Anisotropy of unsaturated hydraulic properties of compacted mineral capping systems seven years after construction

Abstract

The mechanical compaction of soil material of mineral landfill systems affects the continuity and connectivity of the complex soil pore network. A horizontally-oriented layering is intended to generate a slope-induced lateral water flow out of mineral capping systems that is sufficient to minimise the statutory required vertical percolation through the underlying waste body and the potential leachate.In this case, soil compaction affects both the porosity and water retention as capacity values and the hydraulic conductivity as intensity parameter. The idea of this study was to combine information on both soil properties in an extended anisotropy factor based on the soil water diffusivity. The analysis is focused on the direction-dependent soil hydraulic properties of a mechanically compacted landfill capping system.In particular, the volume fractions were related to the fractional capillary potential for each of the characteristic pore size classes. Three different soil profiles of top, middle, and bottom slopes of the mineral capping system of the Rastorf landfill in Northern Germany were sampled seven years after construction. Undisturbed soil cores of 100 cm3 and 438 cm3 were extracted in vertical (ver) and horizontal directions (hor) in depths of 20, 50, and 80 cm representing the main layers. The soil water retention and unsaturated hydraulic conductivity, K, functions were determined by suctions plates, permeameter, and the evaporation method. In the coarse pores range (pressure head values of h ≥ −300 hPa), the standard anisotropy ratio, AR, (K(Se)hor/K(Se)ver) as a function of effective saturation, Se, in the sealing liner in 80 cm depth was larger than 1, indicating higher horizontal than vertical K(Se) values. Thus, AR-values above 1 in the range close to water saturation especially in 80 cm depth suggest the tendency of lateral water flow out of mineral capping system due to a sufficient hydraulic potential and thus its reasonable functionality, even seven years after construction.The anisotropy factor was extended in two steps; for AR* and AR**, the pore size class-related matric flux potential, ϕ, and the soil water diffusivity, D(θ), were proposed to combine intensity parameters with capacity-based volume fractions of pore size classes and the fractional capillary potential. The ϕ- and D(θ)-weighted anisotropy ratios, AR* and AR**, indicate that anisotropy increases with the volume fraction of macropores (r2AR* of 0.69−0.77; r2AR** of 0.71−0.80) and wide coarse pores (r2AR* of 0.57−0.78; r2AR** of 0.79−0.89) in both directions. The results suggest that by combining both the intensity and the capacity parameters of the soil hydraulic properties in an extended anisotropy ratio improves the information on compacted mineral capping systems.