Retention behaviour of a soil
In order to investigate the retention properties of the clay rocks a new device has been developed, a sorption bench. This device permits to applied high suction values at different temperatures by means of vapour equilibrium method. The general scheme and a picture of this device are presented in the following figures. The samples are placed in cylindrical desiccators within which a salt solution regulates the relative humidity. All these desiccators are immersed in a thermo-regulated bath.
General scheme of the sorption bench in its environment
Suction Control
Vapour equilibrium technique, which is used in the sorption bench to apply the suction, consists in controlling the relative humidity of a closed system in which the soil is immersed. By this way, soil water potential is applied by means of migration of water molecules through the vapour phase from a reference system of known potential to the soil pores, until equilibrium is achieved.
The sorption bench contains 8 desiccators. In each, a saturated salt solution regulates the relative humidity of the atmosphere. The salt saturation ensures a constant relative humidity; saturation was easily eye-checked by ensuring that solid salt was still present within the solution at the end of the test. The relative humidity of the reference system was checked by a psychrometer sensor installed in the container solution. The accuracy on the relative humidity depends on the salt used but this value is less than 0.5%.
Sorption bench
Temperature Control
In this type of experimentation the control of the test thermal conditions has to be precise because the control of the relative humidity by saturated salt solution is sensitive to temperature. In addition one of our aims is to provide an experimental tool able to determine WRC for any temperature.
Here the desiccators are immersed in a thermo-regulated bath which leads to perform tests at any temperatures in the range of 10 to 90°C. The temperature of the bath is regulated by two immersion heaters. The total heating power is 4000 W and the total rate flow is about 10-3 m3.s-1. The insulation is ensured by a 4 cm thick glass wool.
The powerful heating system, the water circulation and the high capacity of the bath (more than 0.2 m3) guarantee a good thermal accuracy of the sorption bench (less than 0.1°C).