Soil Suction Measurement of Unsaturated Soils with a Sensor Using Fixed-Matrix Porous Ceramic Discs

Abstract

This paper presents suction measurements in case of unsaturated soils using a commercially available water potential sensor. The sensor uses ceramic discs of predetermined pore-size distribution (i.e., fixed-matrix) for suction measurement and a surface mounted thermistor to take temperature readings. The ceramic disc assembly of the sensor is brought in contact with a soil for which the suction measurement is required. The sensor measures the water content of the ceramic discs and further uses the water retention characteristic of the ceramic discs to determine suction. The suction thus determined is equal to the suction of the soil which is in contact with the sensor. Soil–water mixtures were prepared from two clays (Speswhite kaolin and MX80 bentonite) with a diversified range of plasticity properties and mineralogy. Suction measurements of the soil–water mixtures were carried out at several water contents. The measured suctions of the clays were compared with the test results from a dew-point potentiameter. The test results showed that, very high suctions up to more than 10,000 kPa and very low suctions up to about 10 kPa can be measured using the sensor. The suction equilibrium time for the soils studied was found to vary between a couple of hours to several days depending upon the magnitude of suction and initial state of the sensor (i.e., wet or dry). Comparisons of the test results from the sensor with the results from the dew-point potentiameter tests showed some significant differences in case of the bentonite, whereas the agreements between the test results from the two devices were better in case of Speswhite kaolin. The test results clearly showed that under predetermined conditions, the water potential sensor can be used to measure soil suction greater than 1500 kPa within a few hours.