水-热耦合效应下崩壁土壤抗剪强度变化特征与机制研究

Collapsing gully is the most harmful type of soil erosion in red soil hilly region in south China, and the stability of collapsing gully wall determines its severity level. The shear strength is a core indicator for describing the stability of collapsing gully wall. Traditionally, researches on the soil shear strength of collapsing gully wall mainly focus on the effects of soil moisture, while the influence of temperature on it is less concerned. In addition, there is a lack of research on the micro-mechanisms and chemical mechanisms of soil shear strength changes under water-heat coupling effects. This project selects typical collapsing gully to monitor the water-heat migration in collapsing gully wall. The properties of soil samples treated under several water-heat conditions indoors will be analyzed, which will be compared with the undisturbed soil from collapsing gully wall. Technologies associated with pressure membrane instrument, temperature controlled triaxial apparatus, electronic computer X-ray tomography, field emission scanning electron microscope and X-ray diffractometer will be employed. The soil water-heat moving process in collapsing gully wall and the relationship between soil shear strength and water-heat conditions will be investigated. The dynamics of fracture morphology, pore structure, particle morphology and Fe-Al oxides under water-heat coupling effects will be demonstrated. The internal mechanisms of water-heat coupling effects on changing soil shear strength will be revealed. The results from this project are expected to provide a novel understanding of the collapsing process of collapsing gully wall under water-heat coupling effects, and provide scientific support for the prevention and control of collapsing gully.

崩岗是我国南方红壤丘陵区危害最严重的土壤侵蚀类型，崩壁稳定性决定了崩岗侵蚀的严重程度。土壤抗剪强度是描述崩壁稳定性的核心指标。传统上对崩壁土壤抗剪强度的研究，主要关注水分效应，而温度效应对崩壁土壤抗剪强度的影响鲜有研究；此外，对水-热耦合作用下崩壁土壤抗剪强度变化的微观机制和化学机制缺乏研究。本项目选择典型崩岗进行崩壁水-热运移原位监测，选取室内水-热条件梯度处理重塑土样结合野外崩壁原状土样，利用压力膜仪、温控三轴剪切仪、电子计算机X射线断层扫描、场发射扫描电子显微镜和X射线衍射仪等技术，探索崩壁土体水-热运动过程、土壤抗剪强度与水-热条件响应关系，解析水-热耦合作用下崩壁土壤裂隙形态、孔隙结构、颗粒形态和铁铝氧化物的变化过程，揭示土壤抗剪强度变化的内在机制，为理解水-热耦合作用下崩壁土体的失稳崩塌过程提供新的认识，为崩岗的防治提供科学支撑。

传统上对崩壁土壤抗剪强度的研究，主要关注水分效应，而温度效应对崩壁土壤抗剪强度的影响鲜有研究。本项目选择典型崩岗群，在两处崩壁设置了两个水-热监测断面，同时采集崩壁3个土层（红土层、砂土层和碎屑层）土壤样品，分析了土壤理化性质的空间变异性，研究了不同水分-温度条件下土壤抗剪强度的变化规律，探究了温度对土壤水分特征曲线、液塑限和干缩裂隙的影响，明晰了不同形态氧化铁影响土壤微观结构和抗剪强度的机制，主要结论如下：.（1）崩壁表层土壤的游离态和无定形态铁、铝氧化物以及游离氧化锰含量均大于内部土壤。无定形氧化铝、细粉粒、游离氧化铁、细黏粒为影响崩壁抗剪强度的主要土壤因子。.（2）崩壁不同土层土壤的含水量在20%~40%之间波动，温度在15~35℃之间波动。在中含水率（0.22 g g-1）和高含水率（0.30 g g-1）条件下，红土层土壤粘聚力随温度升高显著降低，在低含水率条件下，温度效应不显著；碎屑层与红土层规律类似，但温度对其粘聚力的影响作用不显著。.（3）在中吸力阶段，温度对各自土层土壤水分特征曲线的影响程度最大。温度从15℃升高至40℃时，三个土层土壤的液限和塑限均显著降低，同时土壤的结合水含量均降低。温度从40℃升高至60℃，红土层、碎屑层土壤塑限和红土层、砂土层土壤液限有所上升。.（4）在较低温度条件下，崩壁红土层表层土壤水分蒸发时间长，裂隙发育过程更缓慢，但发育程度高，裂隙结构更复杂。崩岗红土层原状土壤内部裂隙发育相对密集，裂隙率在9.40%~24.85%区间分布，裂隙率、裂隙长度参数、连通性及分形维数与抗剪强度呈显著负相关关系。.（5）无定形铁在添加浓度≥3 g·kg-1时对粘聚力有显著增大作用，赤铁矿-高岭土混合试样、针铁矿-高岭土混合试样与无定形氧化铁-高岭土混合试样的内摩擦角变化幅度均不大。无定形氧化铁在添加浓度≥1 g·kg-1，无定形氧化铁-高岭土混合试样微观接触面积会显著大于纯高岭土。.