As an improvement of the two-layer cover with capillary barrier effect (CCBE) (i.e. fine-grained geomaterial overlying a coarse-grained geomaterial), a new three-layer landfill cover system is proposed and investigated for humid climate. This new system is to add a fine-grained soil (i.e., clay) underneath a two-layer CCBE for preventing rainfall infiltration and gas emission. However, the underlying clay layer may desiccate due to vapor flow which is favored by the overlying dry coarse layer. One of the methods to reduce desiccation induced shrinkage is by utilizing soil conditioners. In recent years, the utilization of nanomaterials has gained increasing attention in a variety of applications. However, the use of nanomaterial as a soil conditioner is not that common yet.

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As an improvement of the two-layer cover with capillary barrier effect (CCBE) (i.e. fine-grained geomaterial overlying a coarse-grained geomaterial), a new three-layer landfill cover system is proposed and investigated for humid climate. This new system is to add a fine-grained soil (i.e., clay) underneath a two-layer CCBE for preventing rainfall infiltration and gas emission. However, the underlying clay layer may desiccate due to vapor flow which is favored by the overlying dry coarse layer. One of the methods to reduce desiccation induced shrinkage is by utilizing soil conditioners. In recent years, the utilization of nanomaterials has gained increasing attention in a variety of applications. However, the use of nanomaterial as a soil conditioner is not that common yet.

The feasibility and effectiveness of this three-layer cover system in preventing rainfall infiltration were investigated by conducting one-dimensional water infiltration tests. This represent the worst-case scenario for a landfill cover. Three column model tests were carried out with different geomaterials, which were silt/gravelly sand/clay, Completely Decomposed Granite (CDG, silty sand)/gravelly sand/Completely Decomposed Volcanic (CDV, silty clay) and Fine recycled Concrete (FC)/Coarse recycled Concrete (CC)/CDV. The soil columns were instrumented with tensiometers, heat dissipation matric potential sensors, and moisture probes to monitor the variations of pore-water pressure and water content with depth. The amount of water volume infiltrated into soil during ponding was also monitored. In addition, transient seepage simulations were carried out to back-analyze the test results and investigate the influence of hydraulic properties on the proposed cover. For the effects of nanomaterial on the shrinkage properties and permeability of clay, two different nanomaterials namely gamma-aluminum oxide powder (γ-Al₂O₃) and nano-copper oxide (CuO) were selected and mixed with clay at different percentages (i.e. 2%, 4% and 6%). Shrinkage tests were carried out by wax method following the ASTM D4943 standard while permeability tests were carried out in flexible wall permeameters following the ASTM D5084 standard.

Based on the experimental results of the water infiltration test, the upper two-layer CCBE of silt/gravelly sand and CDG/gravelly sand is only effective up to a rainfall of 35 and 20-year return period, respectively. Due to the presence of the bottom fine-grained soil layer of clay (saturated permeability, 5.7x10^-9 m/s) and CDV (saturated permeability, 2.4x10^-7 m/s), a rainfall of 1000 and 150-year return period is required for the initiation of water percolation, respectively. This implies that the proposed bottom layer is effective in preventing water percolation into the waste for humid climates where heavy rainfall is expected. This is consistent with the results from both numerical back-analysis and parametric studies that percolation through the proposed cover system was mainly prevented by the bottom fine-grained layer and is found to be the most important component. This study also showed that recycled concrete with sufficient contrast in permeability and particle size can be used as substitute materials for the upper two-layer of the proposed three-layer landfill cover system.

For the effects of nanomaterial on the shrinkability of clay, the measurements show that addition of 6% nano-CuO and γ-Al₂O₃ increases the shrinkage limit of clay by 17% and 8% respectively. A low shrinkage limit is usually associated with large volume change. With addition of 2% nanomaterials, a reduction in the total volume change (Δ?_TOTAL) of amended clay specimens is about 10% and 6% for nano-CuO and γ-Al₂O₃, respectively. Also, the permeability of clay decreased by about 30% and 45% with addition of 2% γ-Al₂O₃ and nano-CuO, respectively. Pore size distribution curves showed that the largest pore size reduced by 20% when clay was mixed with 4% nano-CuO. These materials certainly have the potential to be used as soil conditioners for reducing both soil shrinkage and permeability.