Farhad Salour Doctoral Thesis

3.1.
 
Soil matric suction measurement 
Matric suction is the difference between the pore-air and pore-water pressures within 
the unsaturated soil structure. Matric suction measurement can be done through direct 
and indirect measurements. In the direct measurement method, only the negative 
pore-water pressure is measured as the pore-water pressure is usually equal to the 
atmospheric pressure. A Tensiometer is the most commonly used device for direct 
measurement of negative pore-water pressure in the field. It consists of a high-air-entry 
(HAE) porous ceramic cup that is attached to a pressure gauge with a small plastic tube, 
filled with de-aired water. Once the HAE ceramic cup is in contact with the soil 
particles and the equilibrium is reached, the water in the tube will have the same 
negative pressure as the soil pore-water pressure that can be read using the device 
pressure gauge. This measurement is numerically equal to the matric suction if the 
pore-air pressure is equal to the atmospheric pressure. The indirect measurement of the 
matric suction is done by measuring a different variable than the negative pore-water 
pressure. 
In indirect measurement methods, the electrical or thermal conductivity of a porous 
material, which is a function of its moisture content, is measured. Since water content is 
also a function of the matric suction, the matric suction of the sensor and the 
surrounding soil can be determined using predefined calibration curves (Fredlund et al., 
2012). 
3.2.
 
Axis translation technique 
The negative pore-water pressure that can be directly measured with a tensiometer is 
limited to about -1 atm. (~-100 kPa) as there is the risk that the water in the tensiometer 
may begin to cavitate and interfere the measurements. In order to measure matric 
suctions over this range, the axis translation technique is usually employed in the 
laboratory. In this technique, the soil specimen is exposed to a certain air pressure in a 
closed chamber, and therefore the origin of the pore-water pressure is translated from 
the standard atmospheric pressure to the air pressure of the chamber. Thus, the risk of 
cavitation is avoided as the pore-water pressure does not go below -1 atm. In other 
words, both the pore-air and pore-water pressures are translated, while the matric 
suction (
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a
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) in the specimen is maintained (Hilf, 1959; Fredlund, 1989). 

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