Soil bioengineering using vegetation has been recognised to improve the sustainable performance of landfill covers. To promote plant growth, biochar is widely used as a soil amendment. Recent studies have mainly focused on the influence of vegetation and its interactions with biochar on the hydrological performance of a single-layer soil cover system. However, their effects on water infiltration into layered soil systems, such as a novel three-layer landfill cover, are not clear. In this thesis, an integrated research approach, including full-scale field monitoring, laboratory test and numerical modelling, was conducted to (i) assess the four-year field performance of a novel bare and grassed three-layer landfill cover system, (ii) investigate the biochar effects on plant characteristic...[ Read more ]

Soil bioengineering using vegetation has been recognised to improve the sustainable performance of landfill covers. To promote plant growth, biochar is widely used as a soil amendment. Recent studies have mainly focused on the influence of vegetation and its interactions with biochar on the hydrological performance of a single-layer soil cover system. However, their effects on water infiltration into layered soil systems, such as a novel three-layer landfill cover, are not clear. In this thesis, an integrated research approach, including full-scale field monitoring, laboratory test and numerical modelling, was conducted to (i) assess the four-year field performance of a novel bare and grassed three-layer landfill cover system, (ii) investigate the biochar effects on plant characteristics and soil water retention curve (SWRC) and (iii) study the influence of plant-biochar interactions on water infiltration into the three-layer landfill cover system.

A novel bioengineered three-layer landfill cover system was constructed at the Xiaping municipal solid waste landfill in Shenzhen, China. The site was under humid climates. A slope area with an inclination angle of 30° and a flat ground area were constructed. Sieved completely decomposed granite (CDG) and coarse recycled concrete were used for the lowest and intermediate soil layers for both slope and flat ground areas. The top layer of the slope area was compacted with unsieved CDG, while that of the flat area was filled with unsieved CDG amended with different biochar contents (i.e., 0%, 5%, and 10%). Cynodon dactylon (Bermuda grass) was transplanted on the whole flat ground as well as the half of the slope area. Then, in the laboratory, soil columns vegetated with a shrub species, Schefflera arboricola, were used to study the influence of biochar-plant interactions on soil water retention ability and water infiltration. Furthermore, a new SWRC model considering plant-biochar interactions was developed. Finally, transient seepage analyses were carried out to interpret the field test results and investigate the influence of rainfall intensity on the proposed cover system.

The field monitoring results show that grass can preserve low pore-water pressure in the landfill cover system after heavy rainfall. The grassed cover system at both slope inclinations (0° and 30°) performed satisfactorily in preventing water infiltration under humid climate condition. The middle coarse recycled concrete layer minimized water infiltration into the bottom layer by increasing up to 60% lateral diversion. At the end of 4-year monitoring, the presence of grass reduced cumulative percolation through the three-layer landfill cover system by up to 14%. Grass-biochar interactions further reduced cumulative percolation by up to 20%. The difference between grassed cover systems amended with 5% and 10% biochar contents was negligible. Measured annual percolation through the landfill cover systems met the recommended criterion of 30 mm/year by USEPA. Numerical analysis results are consistent with the findings in field tests and demonstrates the effectiveness of the cover system even under 100-year return period rainfall. Both the field and numerical results indicate that the novel bioengineered three-layer landfill cover system is a promising alternative for humid climate regions where annual rainfall is about 3000 mm.

Biochar enhances the soil water retention capacity by increasing air entry value (AEV) and saturated water content, hence promoting the growth of grass and shrub leaf and root by 26%-80%. The developed new SWRC model for biochar-amended soil can capture the increase of soil void ratio and AEV due to the presence of biochar intrapore and interpore structure. By coupling the model with the existing SWRC model for rooted soil, the effects of shrub- or grass-biochar interactions on SWRC in sandy soil can be well predicted, with the maximum discrepancy less than 10%.