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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
than the thermal cell. Therefore, it is unlikely that anisotropy is
the reason behind these differences.
In the thermal cell calculations, the total power is used and
any losses neglected. A simple finite element analysis was
conducted, and indicated only minor losses. However, if losses
are in fact significant, then the calculated thermal conductivities
would be overestimates. A more thorough analysis would be
necessary to determine whether this is the case.
The presence of contact fluid in the thermal cell test could
potentially be aiding heat transfer. If the thermal conductivity of
the contact fluid is determined, this would give a better
indication as to what effect it could have. This should not be the
main reason for higher thermal conductivity values, as the
volume of contact fluid is comparatively small.
As previously mentioned, significant moisture migration
occurs due to the large temperature gradient applied. As an
additional mechanism for heat transfer, this may lead to higher
measured values of thermal conductivity.
Table 2. Soil properties.
Sample depth (m)
Density (kgm
-3
)
Average moisture
content (%)
8.00-8.45
Top
Bottom
2092
2142
23.4
23.3
10.00-10.45
Top
Bottom
2053
1951
26.9
27.1
19.00-19.45
Top
Bottom
1783
1787
26.3
26.4
5 FURTHER RESEARCH
This study highlights the need for further investigation into the
needle probe and thermal cell methods of thermal conductivity
measurement for soils. With the needle probe, it is still unclear
as to why heating and recovery gave different results for the
thermal conductivity. As mentioned previously, the needle
probe relies on human judgement in the interpretation of the
results. Further research will be carried out to find a method
which eliminates this source of error.
Some possible sources of error in the thermal cell method
require investigation. A more detailed finite element analysis
could be used to determine what power losses might be
expected, so that this could be factored into the thermal
conductivity calculation. The specimens were prepared by hand,
so that the surface in contact with the platen may not be entirely
flat. Tests on standard materials with and without a contact fluid
between the platen and the soil could determine how significant
the effects of this may be on the heat transfer. From the
recovery data, there was a considerable temperature difference
between the top and bottom of the soil for a long time after the
power had been switched off. Clarke et al. (2008) was able to
use the recovery curve to determine the power input, as the
temperature difference was small. The reasons behind this
discrepancy are unclear, so further tests using the thermal cell
on different types of soil with a range of thermal conductivities
will be beneficial.
The soil samples were taken from a borehole where a
thermal response test was later conducted. Other samples were
also taken to another laboratory to test for thermal conductivity
using the thermal cell method. Once the results from these tests
are known, a comparison will be made to the results gathered
from this study.
6 CONCLUSION
Two test methods for thermal conductivity, the needle probe
and thermal cell, have been compared. The needle probe takes
less time to conduct, and the soil is only heated slightly and for
a short period which means moisture migration is not expected
to affect the results. However, hard soil samples may require
predrilling, and the use of contact fluid which can seep into any
existing fissures thereby potentially affecting the thermal
conductivity measurements.
The thermal cell requires very little alterations to the soil
sample, but raises some issues to do with power losses. The
long heating time also means that moisture migrates towards the
top of the specimen. Within the context of energy foundations,
the thermal cell may prove more suitable for measuring the
thermal conductivity of other relevant materials such as grout
and concrete.
7 ACKNOWLEDGEMENTS
The authors would like to thank Harvey Skinner for his help in
the design, build, and instrumentation of the apparatus. The soil
samples were provided by Concept Engineering Consultants
Ltd. The site work has been carried out by Arup, Canary Wharf
Contractors Ltd, and Concept. This work forms part of a larger
project funded by EPSRC (ref EP/H0490101/1) and supported
by Mott MacDonald Group Ltd, Cementation Skanska Ltd, WJ
Groundwater Ltd, and Golder Associates.
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