The Influence of Hydraulic Nonequilibrium on Pressure Plate Data

Abstract

Pressure plates are used routinely to measure water retention characteristics of soils. Plates of varying porosity are used depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15-bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>15 bar), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15-bar plate is less than 1x10-13 m s-1. Low plate conductance coupled with decreasing soil hydraulic conductivities (e.g., <1x10-13 m s-1) at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay placed on 15-bar pressure plates for 10 days or longer, with and without static loads and with and without use of a kaolinite slurry for improved plate contact. Total matric suctions, inferred from peltier psychrometry data, were always less than 1.5 MPa. When sample height was increased from 1.5 cm to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods to pressuremore » plates may be require to measure equilibrium water suctions of soils in reasonable times in the 15-bar pressure range and that loading of the samples and use of kaolinite appear to be ineffective in speeding equilibrium.« less