Extending Water Retention Curves to a Quasi‐Saturated Zone Subjected to a High Water Pressure up to 1.5 Megapascals

Abstract

Core Ideas A quasi‐saturated zone with trapped air was subjected to high water pressure. Saturation was measured using a dielectric soil moisture sensor. Saturation increase due to compression of air was well described by Boyle's law. The water retention curve was extended to the positive water pressure domain. We propose a mathematical model for the water retention curve in the extended domain. On closure of an underground facility, recovery of the geoenvironmental conditions is one of the essential issues for ensuring proper implementation of closure. At the Mizunami Underground Research Laboratory, Gifu, Japan, a tunnel flooding test is underway in which a section of a deep underground tunnel is filled with groundwater and the existing desaturated zone with trapped air is subjected to high water pressure. Water saturation in the unsaturated zone as a function of capillary pressure is well understood. However, the process of a saturation increase due to a further increase in water pressure is less well studied. In this study, a simplified laboratory setup using test sands with trapped air was established in a pressure chamber. The relationship between the positive water pressure up to 1.5 MPa and saturation was measured and the effect of the compression of trapped air bubbles on the change in saturation was investigated. The experimental results showed that the saturation increased further from the initial saturation of about 0.85 at zero suction. Most of the saturation increase occurred up to a water pressure of 0.5 MPa, at which the saturation reached 0.98. At 1.5 MPa, the air bubbles were compressed to a non‐detectable level so that the sands were fully saturated. A mathematical model was established based solely on Boyle's law to define the water retention curve in a broad sense for the domain where the water pressure is positive. The measured water pressure–saturation data closely followed the relationship estimated with the established model.