Hydro-Mechanical Properties of an Unsaturated Frictional Material
Hydro-Mechanical Properties of an Unsaturated Frictional Material
Hydro-Mechanical Properties of an Unsaturated Frictional Material
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36 CHAPTER 2. STATE OF THE ART<br />
- Imped<strong>an</strong>ce <strong>an</strong>d Capacit<strong>an</strong>ce Devices:<br />
Imped<strong>an</strong>ce <strong>an</strong>d capacit<strong>an</strong>ce devices are electromagnetic instruments operating at fre-<br />
quencies within the r<strong>an</strong>ge <strong>of</strong> TDR <strong>an</strong>d GPR. Imped<strong>an</strong>ce or capacit<strong>an</strong>ce values are influ-<br />
enced by the amount <strong>of</strong> water in the soil. Capacit<strong>an</strong>ce devices determine the apparent<br />
capacity in a soil (De<strong>an</strong> et al. 1987, Robinson et al. 1998, Gardner et al. 2001, Kelleners<br />
et al. 2004).<br />
2.4.2 Measurement <strong>of</strong> Soil Suction<br />
Various methods are available to measure suction in <strong>an</strong> unsaturated soil (Ridley & Wray<br />
1996). Soil suction consists <strong>of</strong> two components, namely, matric suction <strong>an</strong>d osmotic suction.<br />
Therefore it has to differentiate between measurement <strong>of</strong> total suction (sum <strong>of</strong> osmotic suction<br />
<strong>an</strong>d matric suction), matric suction as well as osmotic suction. Krahn & Fredlund (1972)<br />
presented data on the independent measurement <strong>of</strong> total, matric as well as osmotic suction.<br />
Techniques for measuring suction are divided in direct (i.e. determination <strong>of</strong> negative pore<br />
water pressure, that requires the separation <strong>of</strong> the gas phase <strong>an</strong>d the fluid phase usually<br />
realized by a ceramic disk or ceramic cup) <strong>an</strong>d indirect methods for measuring matric suction.<br />
Indirect methods (e.g. the suction is determined by measuring the relative humidity, that<br />
is related to the suction) only are available for measurement <strong>of</strong> total as well as osmotic<br />
suction (Agus 2005).<br />
Measurement <strong>of</strong> Total Suction<br />
Total suction in a soil is measured when using for inst<strong>an</strong>ce, the psychrometer, the chilled<br />
mirror hygrometer, the non-contact filter paper method, the humidity control technique or the<br />
resist<strong>an</strong>ce capacit<strong>an</strong>ce sensor. Agus (2005) compared 4 methods for measuring total suction in<br />
a soil. Leong et al. (2003), Tripathy & Sch<strong>an</strong>z (2003) presented two new devices for measuring<br />
relative humidity <strong>an</strong>d temperature in a soil <strong>an</strong>d thus the total suction. The relative humidity<br />
<strong>an</strong>d temperature tr<strong>an</strong>smitter for in situ total suction measurements as well as chilled-mirror<br />
technique with a selectable temperature environment for measurement <strong>of</strong> total suction in the<br />
laboratory were developed. These methods are described below.<br />
- Psychrometer:<br />
In geotechnical engineering practice the Peltier psychrometer (Sp<strong>an</strong>ner 1951) is most<br />
commonly used. It utilizes the Seebeck effect (1821) as well as the Peltier effect (1834)<br />
to measure the total suction. The Peltier effect as well as Seebeck effect, setup, mode <strong>of</strong><br />
operation <strong>an</strong>d calibration procedure <strong>of</strong> psychrometers are described in detail in Fredlund<br />
& Rahardjo (1993b). Psychrometers have been used by m<strong>an</strong>y researchers for measuring<br />
total suction (Krahn & Fredlund 1972, Hamilton et al. 1979, Ridley & Burl<strong>an</strong>d 1993).
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