Capillary Barrier Performance

Capillary barriers typically consist of two layers of granular materials designed so that the contrast in hydrologic properties and sloping interface between the layers keeps infiltrating water in the upper layer. We carried out two bench-top capillary barrier experiments, identical except for the coarse material used in the lower layer.  These experiments were conducted to better understand the behavior of capillary barriers in particular with respect to an engineered barrier system at the potential high-level nuclear waste site at Yucca Mountain, Nevada. We measured hydrologic parameters for both coarse materials using standard methods and found that both materials, although morphologically different (rounds, uniform vs. angular, non-uniform), had very similar hydrologic properties. The rounded sand provided a better functioning capillary barrier than the angular sand, but neither experiment could be characterized as a perfectly working capillary barrier.  Water infiltrated the lower coarse material in both experiments. Nevertheless, in both cases, more than 93% of the infiltrating water was successfully diverted from the lower layer. Our experimental results show that capillary barrier design based on standard hydrologic property measurements might result in incorrect prediction of the system behavior. Moreover, our numerical simulations of these experiments showed that predicted capillary barrier performance was highly sensitive to small changes in model hydrologic parameter values, which were within the likely uncertainty and variation of these values. Based on this work, we believe that other non-continuum processes such as vapor diffusion and film flow could contribute to the observed phenomena, and that these are important aspects to consider with respect to capillary barrier design.

Fine sand over angular coarse sand  (with large surface area)	Fine sand over rounded coarse sand (typical quartz sand surface area)

Many studies show that vapor will condense, i.e., adsorb to porous materials even though the porous material has the same temperature as the vapor. Kutilek and Nielsen [1994] stated that the nature of soil water adsorption through hygroscopicity is completely different from the simple process of vapor condensation to its liquid phase. Parker [1986] explains that when a dry hydrophilic porous medium is placed in an atmosphere containing water vapor, isothermal water adsorption will increase with increasing vapor pressure until the pore space becomes fluid-filled. The adsorption phenomena are generally classified as being either physical (based on electrostatic and van der Waals attraction forces between the solid surface and water molecules), or chemical (based on rearrangement of electrons and consequent formation of strong chemical bonds), Parker [1986], Nitao and Bear [1996]. The adsorbed water layers coating the solid grains grow into films, and eventually adsorbed films in adjacent pore spaces will coalesce and form a continuous liquid phase in the pore. This process is generally referred to as capillary condensation, e.g. Derjaguin and Churaev [1976], Tuller et al. [1999]. For a system of two surfaces (or a slit) a critical film thickness will develop at which the adsorbed films on the opposing surfaces will become unstable, grow without bounds and coalesce, Christenson [1994], Iwamatsu and Horii [1996], Tuller et al. [1999], and Kohonen and Chistenson [2000]. According to Easton and Machin [2000] there is no well-defined limit to the amount of vapor that can be absorbed for a wetting fluid, however, Tokunaga and Wan [2001] suggest that they range in thickness from tens of nanometers to ~1mm. The adsorption of water vapor is aided by the vapor pressure deficiency, which exists over a concave surface (air-water meniscus in a pore) compared to the vapor pressure over a free, flat water surface, Bear [1988].  The adsorbed film layers are known to have different fluid structure (e.g. density, viscosity) than the bulk fluid, Nitao and Bear [1996], Miyahara et al. [1997], and Muller [1998].  In our capillary barrier experiments, a scenario as that illustrated in the above figure could be responsible for the observed differences in model and experimental results. Initially, water vapor flows through the sand and some of the vapor adsorbs on the grains, while the rest flows or diffuses through the open space between grains, Tzevelekos et al. [2000], Figure a and b. At increasing vapor pressures water will continue to be adsorbed onto the grains in multiple layers as long as sufficient water vapor is provided by vapor diffusion for this process to take place, Figure c. Eventually the films on adjacent grains coalesce in capillary condensation, initially in the finer pores (Figure d) resulting in enhanced permeability and actual capillary action. Eventually, also the larger pores will fill and contribute to the permeability (Figure e and f).

See our AGU Fall 2001 presentation for more information.