1 INTRODUCTION Field measurements of suction in unsaturated soils have been made using different approaches: directly (using tensiometers) or indirectly (using techniques such as porous blocks) or by collection and meas- urement of suction in recovered samples. A direct approach is always to be preferred as the measure- ment is made in situ and avoids errors in defining indirect relationships (e.g. between suction and resis- tivity or thermal conductivity). It also avoids con- cerns that the quality of the suction measurement on recovered samples may be jeopardized because measurements are made in a different stress or water content condition. Conventional tensiometers have been widely used for direct measurement of suction in the field, but they have a cavitation limit of -100 kPa. On the other hand, high capacity tensiometers (i.e. tensiometers capable of measuring pore water pressures lower than -100 kPa) have been mainly used for the meas- urement of suction in the laboratory rather than in the field. This paper describes the use of a commer- cial high capacity tensiometer, manufactured by Wykeham Farrance Limited, for the measurement of suction in the field. This tensiometer has been de- veloped through collaboration between Wykeham Farrance Limited and Durham University and can measure water pressures directly down to -1.2 MPa or even lower (Lourenço et al. 2006). Like other high capacity tensiometers that can be found in the litera- ture (e.g. Ridley & Burland, 1993), the design of the Wykeham Farrance – Durham University tensiome- ter includes a high air entry value porous filter, a wa- ter reservoir and a pressure transducer (see Figure 1). Figure 1. Wykeham Farrance – Durham University high capaci- ty tensiometer (after Lourenço et al 2006). Previous field observations using high capacity tensiometers (Ridley et al., 2003) have used “spot” measurements where the tensiometer has been placed in contact with the soil to take a suction read- ing at a particular time, i.e. the tensiometer was not left in place to take continuous readings with time. Cui et al. (2008) have used high capacity tensiome- ters for the continuous measurement of suction but their system does not allow installation of multiple tensiometers at different depths at the same location. The system reported in this paper provides multiple Paper published in: J. Mendes, D. Gallipoli, D.G. Toll, C.E. Augarde (2008). A system for field measurement of suction using high capacity tensiometers. In Proceedings of 1st European Conference on Unsaturated Soils (Unsaturated Soils: Advances in Geo-Engineering): 219-225. Durham, UK, 2-4 July 2008, CRC Press/Balkema, ISBN: 978-0-415-47692-8 A System for Field Measurement of Suction using High Capacity Tensiometers Mendes, J., Toll, D.G., Augarde, C.E. School of Engineering, Durham University, Durham, UK Gallipoli, D. Department of Civil Engineering, University of Glasgow, Glasgow, UK ABSTRACT: This paper presents a new system to measure suction in the field using high capacity tensiome- ters recently developed through collaboration between Durham University and Wykeham Farrance Limited. The system comprises a borehole probe locator where five tensiometers can be inserted allowing the meas- urement of suction at different depths. Since the tensiometers are left in place, rather than being used for a single “spot” measurement, suctions can be observed continuously with the aid of a logger and a computer. This enables the measurement of variations of suction due to seasonal changes and the observation of the im- mediate response to a rainfall event. Two borehole probe locators have been installed at different points in an embankment to measure suction in the fill material. The instrumented embankment was built for research purposes at Nafferton farm, near Newcastle, UK, as part of a cooperative project (BIONICS) investigating the biological and engineering impacts of climate change on slopes. The paper describes the installation and some preliminary observations obtained using the system.