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 (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15‐bar plate is <3 × 10−11 m s−1 Low plate conductance coupled with decreasing soil hydraulic conductivities 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 d or longer, with and without static loads and with and without using a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always <1.0 MPa. When sample height was increased from 1.5 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 other than pressure plates may be required to measure equilibrium water suctions of soils in reasonable times in the 1.5‐MPa (15‐bar) pressure range and that loading of the samples and use of kaolinite slurry appear to be ineffective in speeding equilibrium.

Equilibrium, non-equilibrium and residual water: Consequences for soil water retention

“…The point for log P a = 4.2 shows that this soil is not in thermodynamic equilibrium because it is above the GG curve. This effect appears to be the same as that which was described as a problem by Bittelli andFlury (2009), Cresswell et al (2008) and Gee et al (2002).…”

“…In particular, the use of the pressure cell apparatus, usually using porous ceramic plates (but sometimes membranes), has been found to be problematic. At high cell pressures, corresponding to low values of water potential or large values of pore water suction, it has been found that the water in the soil samples does not equilibrate with the air pressure in the cell as expected (Bittelli and Flury, 2009;Cresswell et al, 2008;Gee et al, 2002). In these cases, the expected water potential was the cell pressure and the value of potential reached was measured by psychrometry.…”

“…It has been suggested that it may be better to de-water soil samples on pressure plates at an applied air pressure of P a = 15000 hPa, and then to measure the potential of the residual water using psychrometry (Bittelli and Flury, 2009;Cresswell et al, 2008;Gee et al, 2002). However, we envisage that isolated micro-bodies of water can be left behind in a heterogeneous soil structure over the whole range of pressures up to the maximum value applied.…”

“…Several possible causes have been proposed for this effect including: poor contact between the soil samples and the porous ceramic plate, blocking of the pores in the ceramic plate and insufficient time allowed for equilibration. Gee et al (2002) found that improving the soil-to-plate contact either by loading the samples with weights or by the use of a kaolin slurry between the samples and the plate had no effect.…”

Measurement of Matric Suction Using Tensiometric and Axis Translation Techniques

“…The ASTM D-6836-02 standard establishes that the equilibrium time should be monitored according to the level of suction and states that for suction less than 500 kPa it is considered equilibrated the system that does not drain any water for at least 24 h, for suction between 500 and 1,000 kPa this time should be 48 h and for suction greater than 1,000 kPa no water should be drained for at least 96 h. Many authors monitored the equilibrium time behaviour, some of them reporting that the equilibrium is not reached (e.g. Gee et al 2002) and others reporting that the equilibrium is reached (e.g. Leong et al 2004).…”