Field instrumentation for real-time measurement of soil-water characteristic curve

Abstract

Suction measurement has a vital role in unsaturated soil analysis. However, measuring soil suction remains a challenging task due to a number of issues such as the limited range of suction measurement, cavitation, or long equilibrium time. It is even more challenging when the suction measurement is to be carried on the field. Hence, the development of a new suction measurement device which is able to measure high suction range for a long duration without tedious maintenance and yet portable enough for site measurement is required. In this study, a new sensor which is referred to as NTU Osmotic Tensiometer was developed along with the method to correct for decay and temperature. The NTU osmotic tensiometer is based on polymer swelling capacity in order to measure in-situ soil suction in real time for a very long duration. It requires minimum maintenance as the polymer-based sensor is not affected by the cavitation phenomenon. However, correction for decay and temperature is of paramount importance and therefore explained in this paper. Verification of the NTU osmotic tensiometer was carried out by comparing the field measurement results from the NTU osmotic tensiometer and the small tip tensiometer. The performance of the NTU osmotic tensiometer was found to be comparable with that of the small tip tensiometer, but the NTU osmotic tensiometer is able to measure more than 100 kPa soil suction. Therefore, it is possible to obtain the field soil-water characteristic curve by combining the measured in-situ soil suction from the NTU osmotic tensiometer with the measured in-situ water content from the moisture sensor as illustrated in the paper.