Effects Of Vegetation Roots On Landfill Final Covers

Closure cover system is one of important parts of the landfill site, which located at the surface of landfills. It is the last safeguard of landfills. Landfill final covers reduce the infiltration of rain into the garbage, avoiding a sharp increase of leaches. Wastes produce a lot of hazardous substances during the degradation process. Landfill final covers block the rapid proliferation of hazardous substances, making a safe environment for the nearby residents. Vegetation could grow on the landfill final covers, which makes the surroundings more harmony.Closure cover system includes gas discharge guiding layer, clay-liner, drainage layer and plant surface soil layer. As the most important part of the landfill final cover system, clay-liner’s performance influences the long-time serving performance of landfill final covers. Many researches show that the clay-liner is easy to crack. The fissured network forms in soil mass, while cracking. Cracks, as one kind of the macro pores in soils, provide pathways for water flow, enhancing the permeability of soil mass. Cracking of the clay-liner influences the workability of landfill final covers. This influence should be further studied. In the restoration process of the landfill site, vegetation grows on the landfill final covers. While growing, vegetation generates a large underground roots system. Plant roots form a large amount of pores in soil mass. These macro pores enhance the permeability of soil mass a lot. Soil particles are cohered together by roots to improve soil’s capacity of anti-erosion and limit cracking. Vegetation roots affect landfill final covers in various ways. And this effect should be more explicit.Soil columns are compacted layer by layer. Vegetation grows in the compacted soil. Another controlled group with no vegetation in the compacted soil is set up, used to contrast the surface cracking. The binary image method and the image processing technique are adopted to analyze cracking on the surface during vegetation growing. Fissure ratios are calculated. Comparisons between the fissure ratio of the soil column with vegetation and crack and the fissure ratio of the soil column with crack but no vegetation tell that, when vegetation growing in soil mass, cracking could be limited for a certain amount.Soil column tests are applied to simulate water transport behavior in the clay-liner. Firstly, an infiltration experiment of soil column is conducted. In this soil column, no vegetation and no cracking exist. This test is to simulate the clay-liner just constructed in fields. Secondly, another soil column with surface cracking but no vegetation is set up. This soil column test aims at simulating the clay-liner that is not protected by the plant surface soil layer and has gone through some climatic and moisture changes. Thirdly, vegetation grows in the compacted soil. While vegetation growing, some cracks develop on the surface of the compacted soil. Then, an infiltration experiment of this soil column is conducted. This soil column test means to simulate the clay-liner that is covered by the plant surface soil layer and cracks on the surface, after experiencing climatic changes. Fourthly, one infiltration experiment of soil column with vegetation roots but no cracking is conducted. And this soil column test is to simulate the clay-liner that is protected by the plant surface layer and nearly dose not crack on the surface. The results of the soil column tests show that cracks and vegetation roots provide preferential pathways for water flow, enhancing the permeability of soil mass.The tracing technology is applied to observe water flow along vegetation roots in the soil column with vegetation roots but no crack. The brilliant blue is used as the dyeing tracer. Qualitative assessments are conducted to evaluate water preferential flow along vegetation roots. With the depth increasing, the stained area is decreasing gradually. In the soil mass adjacent to vegetation roots, the dyeing tracer transport more rapidly. Preferential flow along vegetation roots is not negligible.