Uncertainty of suction measurement with tensiometers

Climate change and environmental variations can significantly influence the pore-water pressure and shear strength of the shallow depth unsaturated soils. Small variations of negative pore-water pressure (suction) during wetting and drying cycles can cause slope failures and landslides. This highlights the importance of accurate and continuous observation and measurement of soil suction during wet and dry seasons. Among the available direct suction measurement methods, tensiometer is the most reliable method for laboratory and in-situ measurement. Recently, high-capacity tensiometers (HCTs) have been developed for measurement of soil suction up to 2 MPa. HCTs have received significant interest in laboratory testing of unsaturated geomaterials mainly due to their fast response time, and ease of transport and installation in geotechnical apparatuses. However, despite various design and experimental procedures adopted to improve the HCTs’ performance, uncertainties and inconsistencies still exist with regards their measurement accuracy, range and duration, the latter being a function of cavitation occurrence before pressure equalization. In this work, an experimental program is adopted to address these uncertainties. Several prototypes of an in-house designed HCT are constructed. Novel techniques are employed to minimize the potential of cavitation occurrence during suction measurements, hence, improving the measurements range and duration. These prototypes are tested under similar controlled testing conditions with the aim to evaluate the factors causing identical tensiometers having inconsistent measurements. The obtained results are further compared with the results of an indirect suction measurement technique, namely filter paper, carried out under the same testing conditions. It is found that, for the adopted design and experimental procedure, a relative tolerance range in suction measurements is quantifiable and can be implemented in practical design and analysis.