A Modified Osmotic Diaphragmatic Oedometer for Investigating the Hydro-Mechanical Response of Unsaturated Soils

In this paper, an osmotic diaphragmatic oedometer was introduced to determine the hydromechanical behavior and the coefficient of lateral pressure of unsaturated expansive soils at constant suction values. Adopting the osmotic method to impose the matric suction, the modified oedometer apparatus accommodated the circulation of the polyethylene glycol (PEG) solution beneath the soil sample. A semi-permeable membrane was introduced between the soil and PEG solution to prevent the PEG swap between the solution and the soil. Three very thin circular diaphragms were engraved in the approximately rigid ring of the oedometer and three linear variable differential transformers (LVDTs) were installed horizontally in contact with them to measure the horizontal deformation of the diaphragms. The diaphragms were calibrated using known water isotropic pressures and the horizontal pressures and their corresponding deformations were correlated. Vertical displacements of the studied soil and horizontal deformations of the diaphragms were recorded using an auto logging device during the consolidation process. Consolidation response and the coefficient of the at-rest lateral pressure of an artificially prepared sand-bentonite mixture were determined utilizing the modified oedometer at various matric suctions. Drying-wetting cycles followed by constant suction consolidation tests were imposed on the studied soil. Water retention curve of the soil was determined and a novel approach was employed to calculate the effective stress from the volume change behavior of the samples. Experimental results showed that the amount of suction hardening is negligible for the dynamically compacted studied soil. In addition, K0-parameter decreases significantly with the suction increase and drying-wetting cycles stiffen the soil and reduce the amount of the horizontal pressure of the soil specimen.