A preliminary exploration of the micro-scale behaviour of capillary barrier effect using microfluidics

Capillary barrier effect (CBE) is employed in a large number of geotechnical applications to decrease deep percolation or increase slope stability. However, the micro-scale behaviour of CBE is rarely investigated, and thus hampers the scientific design of capillary barrier systems. This study uses microfluidics to explore the micro-scale behaviour of CBE. Capillarity-driven water flow processes from fine to coarse porous media with different pore topologies and sizes were performed and analysed. The experimental results demonstrate that the basic physics of CBE is the preferential water movement into the fine porous media due to the larger capillarity. The effects of CBE on water flow processes can be identified as delaying the occurrence of breakthrough into the coarse porous media and increasing the water storage of the fine porous media. The CBE can impede the increase of the normalized length and decrease the normalized width of the water front, suggesting that the two normalized parameters are potential indicators to assess the performance of CBE at micro scale. CBE can be formed in square and honeycomb networks with the ratio of coarse to fine pore throat width larger than 2.0 when gravity is neglected, and its performance can be affected by pore topology and size.

Numerical model for non-Darcy flow through coarse porous media using the moving particle simulation method

A numerical model for non-Darcy flow, which occurs when water moves through coarse porous media under high Reynolds number, is developed. The governing equation for incompressible viscous flow through porous media is composed of a continuity equation and a momentum equation, which is the Navier-Stokes equation with an additional non-linear resistance term based on Forchheimer?s law. For the discretization scheme, moving particle simulation method is employed. In order to assess the model validity, seepage experiments in different kinds of coarse porous media are implemented, and then reproducibility of the numerical results is examined. From the results, it is found that the computational flow velocities at middle part of porous media are in good agreement with experimental ones while velocities at outflow end are overestimated.

Unsaturated flow in coarse porous media

Design of effective capillary barrier systems requires a thorough understanding of the soil–water interactions that take place in both coarse- and fine-grained unsaturated soils. Experimental observations of water flow through coarse porous media are presented to gain greater understanding of the processes and mechanisms that contribute to the movement and retention of water in coarse-grained unsaturated soils. The use of pendular ring theory to describe how water is held within a porous material with relatively low volumetric water contents is explored. Experimental measurements of seepage velocity and volumetric water content were obtained for columns of 12 mm glass beads using digital videography to capture the movement of a dye tracer front at several infiltration rates. An estimated curve for hydraulic conductivity versus matric suction is shown and compared to a theoretical curve. The method is shown to provide a reasonable predictive tool.Key words: soil-water characteristic curve, hydraulic conductivity curve, water permeability function, capillary barrier, matric suction.