Civil Engineering Reference
In-Depth Information
Fig. 14.8
Time domain reflectometry probe.
of quicker drying techniques than are specified in BS 1377. The most common of these tests are described
below. Nevertheless it is standard practice to calibrate these results occasionally by collecting suitable soil
samples and determining their water content values accurately by oven drying.
Time domain reflectometry (TDR)
The TDR method for the measurement of soil water content is relatively new and an early review of the
method was given by Topp and Davis (
1985
). The technique involves the determination of the propagation
velocity of an electromagnetic pulse sent down a fork-like probe installed in the soil (Fig.
14.8)
. The veloc-
ity is determined by measuring the time taken for the pulse to travel down the probe and be reflected
back from its end. The propagation velocity depends on the dielectric constant of the material in contact
with the probe (i.e. the soil). The dielectric constant of free water is 80 and that of a soil matrix is typically
3 to 6. Hence as the moisture content of the soil changes, there is a measurable change in the dielectric
constant of the system, which affects the velocity of the pulse. Therefore, by measuring the time taken
by the pulse we can establish the moisture content of the soil around the probe.
Soil moisture capacitance probe
Descriptions of the capacitance technique for determining soil moisture content are given by Dean
et al.
(
1987)
and Bell
et al.
(
1987)
. The probe utilises the same principle as TDR, i.e. that the capacitance of a
material depends on its dielectric constant. The probe is used inside a PVC access tube installed vertically
into the soil. Indirect measurements of the capacitance of the surrounding soil are determined at the
required depth and these are translated into soil moisture content using a simple formula. A description
of the use of a capacitance probe for measuring soil moisture content is given by Smith
et al.
(
1997
).
Neutron probe
Nuclear moisture gauges have been discussed in the previous section.
Speedy moisture tester
A known mass of the wet soil is placed in a pressurised container and a quantity of calcium carbide is
added, the chamber then being sealed and the two materials brought into contact by shaking. The reac-
tion of the carbide on the water in the soil produces acetylene gas, the amount (and hence the pressure)
of which depends upon the quantity of water in the soil. A pressure gauge, calibrated for moisture content,
is fitted at the base of the cylinder and from this the moisture content can be read off as soon as the
needle records a steady level. A characteristic of the apparatus that must be corrected for is that the value
of moisture content obtained is in terms of the wet weight of soil.
