Experimental Validation of Terzaghi’s Effective Stress Principle for Gassy Sand

Gassy soils are a special case of unsaturated soils, for which the gas phase exists in the form of occluded bubbles of a size that allows the bubbles to fit within the void spaces without distorting the soil structure. The applicability of the original form of Terzaghi’s effective stress principle was tested for this class of soils. A series of globally undrained tests on gassy specimens, as well as drained effective stress path (ESP) tests on fully saturated specimens, was presented to validate this hypothesis. In a gassy test, the effective stresses were computed with the assumption that the Terzaghi’s effective stress principle was valid. In an ESP test, the effective stress path obtained from the experiment on an identical gassy specimen was applied to a saturated specimen with the drainage lines opened. Two groups of tests covering a degree of saturation ($S_r$) from 92 to 100% were performed. The application of the same effective stress path in the gassy and the ESP tests resulted in the same axial versus volumetric strain response, and therefore it was in accordance with the postulated validity of the Terzaghi’s effective stress principle under gassy states. Such results were found to be valid within the entire range of degrees of saturation explored in this study, and they provide new support for the use of the original definition of Terzaghi’s effective stress in geotechnical analyses for gassy deposits.