膨胀土卸荷蠕变特性及其非线性蠕变模型

鉴于膨胀土滑坡往往表现为长期性、渐进性等与时间相关的特性，利用GDS应力路径三轴仪对膨胀土进行了三轴卸荷蠕变试验。试验结果表明：当偏应力较小时，膨胀土的蠕变曲线仅出现瞬时变形和衰减蠕变；当偏应力达到一定值时，其蠕变曲线也呈现衰减蠕变、稳态蠕变和加速蠕变3个阶段，但其加速蠕变阶段的特征与一般岩土体不同，其蠕变速度近乎常数。膨胀土的应力?应变等时曲线显示，膨胀土卸荷蠕变具有非线性特征，且其非线性程度与蠕变时间和应力水平相关，蠕变时间越长、应力水平越高，其非线性程度越高。基于非线性流变力学理论，提出了一种非线性四元件蠕变模型，将标准线性体与一个非线性黏壶串联，该模型可描述等围压三轴压缩应力状态下膨胀土轴向应变随时间的演变规律。根据膨胀土卸荷蠕变试验结果，采用曲线拟合法对三维非线性模型的参数进行反演识别。拟合曲线和试验曲线对比显示，两者吻合良好，说明该模型可以很好地描述膨胀土的蠕变特性。此外，基于该蠕变模型获取了膨胀土的临界破坏应力，其与常规剪切破坏应力的比值随着固结压力的减小而减小，表明越接近坡面的土层越容易发生蠕变破坏。

Creep properties of expansive soil under unloading stress and its nonlinear constitutive model

Since the slide of expansive soil slope is generally characterised by time-dependent properties such as chronicity and gradualness, triaxial unloading creep tests are carried out on the expansive soil specimens by GDS stress path triaxial apparatus. The test results show that the creep curve of expansive soil only shows transient deformation and attenuation creep with low deviatoric stress. When the deviatoric stress reaches a certain value, the creep curve also exhibits attenuation creep, steady creep and accelerated creep, and the velocity in the accelerated stage is nearly constant, different from common soil. Meanwhile, isochronous stress-strain curve of expansive soil indicats that its creep process has nonlinear characteristics, and the nonlinear degree is related to creep time and stress level. When the creep time is longer and the stress level is higher, the degree of nonlinearity is higher. Based on the nonlinear rheological theory, a new nonlinear four-element creep model is presented, in which the standard linear body is connected in series with a nonlinear clay pot. The proposed model can describe the evolution of the axial strain of expansive soil with time under constant confining pressure and triaxial compression stress. According to triaxial compression creep test results of expansive soil, the parameters of the proposed model are inversed by using curve fitting method．The theoretical curves accord well with the test curves, which indicate that the proposed model can describe the creep property of expansive soil. Furthermore, critical failure stress can be obtained based on the creep model. As the confining stress decreases, the ratio of the critical failure stress to conventional shear failure stress decreases, which indicates that creep likely happens on the expansive soil layer closer to slope surface.