On the equilibrium of suction and pressure plate tests

Abstract

Pressure and suction plate tests are among the most common techniques for determining the soil–water characteristic curve. The required testing period depends on many parameters and equilibrium times may vary from a few minutes to several weeks. This paper presents a numerical evaluation of the equilibrium process, considering the influence of soil type, equipment specifications, the use of thin high air-entry value (HAEV) membranes, and the employment of the interrupted outflow method (IOM). A fully coupled numerical model was developed and verified for the analysis of the conservation and flow of water and air in the specimen-HAEV interface system. Verification and sensitivity analyses showed, however, that the suction and pressure plate tests may be adequately represented using a simplified decoupled version of the full model. The results of the sensitivity analysis showed not only the role of the HAEV interface characteristics, but also the effect of soil type, suction value, suction step magnitude, and the suction path. It was found that the IOM offers greater control over testing times, but may cause undesirable suction reversals within the specimen. Finally, a simple method is presented to estimate the equilibrium times using basic testing parameters.