垃圾填埋场覆盖层植物-土-水相互作用机理及长期服役性能评价

Soil cracking poses a great threat on the deterioration of the long-term performance of landfill covers. Vegetations are believed to play an important role in restricting cracks of superficial soils and the associated preferential flow. However, the influence of the vegetation is still unclear. This project aims to investigate vegetation-soil-water interactions under complicated climates and the impact of such interactions on the long-term performance of landfill covers. Based on previous research on unsaturated flow through cracked soils, this project will go a step further to investigate the mechanisms of water migration through soil covers considering two aspects. One is the positive effect of vegetation which helps restrict cracking of soils and enhance the matric suction in soils. Another is the negative effect induced by the preferential flow through vegetation roots. This study will provide a systematic theoretical framework for the on-site monitoring and laboratory testing of flow in structured soils and long-term performance evaluation of landfill covers. The influence of vegetation on cracking and moisture migration will firstly be investigated in-situ. Then a set of apparatus for testing saturated-unsaturated flow in discontinuous medium will be developed. An experimental procedure to measure the coefficient of permeability for soils including roots and cracks will also be proposed. On the basis of the experiment results, the constitutive equation of the discontinuous medium will be developed and implemented in the following numerical analysis. Finally a numerical method will be established and verified to investigate the mechanisms of covers under evapotranspiration and precipitation conditions. These studies will provide a new way to accurately describe the hydraulic properties of discontinuous medium, which tackles the difficult problem of simulating cracking and roots in covers. The research can also provide an effective method to predict the long-term performance of covers. Furthermore it is expected to provide theoretical basis for the analysis of leakage and contamination in landfill covers.

土体开裂是导致垃圾填埋场封场覆盖层长期服役性能恶化的根本原因，覆盖层表面种植的植物对浅层土体开裂及优势流入渗有重要影响，但其有效性还有待研究和评价。本项目以前期裂隙土的非饱和渗流研究为基础，针对植物限制土体开裂和提高土体吸力的正面作用及其根系作为优势通道的负面作用两个方面深入研究复杂气候条件下植物-土-水的相互作用机理及其对覆盖层长期性能的改善。本项目拟通过现场试验、室内试验及数值分析对植物根系的限裂作用、含根系裂隙土的非饱和渗流以及复杂非连续介质的入渗机理展开系统深入的研究，揭示植物-土-水的相互作用规律和机理，建立含根系裂隙土的渗透本构关系和入渗控制方程，提出覆盖层长期服役性能的评价方法。本研究将提供精确描述非连续介质渗透特性新的理论和试验方法，解决目前覆盖层评价中难以考虑土体开裂和根系影响的难题，为垃圾填埋场封场系统的渗漏和污染分析提供理论依据，为进行覆盖层长期性能评价提供有效方法。

土体开裂是导致垃圾填埋场封场覆盖层长期服役性能恶化的根本原因，覆盖层表面种植的植物对浅层土体开裂及优势流入渗有重要影响，但其有效性还有待研究和评价。本项目以前期裂隙土的非饱和渗流研究为基础，针对植物限制土体开裂和提高土体吸力的正面作用及其根系作为优势通道的负面作用两个方面深入研究了复杂气候条件下植物-土-水的相互作用机理及其对覆盖层长期性能的改善。本项目拟通过现场试验、室内试验及数值分析对植物根系的限裂作用、含根系裂隙土的非饱和渗流以及复杂非连续介质的入渗机理展开了研究，揭示了植物-土-水的相互作用规律和机理，建立了含根系裂隙土的渗透本构关系和入渗控制方程，进行了覆盖层长期服役性能的评价。本研究通过室内土柱模型试验研究得到植物根系能够减缓土体裂隙的开展，土柱染色剂示踪试验的结果表明土体中只有植物根系附近存在染色情况，进而证明了植物根系的优势流作用。通过土体单元试验研究了水分在裂隙中形成优势流过程中裂隙与土体间的水量交换过程，得出裂隙与土体间的界面渗透系数比土体饱和渗透系数大一个数量级。通过在现场建立含有三种不同覆盖层：无植物覆盖层、狗牙根草覆盖层和香根草覆盖层的试验场地，经过长时间对三种覆盖层不同深度处含水量和基质吸力的监测得到狗牙根草能够有效减少土体的渗透特性，相对于无植被覆盖层狗牙根草覆盖层渗透系数下降了一个数量级，明显提高了植被覆盖层的防渗性能，而香根草根系能显著提高土体的渗透特性，在土体中形成优势流作用。通过对现场试验场地三种不同覆盖层土体进行取样，进行室内渗透试验。基于渗透试验数据和对土样根系特征的统计，提出适用于含根系土体非饱和渗透系数计算方程，并将此方程与数值模拟结合，分析了暴雨作用下含植被覆盖层边坡稳定性问题。通过有限元软件建立了考虑植被蒸腾作用、土体蒸发作用、含根系土体渗透特性的植被覆盖层数值模型，分析了植被覆盖层长期服役性能。