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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4.6. EQUIPMENT USED 89<br />
allel tr<strong>an</strong>smission line, is measured. The speed depends on the dielectric const<strong>an</strong>t surrounding<br />
the tr<strong>an</strong>smission line. The higher the dielectric const<strong>an</strong>t, the slower the speed. Because <strong>of</strong><br />
the signific<strong>an</strong>t difference between the dielectric const<strong>an</strong>t <strong>of</strong> air (ka = 1), water (ka = 2...4)<br />
<strong>an</strong>d solids (ka = 81) the traveling speed <strong>of</strong> <strong>an</strong> electromagnetic pules <strong>of</strong> energy along parallel<br />
tr<strong>an</strong>smission line in a soil is dependent on the volumetric water content in the soil. Thus<br />
the time required for a electromagnetic pulse to travel down a known length (in this case 80<br />
mm) <strong>of</strong> tr<strong>an</strong>smission line is measured <strong>an</strong>d then used for calculation <strong>of</strong> the dielectric const<strong>an</strong>t.<br />
Fig. 4.15 shows a typical measurement <strong>of</strong> a TDR sensor. The dielectric const<strong>an</strong>t is correspond-<br />
ing to a certain volumetric water content in the soil. Using TDRs, volumetric water content<br />
in a r<strong>an</strong>ge <strong>of</strong> 0 to 100% is measured. The TDRs have <strong>an</strong> accuracy <strong>of</strong> ±2% full scale.<br />
The dielectric const<strong>an</strong>t-volumetric water content relationship has been established by care-<br />
ful measurements <strong>of</strong> ka in a test cell with known volume. Before it was checked that all TDR<br />
sensors measure the accord<strong>an</strong>t dielectric const<strong>an</strong>t for air as well as water. The results are<br />
presented in Fig. 4.16 where the top diagram shows the results <strong>of</strong> the measurements in the<br />
Effective voltage (-)<br />
Cable 3 Rods<br />
Figure 4.14: Time Domain Reflectometry Sensor (TDR) mm 80 TDR<br />
Reflection <strong>of</strong>f <strong>of</strong> end <strong>of</strong> waveguides<br />
Time<br />
Tr<strong>an</strong>sit time in waveguides Energy level as it<br />
(ns)<br />
waveguides Ch<strong>an</strong>ges in imped<strong>an</strong>ce as the enters<br />
travels along the waveguides<br />
Figure 4.15: Typical output <strong>of</strong> TDR sensor<br />
pulse
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