Development of a New Tablet-Filled Osmotic Tensiometer for High Soil Suction Measurement

The development of reliable instruments for the measurement of soil suction is important for the study of unsaturated soil mechanics. This paper proposes a method to prepare new polyacrylamide (PAM) tablets based on free radical polymerization and uses the proposed PAM tablet to develop a new type of osmotic tensiometer (OT) (i.e., the PAM tablet–filled OT) for high soil suction measurement. Three batches of PAM tablets with different dimensions, mass, and swelling capacity were synthesized by altering the compositions of the precursors in the solution prepared for polymerization. The relationship between the water potential of the PAM tablet and the volume fraction of the PAM molecules within the tablet indicates that the measuring range of the tablet-filled OT is expected to be improved by increasing the mass of the tablet filled in the OT. The 250-day pressure observation of three tablet-filled OTs shows that a stable measuring range exceeding 2,000 kPa can be maintained for a long time with a negligible decrease in the pressure decay rate. The performance of the tablet-filled OT in the soil suction measurement was proved comparable to the particle-filled OT (i.e., the OT filled with PAM particles) by using them together with the WP4C dewpoint potentiometer to measure the soil-water characteristic curve (SWCC) of a residual soil. Considering the advantages of the synthesis method and the higher measuring range of the tablet-filled OT over the traditional particle-filled OT, this new instrument will support the related research on unsaturated soils.