Quantification of Air Diffusion Through High Air-Entry Ceramic Disks

Suction control for testing unsaturated soils requires the use of high air-entry ceramic disks (HAED). HAEDs provide a medium for water in a soil specimen to migrate as required regardless of the applied pore air pressure. Even though the HAEDs are rated for a specific bubbling pressure, some air diffuses through the HAEDs, even at pressures below the rated capacity. Diffused air has a tendency to accumulate underneath the HAEDs resulting in an error in the specimen water content measurements. Diffused air has the potential to de-saturate the bottom of the HAED thereby retarding the flow of water through the disk. In this study, commonly used HAEDs were tested with the Fredlund soil-water characteristic curve (SWCC) device to measure the diffusion rate at different pressures. Three-inch diameter HAEDs rated at 100, 300, 500, and 1,500 kPa bubbling pressures were tested at different percentages of their rated pressures (e.g. 50 percent, 95 percent) to determine the diffusion rate. Several HAEDs from the same manufacturer were tested for each bubbling pressure. The hydraulic conductivity of the HAEDs was also measured at different testing stages to determine the effects of air accumulation underneath HAED disks. Diffused air volume rates are presented graphically. Conclusions and recommendations for the frequency of flushing are also presented. In summary, the amount of diffused air generated when using 1- or 3-bar, 5-bar, and 15-bar is: negligible, small, and large, respectively.