低应力和湿化路径下膨胀土的持水－变形－破坏耦合行为研究

Typical deformation and failure of expansive soils usually occur under conditions of wetting and low confining pressures. However, the mechanisms of coupling between mechanical and hydrological behaviors under those conditions remain obscure. The existing models are not capable to effectively describe such deformation and failure phenomenon and their dependencies with soil water retention. As such, this project is aimed to investigate the soil water retention, volume change, and strength behavior of expansive soils through systematically designed hydrological-mechanical coupled testing under varying saturation conditions along wetting path. The experiment program will be involved with instruments including GCTS USTX-2000 double-cell unsaturated soil triaxial test system, and Fredlund unsaturated soil oedometer. Theoretical study will establish: (1) a new model of soil-water characteristic curve (SWCC) considering drying and wetting boundary and scanning behaviors of hysteresis; (2) a new deformation model describing associated behaviors of volume change induced by drying/wetting and by external loading; and (3) a new shear strength model simulating degradation of shear strength with volume increase and suction decrease. A user-defined development in GeoStudio software will be followed to apply the shear strength model into limit equilibrium method for stability analysis on slopes of expansive soils. The proposed deformation model and SWCC model will be employed with finite-element-method based analysis on seepage-deformation coupling behavior under variably saturated conditions. The engineering implication of this proposal is to improve the prediction capability of related expansive soil problems, e.g. rainfall-induced landslides and deformations of pavement.

膨胀土典型变形与破坏现象通常发生在低应力和湿化路径共同作用下。然而由于膨胀势的存在，应力和水作用下浅表层膨胀土力学行为相当复杂，规律尚不清楚；现有模型还不能有效描述相应变形与破坏行为及其与持水状态的相关性。项目拟对浅表层膨胀土开展系统的水力耦合试验，研究低应力状态下由非饱和湿化至饱和全过程中持水、变形、破坏行为及其耦合效应。试验平台主要有GCTS双压力室非饱和三轴试验系统和Fredlund非饱和固结仪等。基于试验研究，项目将构建浅表层膨胀土水力耦合本构模型，包含：(1)涵盖脱/吸湿边界线和扫描线的土水特征曲线方程；(2)能描述干湿变形与应力变形耦合行为的体变模型；(3)能描述强度随体胀和吸力降低而衰减的抗剪强度方程。最后，将所提本构方程通过二次开发应用于膨胀土边坡稳定性分析与非饱和/饱和渗流－变形耦合分析。项目成果将为降水导致的浅表层膨胀土工程破坏与灾害的稳定分析与预测预判提供科学依据。

膨胀土典型变形与破坏现象通常发生在低应力和湿化路径共同作用下。然而由于膨胀势的存在，应力和水作用下浅表层膨胀土力学行为相当复杂，规律尚不清楚；现有模型还不能有效描述相应变形与破坏行为及其与持水状态的相关性。项目对浅表层膨胀土开展系统的水力耦合试验，研究低应力状态下由非饱和湿化至饱和全过程中持水、变形、破坏行为及其耦合效应。试验平台主要有GCTS双压力室非饱和三轴试验系统、吸力控制非饱和土直剪仪、动态非饱和土环剪仪、Fredlund非饱和固结仪等。结果表明：（1）低应力下原状样峰值强度主要受土体结构性控制。峰值与残余强度比可作为反映土体结构性强弱的一个显性指标。整个低中高应力水平，原状、压实、泥浆固结样峰值强度线均为线性，且有效内摩擦角基本一致。原状与压实样的结构性体现于峰值强度有效凝聚力。整个低中高应力水平原状与压实样残余强度基本相同；低应力水平下残余强度线表现出强烈非线性；采用幂函数可很好描述低中高应力水平下的残余强度特征。（2）原状、压实、泥浆固结样压缩曲线相交于压力1200 kPa对应点；压实样压缩曲线总位于原状样与泥浆固结样之间。泥浆固结样压缩曲线与固有压缩线ICL经验公式几乎重合；原状与压实样压缩曲线均穿越ICL；原状样膨胀敏感性大于压实样，表明原状样颗粒间胶结更强。（3）采用TRIM试验方法获得6种密度下脱/吸湿过程的HCF模型参数，基于所获HCF模型参数在Hydrus中开展降雨入渗数值分析表明，土体密度对非饱和渗流影响显著，但密度变化对湿润锋前进速度快慢影响不具单调性；水力滞回对渗流分析结果影响明显；采用形式简单的HCF模型如VGM模型，采用α、n与e的经验公式，对不同密度、不同脱/吸湿路径采用对应的模型参数值，是综合考虑密度与水力滞回对非饱和渗流影响的可行方法。（4）采用相对湿度95 %下吸湿重力含水率w95作为膨胀潜势判别标准并验证其可行性。(5) 提出了一种黏土真实缩限值的确定方法。项目成果将为降水导致的浅表层膨胀土工程破坏与灾害的稳定分析与预测预判提供科学依据。