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Capillary barriers, consisting of fine-over-coarse soil layers, are being considered as an alternative cover component for waste-disposal facilities, especially in dry climates. Infiltrating water is removed from the fine layer by evaporation or transpiration, or percolation into the coarse layer (failure). If the fine-coarse interface is sloped, water in the fine layer can also drain laterally under unsaturated conditions. The effectiveness of two capillary barriers in laterally diverting water was tested. The barriers were 7 m long and 1.2 m thick, built on a 10% slope. One had a homogeneous fine layer, while the fine layer of the other was layered to increase its ability to divert water laterally. The barriers were first subjected to constant infiltration (10 mm/day) followed by exposure to ambient climatic conditions. The layered capillary barrier was successful in laterally diverting water near the interface and did not permit any water to enter the coarse layer. In contrast, the homogeneous capillary barrier failed over its entire length. These results indicate that a significant lateral diversion capacity can be designed into capillary barriers, greatly increasing their effectiveness.

The Geocomposite Capillary Barrier Drain (GCBD) has been developed and tested to prevent positive pore pressures from developing by laterally draining water while it is still in tension. The GCBD consists of two key layers that function as long as the water pressures in the system remain negative: (1) a transport layer that laterally drains water and (2) a capillary barrier layer that prevents water from moving downward. prototype GCBD systems have been tested in a 3 m long lateral drainage test apparatus. For most test conditions, the GCBD systems drained water under negative pressures at a rate sufficient to prevent any positive water pressures from developing in the overlying soil. Further, the drain system served as a barrier as it prevented downward flowing water from moving into the underlying soil.

Ross` solution for the divergence capacity of a capillary barrier is modified to incorporate constant hydraulic conductivity anisotropy of the fine layer. The analysis assumes steady infiltration and that the principal directions of the hydraulic conductivity tensor are parallel and normal to the fine-coarse contact. The solutions for divergence capacity and length have two terms. The first term, similar to the solution of Ross, represents the diversion due to the presence of the capillary barrier and is proportional to the hydraulic conductivity parallel to the contact. The second term, which arises from infiltration into an anisotropic medium independent of the capillary barrier, is a function of the fine layer thickness and will be small for engineered barriers. Thus an anisotropic fine layer with a greater hydraulic conductivity in the direction parallel to the fine-coarse contact will extend the performance of a capillary barrier as measured by its divergence capacity and length. 6 refs., 2 figs.