A comparative study of high capacity tensiometer designs

The High Capacity Tensiometer (HCT) is a soil sensor that can measure large negative pore-water pressures, i.e. in excess of −1 MPa, and is a key instrument for monitoring ground-atmosphere interactions. The design of HCTs has not changed since the first prototype was proposed in the 1990s and still includes three main components, namely a high air-entry value porous filter, a small water reservoir and a pressure transducer, which are all encased inside a protective sheath. Despite many successful examples of measurement of pore-water tension in the laboratory and the field, there are essentially no commercially available HCTs for industrial applications. The rare utilization of HCTs in engineering practice has always relied on bespoke devices that have been purposely designed by research laboratories or suppliers of soil sensors. The dissemination of HCTs into engineering practice has been further hindered by the relatively poor understanding of sensor design and its impact on the reliability of measurements. To overcome such gap of knowledge, this paper explores the influence of distinct design variables (i.e. the porous filter, the pressure transducer, the water reservoir size and the protective casing) on the measuring range of HCTs. Seven prototypes were manufactured with different combinations of the above design variables showing that, if HCTs are properly saturated, the air entry value of the filter has the strongest influence on the measuring range while the effects of reservoir size, pressure transducer and protective casing are relatively modest. It is expected that the results from this work will guide future design of HCTs, thus contributing to the development of resilient sensors for geotechnical applications.