We are evaluating two engineered cover designs: a GCL on asphalt and a capillary barrier (Figure 3). Although the GCL/asphalt barrier resembles traditional resistive barriers, the use of asphalt instead of compacted clay should avoid problems with desiccation and root penetration. Both cover systems are 3 m thick and were constructed to grade by backfilling an excavated trench with 0.15-m lifts. The surface of each lift slopes at 2 percent (Figure 4). The asphalt cover design consists of 0.3 m of top soil, 1 m of compacted native sediment, a geosynthetic clay liner (GCL, bentofix), a 5-cm asphaltic surface on 20 cm of apshaltic concrete, and 1.46 m of compacted native sediment (Figure 4). Native sediments were compacted to obtain a hydraulic conductivity <10-5 cm s-1. The capillary barrier design consists of 0.3 m of top soil, 1.4 m of compacted native sediments, 0.3 m of clayey sand, 0.9 m of gravel, and 0.15 m of sand material (Figure 4). All fine texture materials were compacted to 90 percent of modified Proctor density (ASTM D-1557-91). Both covers were vegetated by native grasses in the summer of 1998 and irrigated, and a pea-gravel mulch was tilled into the upper 20 to 25 cm of top soil to minimize erosion. Test plots will be subjected to natural and enhanced precipitation conditions. These experiments result in a total of four plots (Figure 3): irrigated- and nonirrigated GCL/asphalt- and capillary-barrier deigns.

Each plot is 17 m2 and separated from adjacent plots by a berm and a ridge line. Berms on the GCL/asphalt were inset from the surface berms and create a 15-m2 drainage area. The two cover designs are separated in the subsurface below the level of the asphalt by a vertical polyethylene liner. The two designs differ in thickness of native sediments above the barriers (1.3 m above the asphalt and 2.0 m above the capillary barrier), affecting water-retention capacities of the covers.

Figure 3. Plan view of the test plots indicating the relative positions of the instrument silo and the instrument signal cable trees. Instrument cables are routed from the trees to the silo through conduits located about 0.6 m below the surface. Each plot has a 17-m2 surface area.


Figure 4. Cross-sectional diagram of the GCL/asphalt- and capillary-barrier designs. Columns of small circle symbols indicate approximate depths where thermocouple psychrometers (TCP) are installed. Larger circle symbols indicate approximate depths of the horizontal neutron probe access tubes (NPAT).

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