黄土非饱和渗透性参数的测试方法研究

The water seepage in loess, both down motion such as rain and irrigation water, and up motion such as capillary water, can trigger varieties of geo-hazards. Moisture in loess migrates in unsaturated state. Soil water characteristic function and unsaturated permeability function are the essential parameters to reflect the permeability of unsaturated soils, but direct measurement of the functions is time-consuming and technically difficult, so it has been affordable to find convenient methods to determine them. In this project, a fine soil column test equipment will be designed based on the filter paper method for measuring the functions. A sets of loess specimens within metal ring are overlapped vertically and sandwiched with filter paper between each two of them. A Mariotte bottle is installed on the top of the column to provide a constant water head and a ceramic disk with definite coefficient of permeability is used to control the flow quantity in the test. With the equipment, the vertical profiles of moisture and the permeability are measured； while the soil water characteristic function and unsaturated permeability function can be calculated by reverse-analysis. The functions of the loess and paleosol in a vertical loess profile and the loess at the other two sites will be measured with the method. Meanwhile, the soil water characteristic functions will be measured with pressure plate and the pore size distribution curves will be measured with mercury penetration test and microstructure image statistics methods which are applied to validate and analyze rational of the test method. The measured functions are applied to predict the seepage of the rain fall at the sampled vertical profile and compare with the monitoring results to check the availability. The method is easy to operate and can provide the key parameters for unsaturated permeability calculation.

黄土地区降水、灌溉水的下渗和毛细水的上升都可诱发不同类型地质灾害，而黄土中水分多以非饱和态运移。土水特征函数和渗透性函数是反映非饱和渗流的两个重要土性参数，但直接测定这两个函数耗时长、技术难度大，为此人们一直在寻求更简捷的测试方法。本项目设计了一种基于滤纸法的精细土柱试验装置。在一系列垂直叠置的环刀土样间夹标准滤纸，顶部用Mariotte瓶维持稳定水头，利用不同渗透性的陶土板控制渗透率，可测得不同时段含水率和基质吸力垂直剖面，通过反分析可获得土水特征函数和渗透性函数。利用该方法对一现场剖面黄土和古土壤及另两个场地马兰黄土的土水特征函数和渗透性函数进行测试；同时用压力板仪测定其土水特征曲线，用压汞法及微结图像统计测定其孔隙分布曲线，用以分析检验该方法的合理性和可靠性。再将实测函数用于现场剖面降雨的入渗过程预测，与监测结果对比检验其有效性。此方法易操作，可为非饱和渗流计算提供必要的参数。

处于干旱-半干旱区的黄土地区地下水埋藏深，地表水以非饱和渗流的方式向下运移，该区内滑坡、崩塌、地基湿陷等地质灾害的发生与非饱和渗流息息相关，因而研究降雨、灌溉水等地表水的入渗规律和模式，对黄土地区地质灾害的防治具有指导意义。土水特征函数和渗透性函数是反映非饱和渗流的两个水力学特性参数，准确获取这两个参数是解决非饱和渗流问题的关键。本项目研发了一种小型土柱试验装置，该装置用叠置的环刀组成土柱，大大减少了传统土柱体积；用滤纸法直接测定土的基质吸力，用环刀测定小土柱的含水率，能同时获得土水特征函数和渗透性函数。同时，利用压力板仪单独测定土水特征函数，并与小型土柱试验结果进行对比，验证了该试验装置及其测试方法的可靠性。根据反复试验，提出了该装置测量非饱和土水力特性参数的规范化操作方法，便于推广应用。建立了全吸力范围内用孔隙分布曲线预测黄土土水特征函数及渗透性函数的预测模型，与小型土柱试验结果、滤纸法及压力板仪法试验结果进行综合对比，验证了预测模型的准确性。指出孔隙分布曲线预测黄土渗透性函数的方法适用于低基质吸力部分的毛细水，而不适用于高基质吸力部分的吸附水，针对试验所用黄土，提出用实测非饱和渗透数据标定吸附水的渗透性特征，修正了用孔隙分布曲线预测渗透性函数的方法，得到了全吸力范围的渗透性函数预测模型。基于本项目的技术成果，测定了正宁黄土剖面上三层黄土与三层古土壤的水力特性参数，建立了水力特性参数与其微观孔隙结构的内在关系。根据原位监测和地层分布情况，在VADOSE/W软件中构建了实际黄土-古土壤互层的一维土柱模型，并将实测的6层黄土和古土壤的水力特性参数应用于非饱和渗流控制方程中，分析了降雨和温度等气候变化对水分入渗的影响，结合实际监测结果，揭示了厚层黄土的水热迁移模式及地下水补给特征。本项目的研究成果解决了非饱和水力特性参数难以测定的问题，深化和完善了对非饱和黄土水力特性的内在机理认识，为黄土地区地表水下渗引发的地质灾害和工程问题的预防治理提供了技术进而理论支撑。