Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.4. EQUIPMENT AND MEASUREMENT TECHNIQUES FOR TESTING
UNSATURATED SOILS
relation to soil water content are described in Topp et al. (1980), Topp & Reynolds (1998),
Ferré & Topp (2002). The electromagnetic method makes use of the difference between the
high dielectric constant of water (ka = 80) and the low dielectric constant of the other soil
components as air (ka = 1) and the solids (ka = 2 − 4). The large difference allows to draw
conclusions to the volumetric water content (Selig & Mansukhani 1975, Topp et al. 1980,
Look & Reeves 1992).
- Time Domain Reflectometry (TDR):
In the measurement of water content the technology of time domain reflectometry
(TDR) received the most attention (Topp 1987) and the overview is focused on this
method. The first who applied TDR to measurements in soil were Davis & Chudobiak
(1975). From that time on TDR method has been used frequently to measure water
content at several scales and under several conditions (Topp & Reynolds 1998, Robinson
et al. 2003) and has become a standard method for measurement of water content in
unsaturated soil in the field and in the laboratory. Topp et al. (1980) tested 4 mineral
soils and found insignificant influence of properties such as salinity, temperature, texture
and dry density on the dielectric constant. A calibration function was obtained by Topp
et al. (1980), which originally was believed to apply universally to all soil types. Similar
equation was proposed by Ledieu et al. (1986). Later numerous authors (Herkelrath
et al. 1991, Malicki et al. 1996, Suwansawat 1997) showed that the composition of solids
of the soil, surface electrical conductivity and density of a soil can significantly effect the
relationship between dielectric constant and volumetric water content. Thus although
the universal equation by Topp works well for soils composed of quartz, a unique rela-
tionship between the dielectric constant and the volumetric water content does not exists
for all type of soils (Roth et al. 1992, Dasberg & Hopmans 1992, Suwansawat 1997). A
soil specific calibration is recommended (Benson & Bosscher 1999, Suwansawat & Ben-
son 1999). Recommendations regarding calibration procedure and experimental setup
are given in Suwansawat & Benson (1999). Siddiqui & Drnevich (1995) extended the
TDR measurements to geotechnical application and related the TDR measurements to
water content and dry density. A method for determination of soil water content as well
as dry density using TDR measurements is described in detail in Yu & Drnevich (2004).
- Ground Penetrating Radar (GPR):
The physics of the GPR method is identical to the physics of the TDR method (Weiler
et al. 1998). GPR method is widely applied in geosciences and was recently developed to
measure water contents in soils (Davis & Annan 2002, Huisman et al. 2003). Compared
to TDR method, GPR method provides data from larger spatial region.
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