膨胀土地区大量工程实践表明，发生在膨胀土边坡浅表层一定深度范围内的牵引式滑坡现象非常普遍，对于该种渐进性边坡破坏模式，传统极限平衡分析无法反映其失稳状态和破坏机制。借鉴温度场中温度膨胀系数的定义和数值处理方法，以室内无荷/有荷膨胀率试验成果为依据，对膨胀土的湿度膨胀系数进行了反演拟合，得到低荷载条件下含水率-膨胀系数近似符合线性关系。基于降雨作用下膨胀土边坡破坏的大型静力物理模型试验成果，采用FLAC非饱和二相流模型模拟降雨引起的边坡湿度场分布变化状态，引入含水率-膨胀系数的线性关系，提出一种适用于膨胀土边坡的湿度场-膨胀变形场-应力场的多场耦合数值分析方法，并藉此对物理模型试验进行仿真。结合模型试验和多场耦合数值分析，阐述膨胀土边坡浅层渐进性破坏机制的本质是水力边界条件变化引起的膨胀土膨胀变形作用。

The progressive failure of expansive soil slope at shallow depth occurs frequently; but the traditional limit equilibrium theory can not reflect properly particularities of shallow instability of expansive soil slope. Based on a series of large static physical model tests of the compacted expansive soil slope, this paper firstly simulated the moisture field distribution by the unsaturated model of two-phase flow of FLAC; and then, by referencing the temperature expansion theory, a back analysis of the moisture swelling coefficient of expansive soil is conducted according to the indoor expansion tests with or without vertical loading. Finally, a multifield coupling numerical analysis method with moisture field, swelling deformation field and stress field is proposed. Combining the physical model tests with multifield coupling numerical analysis, the expansion failure mechanism for the shallow instability of expansive soil slope is expounded.