Tensiometers

One of the most common devices for measuring suction is a tensiometer. A ten­siometer consists of a fine porous ceramic cup connected by a tube to a vacuum

gauge (see Fig. 3.13). The entire device is filled with de-aired water. The porous tip is placed in intimate contact with the soil and the water flows through the porous cup (in or out) until the pressure inside the ceramic cup is in equilibrium with the pore water in the soil. The reading on the pressure measuring device, once corrected for the water column in the device, is the matric suction (Apul et al., 2002). The water pressure that can be measured by this method is limited to approximately -90 kPa, otherwise water will begin to boil inside the tensiometer (“cavitation”). Tensiome­ters have been found to provide the best measuring technique for low-range suction as they measure the pore pressures directly and respond promptly to pore water pressure changes (Rahardjo & Leong, 2006).

Lately, “high-capacity” tensiometers have been developed (e. g. Ridley & Burland, 1993; Guan& Fredlund, 1997; Tarantino andMongiovi, 2001). Whencou — pled with specialised operating procedures, for example, cyclic prepressurization techniques, they have been shown to be applicable for matric suction up to 1500 kPa. Comparisons with established measurement systems have shown high-capacity ten­siometers to be relatively reliable and quite rapid in terms of response time.