1202
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 5. Comparison between the Coiled TDR (with a steel tensiometer
housing) and a conventional 3 prong TDR device for Leighton Buzzard
sand, Birtley Clay and a very loose organic soil.
neglected, a different optimal value of
K
house
would be achieved
that would provide a closer fit to the observed values.
Ignoring the anomalous results for sand and comparing
calculated and measured water contents it was found that the
ceramic probe gave an accuracy for water content determination
of ±0.047. This resulted in an
R
2
value of 0.966 for
K
soil
when
compared to the actual
K
a
found from known
θ
. Likewise for
the stainless steel probe, accuracy was found to be ±0.075 with
an
R
2
value of 0.937. Improved accuracies can be obtained from
direct calibration, rather than applying a mixing model.
It can be seen that Topp’s equation does not provide a good
fit to the results (from either device) for the very loose organic
soil. It is known that Topp’s equation is not appropriate for high
volumetric water contents (>0.5).
4 CONCLUSIONS
The design and laboratory testing of new devices for water
content measurement are described. A flexible multi-electrode
resistivity system has been developed to acquire resistivity data
using different arrays, including a resistivity probe. The novel
coiled TDR device uses a two-pronged TDR wrapped around
the body of the Durham University high capacity tensiometer.
The devices have been developed to carry out experimental
studies to monitor water content changes in unsaturated soil
specimens submitted to drying and wetting cycles.
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