New Osmotic Tensiometer Development

Abstract

Abstract Climatic variations induce negative pore-water pressure or soil suction oscillations in unsaturated soil with daily and seasonal cycles. The dramatic suction changes of unsaturated soil in shallow depths can lead to engineering failures. However, suction measurements using conventional water-based tensiometers are limited to 98-kPa suction that is due to water cavitation. Therefore, they are not able to capture variations of high suction values near the ground surface. A newly proposed osmotic tensiometer was developed using a cross-linked polymer-water solution to allow the high suction measurement range and provide a reliable and consistent suction response under long-term field monitoring. The unique swelling behavior of the polymer-water solution extends the suction measurement capacity to suction values above 1.5 MPa. Laboratory verification and calibration were carried out with centrifuge (covers the lower range of suction measurement from 0.66 to 250 kPa) and WP4C potentiometer (covers the higher range of suction measurement beyond 250 kPa). The proposed osmotic tensiometer can measure rapid suction changes during drying and wetting paths. Long-term laboratory monitoring using the submerged osmotic tensiometer in a deionized water reservoir demonstrated reliable and consistent suction measurements. Osmotic pressure decay was observed after a long testing period because of the leakage of polymer particles through the high air-entry ceramic disc. A series of parametric studies was performed to explore the optimal tensiometer configuration to minimize the pressure decay. Polymer adsorption measurements from ultraviolet-visible spectrometry were used to quantify the polymer leakage. The newly proposed osmotic tensiometer is expected to capture field suction changes that are due to rainfall precipitation (wetting) and evaporation (drying) and provide long-term, accurate, and consistent suction measurements for geotechnical engineering applications.