The measurement of soil-water retention curves using the tensiometer method

Abstract

The soil-water retention curve (SWRC) is an essential tool in geotechnical engineering and agriculture used in the analysis of unsaturated soil conditions such as those found in tailings dams and the water retention capacity of agricultural lands. The SWRC describes the relationship between the soil matric suction and the water content of an unsaturated soil. The SWRC can be used as a basis to model various unsaturated soil parameters, such as the hydraulic conductivity and shear-strength functions. It is, therefore, vital that an accurate characterisation of the SWRC of a soil be made. Until recently, the methods used to obtain the SWRC have involved obtaining point measurements along the SWRC and fitting an empirically derived curve through these points. These procedures can take weeks to complete and usually rely on indirect methods of measuring soil suction. These factors have limited the widespread use of the SWRC, and therefore, the adoption of unsaturated soil mechanics into modern geotechnical engineering practice. The development of high-capacity tensiometers (HCTs), the only devices capable of directly measuring high soil suctions, has enabled researchers to significantly expand the study into unsaturated soils and the determination of SWRCs. Toker et al. (2004) introduced a rapid method for determining continuous SWRCs using an HCT and a digital laboratory balance by continuously monitoring the mass and suction generated in a drying a soil sample. This method was investigated by first developing a new low-cost tensiometer capable of measuring suctions up to 1150 kPa. Simple procedures were developed to reliably saturate and calibrate the tensiometer without the need for specialised equipment. The SWRCs of five different soil types were determined using the continuous drying method and then compared to the SWRCs determined from point measurements by the filter paper method. A novel way of volume change measurement was also incorporated into the method to produce SWRCs for soils that undergo shrinkage during drying. Excellent agreement between the methods was found for both the granular soils and fine-grained clayey soils tested. The method reduced the time taken to determine SWRCs from weeks to mere hours.