Preferential flow in vertically oriented, unsaturated soil layers
Abstract
Preferential flow paths develop where particular areas of a geologic profile become more conductive than the surrounding material. Research has been conducted in recent years on preferential flow and the flow of water through unsaturated soils. Unfortunately, the research has been completed by different disciplines of science and engineering, resulting in a wide range of terminology and research objectives.
A field program conducted during the excavation of a large waste rock pile at Golden Sunlight Mine in Montana defined the structure of the pile to consist of steeply-dipping, fine and coarse layers. The fine layers located near the top of the pile were wet and oxidized while the coarse layers were dry and unoxidized. The results of the field program indicated the development of preferential pathways through the fine-grained layers.
A column study was developed to investigate the potential development of preferential flow in vertically layered, unsaturated systems. To achieve this objective, a column was constructed that enabled the amount of lateral flow between two adjacent materials to be quantified and related to the applied surface flux and the hydraulic properties of the individual materials under steady-state conditions.
The results of the column study and subsequent numerical modelling program showed that water prefers to flow where water exists. In unsaturated systems, a fine-grained soil has smaller interparticle voids and is able to maintain fluid-filled pores at suctions greater than that of a coarse-grained material. Once suctions exceed the air entry value of the material the largest voids begin to drain, air enters the system and the hydraulic conductivity decreases. The decrease in hydraulic conductivity with increased suctions for each material is dependent on the distribution of pore sizes. In an unsaturated system it is this mechanism of decreasing hydraulic conductivity with increasing suction that can result in a fine-grained material becoming more conductive than a coarse-grained material.
When a surface flux is applied to a vertically layered, unsaturated system under steady state conditions, the preferential flow path is determined by the relationship between the applied surface flux rate and the saturated hydraulic conductivity of the fine layer. If the applied flux rate is greater than the saturated hydraulic conductivity of the fine material, the equilibrium suction that forms within the column results in the coarse layer becoming the preferential flow path. Reducing the surface flux to a rate less than the saturated hydraulic conductivity of the fine material results in an equilibrium suction where the fine layer becomes the path of preferential flow. It is critical that the interaction between the hydraulic properties of materials within a system be quantified in order to predict the behaviour of the system.
Following the analysis of the fine and coarse sand column, another column was constructed using fine and coarse waste rock. The results from the second column experiment showed that when the applied surface flux was reduced to a rate of 5.56 x 10-8 m/s (i.e., 1753 mm/year), the fine waste rock layer became the path of preferential flow.