Hydro-Mechanical Properties of an Unsaturated Frictional Material

4.6. EQUIPMENT USED 91
200 mm
4 x 50 mm
300 mm
TDR 1
TDR 2
TDR 3
Figure 4.17: Container for TDR sensor calibration
plastic container of 200 mm height and 300 mm diameter (see Fig. 4.17). 3 TDR probes with
a distance of 50 mm were horizontally placed in a cylindrical plastic container and measured
the dielectric constant of the wet sand specimen. After receiving constant values in electrical
conductivity the measurement was stopped and the water content was calculated by oven
drying the sand specimen. Knowing the water content the dry density and thus the volumet-
ric water content the dielectric constant was related to the volumetric water content. The
calibration results of the TDR sensors are given in Fig. 4.18, where the output signal (dielec-
tric constant) corresponds to the predefined volumetric water content. Calibration functions
for the experimental results derived for loose, dense as well as loose and dense specimens
are given in Eq. 4.1 to 4.3. Calibration curves (Eq. 4.1 to 4.3) were determined for loose
specimen and dense specimen separately as well as loose and dense specimen combined. But
Volumetric water content (%)
50
40
30
20
10
0
0 10 20 30
Experimental results dense specimen (initial void ratio=0.66) Experimental results loose specimen (initial ratio=0.89) Calibration function loose and dense results
40
function dense results Calibration
Dielectric constant (-)
function loose results
Figure 4.18: Calibration of TDR sensors for different initial void ratios
Calibration