Fluid injection is one of the major triggering factors of instability in unsaturated soils. Robust simulation tools are therefore required to examine hydrologic transients in such a class of inelastic materials. In this paper, the governing equations imposing the balance of mass and momentum in deformable unsaturated porous media are inspected from an analytical perspective. Our main goal is to examine the role of volume collapse on the stability of suction transients. Distinct scenarios are considered in terms of material behavior, distinguishing the case of elastoplastic and viscoplastic materials. It shows that in elastoplastic materials the mathematical ill-posedness of the partial differential equations leads to a diffusive instability of the pore pressure field, signaled by the loss of stress controllability under water injection. By contrast, the introduction of viscosity restores the mathematical well-posedness and ensures the positive diffusivity. Numerical implications of these findings have been explored through simulations of water injection with a 1D finite element solver. It shows that an increase of the soil hydraulic sensitivity (i.e., a marked deterioration of the yield stress upon suction removal) can deteriorate the diffusive stability and prevent the computation of suction changes across a collapsible soil layer. While the incorporation of minor amounts of viscosity can restore numerical stability by suppressing the constitutive singularity. Such numerical results have been corroborated by the computation of local stability indicators based on controllability theory, which proved to be robust diagnostic tools to identify the culprit of runaway instability emerging from coupled hydro-mechanical processes.

中文翻译：

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非饱和土中不稳定吸力瞬态的数值模拟：润湿塌陷的作用

流体注入是非饱和土壤不稳定的主要触发因素之一。因此，需要强大的模拟工具来检查此类非弹性材料中的水文瞬变。在本文中，从解析的角度检查了可变形不饱和多孔介质中施加质量和动量平衡的控制方程。我们的主要目标是检查体积坍塌对吸力瞬变稳定性的作用。在材料行为方面考虑不同的情况，区分弹塑性和粘塑性材料的情况。它表明，在弹塑性材料中，偏微分方程的数学不适定性导致孔隙压力场的扩散不稳定性，表现为注水时应力可控性的丧失。相比之下，粘度的引入恢复了数学适定性并确保了正扩散率。通过使用一维有限元求解器模拟注水，已经探索了这些发现的数值含义。它表明土壤水力敏感性的增加（即吸力去除时屈服应力的显着恶化）会降低扩散稳定性并阻止跨可塌陷土层的吸力变化的计算。而加入少量粘度可以通过抑制本构奇点来恢复数值稳定性。这样的数值结果已经被基于可控性理论的局部稳定性指标的计算所证实，