基于分布各向异性的垃圾多重孔隙渗流及填埋场降水研究

Leachate drawdown is a key point when considering stability, settlement, capacity, anti-contamination and gas collection of a landfill. Setting shafts or French drains is usually used to lower or control the leachate drawdown in a landfill. Municipal solid waste (MSW) is mainly characterized by inhomogeneous multi-porosity seepage. Because of the significant anisotropic property of macropores after being compressed with layered landfill, the internal mechanism of seepage in MSW is totally different from in a homogeneous medium, which leads to the hardship in applying theoretical analysis to engineering practice. In this subject, dye tracing test, slicing and CT scan are intended to be used to study the pore distribution and the permeability of MSW under different confining pressure and age condition from unit to model scale . A generalized model of pore distribution under deformation conditions can be gained through image processing techniques and statistical approach. And a flow theory of landfills can be established based on the pore distribution and anisotropy theory after using the MPMP model or others to study the evolution of permeability characteristics of MSW. In-situ preferential flow characteristics can be revealed to verify the mathematical model and analytical method used in this subject through well dewatering or other in-situ tests, with the help of multi-dye tracing test and ERT method. This subject can offer experimental and theoretical bases for deeper research on the migration law of leachate and optimizing the design of the guide system in a landfill, and can also improve the safety and the level of anti-contamination in a landfill.

渗滤液水位是填埋场稳定、沉降和容量、污染防控及气体收集的关键问题。竖井和盲沟排水是控制填埋场水位的惯常做法，但理论分析往往难以反映工程实际。填埋体以非均匀多重孔隙渗流为主要特征，分层堆填以后，堆体内的孔隙被不同程度地压缩并呈现出显著的各向异性，与均质体渗流在内在机理上存在明显差异。本课题拟结合速冻切片、CT扫描、染色示踪等手段，通过单元体和模型试验研究不同压力和龄期条件下垃圾的孔隙分布和渗流规律；基于图像处理技术和统计学方法提出变形条件下孔隙分布概化模型；采用多孔隙多渗流模型等分析垃圾体导水特性及其演化规律，建立基于孔隙分布及其各向异性的填埋场渗流理论。通过竖井抽水等现场试验，结合染色示踪和ERT等手段，揭示现场垃圾渗流特征，验证本课题提出的理论和分析方法。本课题将为深入认识填埋堆场的渗滤液运移规律，优化设计堆体内导排系统提供试验和理论基础，有助于提高填埋场安全和污染防治水平。

本课题基于Poiseuille 方程，根据上覆压力作用下生活垃圾孔隙形状、孔隙尺寸分布及孔隙排列方向分布特征，提出了基于优先流及各向异性的饱和渗流模型。渗流各向异性值大体随上覆压力增加而增大；0～600 kPa 内计算得新鲜垃圾渗流各向异性值变化范围为1～10。同时通过自制双向渗透仪，测定了不同龄期、不同荷载作用下垃圾双向渗透系数及各向异性值。试验结果显示，当上覆压力由0增加至400 kpa时，竖向渗透系数比水平向渗透系数减小得更多，导致各向异性值从1逐渐增加至12，试验值与理论计算值吻合。.钻取现场垃圾样进行筛分与分拣，同时对不同荷载作用下的垃圾试样进行染色切片与CT扫描测试。结果表明，对于不同埋深的城市固体废弃物，细颗粒(0D，即粒径<2cm)所占比例最大，几乎均超过50%；粗粒中2D颗粒所占比例最高，而塑料在2D颗粒中占比最大；随着荷载的增大，细粒含量增加，大孔隙和优先流通道减小。2D 颗粒在荷载的作用下能定向排列，使渗流路径发生曲折，进而导致渗流各向异性。CT扫描图像能可视化内部优势流通道，平均孔隙排列角度随上覆荷载的增加呈对数减小。.针对我国高厨余垃圾填埋场中水位雍高问题，进行了国内首个填埋场中真空竖井及水平井降水试验。长度50 m的水平井的渗滤液流量从第1天的75 m3/day逐渐衰减至第90天的3 m3/day，根据水位梯度推算影响半径为40-50 m。高密度电法监测结果显示排水影响范围内垃圾电阻率增加6~16%，增加区域呈椭圆形分布，可视化显示出渗滤液运移路径。真空竖井抽水试验结果表明，竖井稳定产流量和周围水位降深会随着井内真空的增加而逐渐增加，真空的水平向影响范围约为15 m。考虑固废优先流效应，建立双孔隙度介质中竖井水气耦合运移模型。采用双孔隙度模型能较好模拟实测抽水流量和水位降深，而单孔隙度模型会过高估计降水效果。.本课题的研究成果发表于国际著名期刊《Géotechnique》、《Waste Management》、《Engineering Geology》及国内《岩土工程学报》等杂志，其中SCI论文9篇，EI论文7篇。多项研究成果被课题组正在编写的国家标准《生活垃圾卫生填埋处理岩土工程技术标准》所采纳，同时应用至杭州、上海及广州等地大型垃圾填埋场，为提升优化填埋场渗滤液导排系统的设计和稳定安全分析提供了理论、数据库和方法基础。