FIRST RESULTS FROM OEDOTENSIOMETRIC TESTS

An oedotensiometer was used to examine to examine the behaviour of sieved sample of a swelling soil (a vertisol) as well as of the same soils treated with solution of Na+ + Ca2+ to simulate the soil changes from excessive irrigation with brackish water. The oedometer test consisted in an infiltration of water from below through a ceramic porous plate at a feeding pressure of +10 cm water and successive drainage under a depression mostly of -112 cm of water. The rate of water entry as well as the swelling rate of the sample were monitored. Preliminary considerations regards the domains in which the shrinkage curve of a swelling soil is subdivided and make hypothesis on the swelling process expected when the infiltration from below of the sample is applied. The results support the hypothesis that when the water pressure is applied some water enters rather rapidly in the larger structural pores and is followed later by the swelling in the smaller pores, responsible for the basic domain. This first conclusion demonstrates that the assumption of a simultaneous movement of solid and liquid components in the sample, which is the base of most theoretical developments for swelling soils, cannot be accepted for the tested samples. Some cases with water clogging on the sample surface confirm a late final swelling of the soil and permitted to evaluate the hydraulic conductivity of the swollen soil. These manifestations are more evident in sodicated soils. The loading of the sample reduces the swelling of the sample and seems to reduce its permeability. The reduction of the feeding water pressure further reduces the sample swelling. The draining process from saturated soil sample shows that most of the process occurs in the large pores of the structural domain. This gives the possibility to evaluate the water diffusivity coefficient for the structural domain of the sample. In draining the soil with the highest sodication there was a variation of soil volume practically equal to the water lost, demonstrating that in this case the shrinking state of the soil was in the basic domain and the soil was in a biphasic condition (no air phase in the wetted soil part). This was the only case in which material coordinate arguments could be applied and the soil completely obeys to swelling properties in quasi equilibrium conditions.