Study of Equilibration Time in the Pressure Plate

The retention curve is an important tool in the analysis of the behavior of unsaturated soils. Its result has been used in models to forecast soil shear strength envelope and in problems that involve water flow. Several types of equipments and techniques have been used for the determination of the soil water retention curve, such as the suction plate, the pressure plate and the filter paper. In this article a study is presented on the equilibration time for the suction imposed on the pressure plate apparatus. Through the use of this equipment, the axis translation technique may be applied, imposing matric suction on soils specimens assembled on the ceramic disk. Usually, suction equilibration time is determined by the observation of the variation of the water level in a burette connected to the ceramic disk. However, it is not always possible to determine the instant of full stabilization. From a gneiss residual soil, specimens have been statically compacted, at different region of the compaction curve: the dry branch, the optimum point, and the wet branch. Equilibration time has been overseen by means of the observation of the water level in the burette, and by the variation of the gravimetric moisture content of the soils specimens. Suction was also measured using a high capacity tensiometer. The results seem to demonstrate the effect of the water exchange between the ceramic disk, the soil specimens and the internal environment of the pressure plate, and it justifies the asymptotic behavior generally observed.