In some cases it may be acceptable to use NRCS estimates of soil permeability after confirming the actual presence of the specific soil on the site during a field investigation. This should be sufficient for pond and OF systems on soils with
in/h 0.1 0.2 0.3 0.4 0.6 0.8 1 cm/h 0.25 0.5 0.8 1 1.5 2 2.5
20 30 40 60 80100 50 80 100 150 200 250
FIGURE 2.4 General relationship between specific yield and hydraulic conductivity for fine-textured soils.
naturally low permeability. Concepts where water flow in the soil is a major design consideration will require field and possibly laboratory testing. Infiltration testing in the field is recommended where infiltration rates are critical to the design. A variety of testing methods are available, as shown in Table 2.20, to measure surface infiltration rates. The reliability of test results is a function of the test area and the zone of subsurface material influenced. This relationship is shown in Table 2.20 by the volume of water required to conduct a single test. As indicated in Chapter 8, the increased confidence resulting from larger scale field tests allows a reduction in the safety factor for the design of some land treatment systems.
A basin test area of at least 75 ft2 (7 m2) is suggested for all projects where infiltration and percolation of water are design expectations. The area can be surrounded by a low earthen berm with an impermeable plastic cover, or aluminum flashing can be partially set into the soil in a circular configuration to define the test area. The use of a bentonite seal around the aluminum flashing perimeter is recommended to prevent leakage of water. Tensiometers at a depth of 6 in. (15 cm) and 12 in. (30 cm) can be installed near the center of the circle to define saturated conditions at these depths as the test progresses (see Figure 2.5). The test basin should be flooded several times to ensure saturated conditions and to calibrate any instrumentation. The actual test run should be completed within 24 hr of the preliminary trials. This final test run may require 3 to 8 hr for coarse-textured soils.
The water level in the basin is observed and recorded with time. These values are plotted as intake rate vs. time. This intake rate will be relatively high, initially, and then will drop off with time. The test must continue until the intake rate
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