Comparison of high capacity tensiometer designs for long-term suction measurements

Abstract

This paper investigates the long-term measurement of negative (tensile) pore-water pressures in soils by using high capacity tensiometers (HCTs). Seven different HCT prototypes were designed and manufactured by using different porous filters, pressure transducers, water reservoirs and protective casings. The ability of these prototypes to record negative pore-water pressures over long times was initially assessed by a series of measurements on small clay samples equalised at different suction levels. These tests were followed by two larger scale experiments in which four HCT prototypes were simultaneously installed inside a lysimeter filled with a sandy soil alongside two standard dielectric permittivity sensors measuring suction and water content, respectively. In one experiment, the soil was left to dry until all four HCTs cavitated while, in another test, the soil was allowed to dry up to an intermediate level of suction before triggering a rainfall, after which the soil was left to dry again until all four HCTs cavitated. In each lysimeter experiment, the readings of the HCTs were reasonably consistent, which suggests that sensor design has little effect on the accuracy of measurements. These experiments also indicated that HCTs are more accurate and exhibit a faster response than standard dielectric permittivity sensors. Moreover, the HCTs incorporating a small water reservoir showed a greater ability to sustain suction over long period of time without cavitating.