A New Multiphase Apparatus for Testing Soil Permeated with Three Immiscible Fluids

Oil-contaminated unsaturated soil is a complex multiphase material. In this study, a new triaxial apparatus was developed to independently control/measure its air, oil, and water pressures. Therefore, the suctions associated with air–water, air–oil, and oil–water interfaces can be determined. The apparatus utilizes the hanging column and axis-translation techniques to control suctions in 0∼5 kPa and 5∼600 kPa. It is suitable for investigating water/oil retention, isotropic compression, and triaxial shear behavior of oil-contaminated soil. Using this new apparatus, five tests were conducted to analyze the saturation–pressure (S–P) relationship of sand and sandy silt at various oil and water contents and to evaluate the validity of the axis-translation technique for soil permeated with three nonimmersible fluids. Results showed that the artificial increase of air pressure in the axis-translation technique does not obviously alter the air–water and air–oil suctions at constant oil and water contents. A popular suction–saturation model, which was proposed based on test results of clean sands, showed a good prediction of the air–oil suction. However, the oil–water suction results of sandy silt do not align with the model prediction, suggesting that the model may need to be revised for soils with some silt and clay particles.