Hydro-Mechanical Properties of an Unsaturated Frictional Material

38 CHAPTER 2. STATE OF THE ART
author differs between 3 types of tensiometers, namely, manometer type of tensiometer,
gauge type of tensiometer and transducer type of tensiometer. The author discusses in
detail the advantages and disadvantages of each type of tensiometer and found that vac-
uum gauge type tensiometers are not recommended for measuring unsaturated hydraulic
gradients. Mercury manometer type tensiometers are probably the most accurate and
robust sensors. Hysteresis in a manometer tensiometer is much less than in a gauge type
tensiometer or pressure type tensiometer. Transducer type tensiometers are well suited
to collect a large quantity of data. Measurements can be performed in predetermined
time intervals and are recorded automatically. The accuracy is excellent but there is
a need for frequently calibration procedure. Working range of standard tensiometers
is limited by the cavitation process, which occurs approximately at 80 − 100 kPa. De-
tails on the preparation, installation and usage of tensiometers are also discussed in the
former work of Cassel & Klute (1986). Ridley & Burland (1993) designed a stainless
steel miniature pore-water pressure transducer with a working range up to 3500 kPa.
The suction probe uses a 15 bar ceramic disc, an electronic pressure transducer and an
electrical connection for measuring the matric suction. The very thin water reservoir
(∼ 250µm) between ceramic disc and transducer avoids the formation of air bubbles and
thus increases the measuring range of the instrument. Evaluation of the suction probe
yields to reliable measurement of matric suctions up to ∼ 1500 kPa. Guan & Fredlund
(1997) developed a suction probe measuring pore-water pressure up to 1250 kPa for
both saturated and unsaturated conditions and tested a glacial till and Regina clay.
They found reasonable agreement between measured values in the suction probe and
those measured by the filter-paper method as well as thermal conductivity sensor. With
decreasing saturation differences in the measurements appeared. Problems in usage of
suction probes arise for the calibration procedure. The calibration is carried out in the
positive range of pore-water pressure (hydraulic pressure) and extrapolated to the nega-
tive range of pore-water pressure (matric suction). A tensiometer similar in conception
was designed by Tarantino & Mongiovi (2002). Additionally their design was modified
to perform calibration also at negative-pore water pressures. Further suction probes
were developed by several researchers, e.g. Mahler & Diene (2007).
- Thermal and Electrical Conductivity Sensor:
Indirect measurements are performed using either thermal conductivity sensors or heat
dissipation suction sensors. Because water is a better thermal conducter than air, ther-
mal properties and electrical properties of an unsaturated soil can be related to the
water content in the soil. Such kind of sensor consists of a temperature sensor and a
heater as developed by Shaw & Baver (1939). Different soils required different calibra-
tion curves to relate the thermal conductivity to the water content. Therefore several
researchers suggested to enclose the thermal conductivity sensor in a porous medium,