An Improved Unsaturated Triaxial Device to Characterize the Pore-Water Pressure and Local Volume Change Behavior of a Compacted Marl

Abstract

Abstract Compacted fills in earthworks are in an unsaturated state, with suctions (negative pore-water pressures) usually ranging from several kPa to several MPa. Characterizing the variation of pore-water pressure is a major concern in the estimation of embankment stability and, at the same time, a big challenge when its value falls below –80 kPa. This technical note presents an improved triaxial system based on the standard triaxial cell, equipped with a tensiometer, a thermocouple psychrometer, and local strain gauges. The main objective of these improvements is to measure the pore-water pressure and local volume changes during isotropic loading. In this study, three compacted specimens were tested under undrained conditions. Pore-water pressure was continuously measured by means of the psychrometric probe in the range of –3,000 to –100 kPa and by the tensiometer in the range of –80 to 1,100 kPa. Volume change was measured by the local strain gauges. Analysis and development of the results from both this study and the literature lead to the conclusions that i) specimens with occluded air bubbles (e.g., at the standard Proctor optimum) behave like a saturated soil, as Bw is close to unity when pore-water pressure becomes positive, and ii) a pore-water pressure coefficient Bw close to 1 is not necessarily a sign of saturation, as soil in an unsaturated state may also have a pore-pressure coefficient close to unity, especially at the beginning of the mechanical loading.