循环次数的增加,剪切过程中的体积应变逐渐增大,而破坏强度则逐渐减小。然后,分别选择椭圆函数和抛物线函数描述p-q平面上的体积屈服面和剪切屈服面,引入相关联流动法则,推导了偏应力增量与轴向应变增量和体积应变增量两者之间的函数关系式。基于三轴试验结果,确定了模型参数随冻融循环次数的变化规律。结果表明,随着冻融循环次数的增加,c、φ、h、K、M1、M2、a呈逐渐减小的规律,而t、n呈逐渐增大的规律,且均可以采用Logistic函数拟合模型参数与冻融循环次数的回归关系。将以冻融循环次数为因子的模型参数表达式代入应力应变增量形式方程,建立了考虑冻融循环影响的修正双屈服面模型,模型计算值与试验实测值基本吻合。

The freeze-thaw cycle represents one of the most common physical weathering processes in cold regions, which can significantly affect the deformation characteristics of soils. An improved constitutive model with double yield surfaces, which can take the freeze-thaw effect into consideration, was developed based on the traditional elliptic-parabolic yield surfaces model in this study. First, a set of freeze-thaw cycling tests and consolidated-drained(CD) triaxial tests were conducted on silty clay from Qinghai-Tibet Plateau. The test results showed that the stress-strain behaviors exhibited strain hardening and shear-shrinkage characteristics. The volumetric strain increased while the failure strength decreased during the shearing process. Then, the elliptic equation and parabolic equation were adopted to fit the volumetric yield surface and the shear one in p-q plane, respectively. The incremental deviatoric stress-axial strain relation and deviatoric stress-volumetric strain relation were subsequently established by following the associated flow rule. The parameters of above relations were valued according to the tested deviatoric stress-axial strain-volumetric strain relationship, and were all found varying by the number of freeze-thaw cycles with certain regularity. The c、φ、h、K、M1、M2 and a all kept decreasing with increasing the number of freeze-thaw cycles, while the t and n kept increasing, and they can all be fitted by the Logistic function. At last, the expressions of above parameters with number of freeze-thaw cycle as variable were included into the incremental constitutive equations, thus an improved model considering freeze-thaw effect was established.The deviatoric stress-axial strain curves and volumetric strain-axial strain curves predicted by the proposed model agreed basically with the experimental results.