Case History on Measuring Field Performance of Compacted Clay Liner
This case history illustrates the use of hydraulic conductivity measurements in the field to demonstrate the short-term performance of a compacted clay liner. The field data come from a test pad constructed to evaluate a compacted clay liner for a proposed hazardous waste landfill near Phoenix, Arizona (Gilbert, unpublished). The compacted clay liner was a soil-bentonite admixture, and the test method was a sealed double-ring infiltrometer (Daniel, 1989; ASTM D 3385-88). A 1.5-m square “ring” was placed on top of the liner and filled with water to a depth of 0.3 m. The infiltration rate (the volumetric rate of flow per unit area seeping into the top of the clay liner under a constant head) was measured by tracking the mass of water lost from the ring as a function of time. The infiltration rate in Figure 4.1 was calculated assuming that all of the water lost was infiltrated down into the clay liner. Tensiometers in the clay liner at depths of 0.15, 0.3, and 0.45 m were used to track the change in matric suction, and therefore pore water pressure, with depth and time as the water infiltrated the liner. When the measured matric suction is zero in a tensiometer, the hydraulic gradient can be estimated by assuming the pore water pressure is equal to zero at the elevation of the tip of the tensiometer.
The infiltration rate, q, is related to the hydraulic conductivity as follows: q = ki, where k is the hydraulic conductivity and i is the hydraulic gradient. Hence, the hydraulic conductivity in Figure 4.1 is calculated by dividing the infiltration rate by the estimated gradient. The calculated or “measured” hydraulic conductivity changes with time for several reasons: (1) the soil swells during the test, so some of the flow lost from the inner ring goes into storage and does not infiltrate through the clay liner; (2) the depth of soil used to estimate the hydraulic conductivity increases with time and the soil displays variability with depth (such as layering); and (3) the hydraulic gradient changes rapidly with time and is more difficult to estimate at the beginning of the test. The reported value is typically taken from the last reading, a hydraulic conductivity between 2 × 10−10 and 3 × 10−10 m/s in this case. Note that this hydraulic conductivity does not necessarily correspond to saturated conditions because of the possible existence of entrapped air, but the measured value probably corresponds reasonably well to the field value since the test conditions are similar to those expected in the field except for the lack of overburden stress on the clay liner. In the field application the applied stress from waste in the actual landfill will tend to increase the degree of saturation and reduce the saturated hydraulic conductivity due to consolidation.
The saturated hydraulic conductivity was measured in the laboratory on small-scale (64-mm-diameter), thin-walled tube samples taken from the clay liner at the end of construction. The laboratory measurements, which correspond to saturated conditions, ranged from 1 × 10−10 m/s to 7 × 10−10 m/s with a geometric mean of 3 × 10−10 m/s. Therefore, in this case, the hydraulic conductivity measured at the field scale (a 1.5-m square area), between 2 × 10−10 and 3 × 10−10 m/s, was both comparable to the laboratory measurements on small-scale samples under saturated conditions and less than the regulatory-specified maximum of 1 × 10−9 m/s.