Engineers Australia

What is the total suction of soil at oven dry?

N. S. Beal


The values of total suction (moisture potential) ascribed to the oven dry condition by various authors are reviewed. An explicit calculation of the limiting total suction imposed by a drying oven is described. The variables are the oven temperature together with the relative humidity and temperature of the ambient air that is necessarily admitted into the oven. The calculation requires reference to psychrometric relationships between relative humidity, temperature and humidity ratio. These relationships are accessible in commonly available psychrometric charts as well as in other forms. For typical laboratory conditions, the limiting total suction imposed by a drying oven will fall in the range 5.8 to 6.0 logkPa (6.8 to 7.0 pF). The direction and magnitude of the change in total suction that occurs when an oven dried soil sample cools from oven temperature to room temperature is also considered by reference to published experimental data and the known temperature trends of the physical phenomena associated with suction. A small set of experimental measurements of the humidity over soil samples cooled from near oven dry to room temperature are reported. These experiments were aimed at the narrow question of the direction and magnitude of the change in the soil suction upon cooling from drying oven temperature. Some published data on the suction attained when a soil is dried to zero moisture content by a room temperature process are also reviewed.