THE UNIQUENESS OF THE MOISTURE CHARACTERISTICS

Experiments, designed to test the non-uniqueness which has been reported in the relationship between moisture content and suction in porous materials during transient flow conditions, are escribed. When a vertical column of sand was brought to uniform moisture content and uniform suction in a draining condition by percolating a constant flow through it until apparent equilibrium conditions ensued and then placed horizontally at the same time as the flow was stopped, the suction was found to decrease with time over a period of a day, indicating that the sand was now wetting up. This decrease was smaller the drier the sand. However, when the column was left in a vertical position with the water percolating through it for three hours after apparent equilibrium conditions were reached, a small adjustment of moisture content and suction was observed; when it was then placed horizontally as in the previous experiments little decrease of suction was observed. No change in suction was observed in either case if the equilibrium condition was reached in a wetting porous material. These experimental results and those of previous workers are explained by the hypothesis that, during drainage, some water is left in pores from which the air-water interface has already retreated to drain through film flow, more slowly, subsequently to the continuous water body. Additional experiments showed that the release of entrapped air may have a small effect on the uniqueness of the moisture characteristics.