Capillary Barriers and Subtitle D Covers: Estimating Equivalency

Accumulating data on traditional compacted soil-surface covers are demonstrating that they are likely to degrade and have reduced effectiveness as long-term barriers; therefore, suitable alternatives are being examined. One possible alternative that is receiving increased attention is capillary barriers. The U.S. Environmental Protection Agency (USEPA) allows for alternatives to be used, but requires that they achieve infiltration and erosion protection equivalent to that of designs contained in design guidance documents. A method of comparing a capillary barrier to a design that features a compacted soil layer that meets the minimum requirements for a solid-waste landfill cover (so-called Subtitle D) under identical, transient conditions is introduced in the present paper, allowing equivalency to be demonstrated. The approach uses daily climatic data rather than monthly or yearly averages, which can provide misleading results. The concept of adding a "transport layer" at the fine/coarse interface of the capillary barrier to laterally drain water and reduce the moisture content is also presented. Numerical modeling results for a variety of climates show that the capillary barriers may be equivalent (or better) compared to a Subtitle D cover at many locations. The inclusion of a transport layer may significantly improve capillary barrier performance.