冻融交替下黄土斜坡水文循环机理及其对斜坡稳定性影响研究

Freezing-thawing landslide chiefly occurs on the loess slopes beside loess platform in China. During the freezing process, hydraulic conductivity (K) of the frozen layer gradually decreases to zero, so that the frozen layer becomes aquitard, thus, groundwater gathers. On the contrary, during the thawing process, K of the frozen layer begins to increase and groundwater discharges. Accordingly, the interaction between changes of kinetic field of groundwater and of shear strain of soils, results in landslide on loess slopes during cyclical freezing and thawing. Combinating field survey with in-situ experiments, indoor soil engineering testing, permeable testing and numerical simulation, the project studies both hydrological cycle mechanism and stability response of loess slopes under alternation of freeze and thawing. It mainly studies the functional relationship among hydraulic conductivity (K), moisture content(θ), and temperature(T) of the frozen layer under different status, then analyzes change laws of kinetic field in groundwater on the loess slopes during freezing-thawing cycle. Moreover, this project tries to construct a hydrological cycle model of loess slopes. Integrating mechanical property of soils, it also construct an assessment of stability and forecast model coupling with moisture content, heat and force on loess slopes, to reveal the hydrological cycle mechanism and stability response mechanism of loess slopes under freezing and thawing cycle, and to provide a scientific evidence and technical support for assessment, preventive treatment and forcast of landslide in the freeling-thawing areas of loess plateu.

冻融型滑坡主要发生在黄土台塬边的黄土斜坡，在冻结过程中，冻结层的渗透系数K逐渐降低至形成隔水层，地下水富集；在解冻过程中，渗透系数K开始增大，地下水排泄。在这冻融循环过程中，黄土斜坡地下水动力场的动态变化与土体剪应变力变化相互作用导致滑坡的产生。本项目通过野外监测、室内土工实验、冻土渗透试验及数值模拟相结合方法，开展冻融交替下黄土斜坡水文循环机理与稳定性响应研究，主要研究冻结层在不同冻融状态的渗透系数K与含水量θ、温度T的函数关系，分析冻融交替作用下黄土斜坡地下水动力场变化规律，建立黄土斜坡水文循环模型、结合土体力学性质，构建黄土斜坡水-热-力耦合的稳定性评价与预报模型，揭示黄土斜坡在冻融循环作用下水文循环机理及对斜坡稳定性的响应机制，为黄土高原冻融区滑坡的评价、防治、预警预报提供科学依据和技术支撑。

项目选择甘肃永靖县黑方台黄土斜坡为研究区，原位监测冻结至融解过程中黄土的温度和含水量，研究冻融条件下黄土斜（边）坡冻结层温度和水分及变化特征及其关系。结果表明：本次监测点黄土冻结深度为52cm，黄土剖面温度表现为三个特征带：温度稳定传递带、温度变动带、温度交替显著带。温度在黄土斜（边）坡冻融过程对水分具有控制性作用，含水量随温度变化划分为三个阶段:小于2℃，含水量随温度呈现非线性递增趋势；2℃-6℃，含水量随温度呈现波动性下降；大于6℃，水分由小幅增加直至稳定，温度影响权重减小。随着冻结土层完全融解，被冻结水分融解后向下运移，最终导致剖面含水量急剧增加，是导致浅层黄土滑塌的重要因素。. 根据达西定律，研制了一种变温下饱和-非饱和土水力参数测定装置，该装置包括实验筒、供水与回水系统和数据采集系统。该装置可以获得不同温度下饱和渗透系数、给水度和土壤水分特征曲线、非饱和渗透系数、比容水量和扩散率等参数，进一步可以获得这些参数与温度的数学表达式；该实验装置所用试样大，减少了小实验样品测试结果的随机性，且测定参数多、自动化程度高，具有推广应用价值。. 通过实验和分析结果表明: 黄土的饱和渗透系数随着温度的增加而递增，冻融交替循环次数也是影响饱和渗透系数的重要因素；黄土非饱和渗透系数和扩散率是温度和含水率的函数，而比水容量是含水率的函数。水动力粘滞系数随温度增加而减小是上述参数增大的关键因素。. 研究结果表明，冻融条件下，黑方台黄土斜坡存在相应的稳定性变化过程，斜坡在经历冻融循环作用下，地下水位反复升降，会使得土体进入塑性区的范围进一步增大，土体遭受进一步破坏，极有可能诱发滑坡。