Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.4. EQUIPMENT AND MEASUREMENT TECHNIQUES FOR TESTING
UNSATURATED SOILS
- Hygrometer:
The use of chilled mirror hygrometer for total suction application was described by Gee
et al. (1992). The hygrometer consists of a metallic mirror, a photodetector, and a
light. When the mirror is cooled down condensation occurs from the ambient water
vapor in the closed chamber. Due to condensed water on the mirror the light reflected
from the mirror is scattered and the intensity of the light is reduced. The intensity is
detected by the photodetector. The dew point temperature is related to the ambient
relative humidity and the total suction in the soil using Kelvin’s law. Leong et al. (2003)
used the chilled mirror hygrometer for measuring total suction in 2 residual soils. The
authors independently measured also the matric as well as osmotic suction of the soils
and found that the device can be used for rapid determination of total suction.
- Filter Paper Method:
Filter paper method is an indirect method, which can be used to measure either total
suction or matric suction. It is assumed that the filter paper comes after sufficient time
into equilibrium with the specific suction of an unsaturated soil. When placing the
filter paper not in contact to the specimen total suction is measured. The water content
of the paper is measured gravimetrically at equilibrium condition. A predetermined
calibration curve relates the water content to the total suction (Fawcett & Collis-George
1967, Houston et al. 1994, Leong et al. 2002).
- Humidity Control Techniques:
Humidity control techniques imply the measurement of water content for specimens of
controlled total suction. The suction is controlled by adjusting the relative humidity in a
closed chamber and the application of Kelvin’s equation. The specimen is placed in the
controlled humidity environment, where water is adsorbed or desorbed until reaching
equilibrium condition in the soil. Lu & Likos (2004) describe two humidity control
techniques in more detail, namely the isopiestic control technique and the two-pressure
humidity control technique.
Measurement of Matric Suction
Matric suction can be measured using direct or indirect methods. Techniques for measuring
matric suction are amongst others, the tensiometer sensor, the thermal conductivity sensor
as well as the electrical conductivity sensor and the contact filter paper method. Whereas
the tensiometer sensor measures the matric suction directly the other techniques measure the
matric suction indirectly.
- Tensiometer:
In the laboratory and in situ tensiometers enable to measure matric suction directly. A
review of theory and the behavior of tensiometers was given by Stannard (1992). The
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