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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4.2
Shaft resistance subject to thermo-mechanical loads
The O-cell is a static form of testing although its application is
inherently different to other existing pile load tests (i.e
Statnamic, anchored loading system, etc.). The O-cell is a bi-
directional, hydraulic driven, sacrificial loading jack installed
within the test pile. It is capable of creating pressures which
subsequently are applied to the pile shaft and base. The cell is
capable of opening or expanding to 150 mm and is usually
attached to the reinforcing cage that is cast within the pile.
Where the O-cell is placed determines the testing schedule of
the pile. The energy pile was subjected to mechanical load tests
on its pile shaft, the first was performed prior to any thermal
loading was introduced to the ground. At the end of each
3 loop ST and 3 loop LT heating and cooling periods the pile
shaft was mechanically loaded and compared to the initial load
test result.
Peak Upper O-Cell (UOC) load before and after thermal
loading was carried out on the energy pile, the upper section of
the pile shaft (10.1 m) was displaced in a upwards direction
with the average displacement of the upper pile shaft for the
mechanical pile tests shown in Figure 3. During loading
(pressurising) of UOC the Lower O-Cell
(LOC) was “closed”
where the middle and lower section of the pile act as one whole
section. This allowed the UOC to use the base resistance and
the lower 6 m of the pile shaft resistance to react against the
upper 10.1 m of the pile shaft resistance. The maximum
applicable load on the 10.1 m pile upper section was
approximately 1885 kN.
During mechanical loading of the energy pile, the shaft
resistance can be variable. To evaluate the mechanical
behaviour due to temperature change, consistent mechanical
behaviour of the pile shaft is required before and after thermal
loading to determine if there is any change in the shaft
resistance. Load/unload cycles were applied until the loading
behaviour was constant, thus, pile shaft reaching its ultimate
residual resistance.
Figure 3. Load vs. pile upper-section average shaft displacement
–
initial, after heating and after cooling.
Figure 3 presents the pile loading tests carried out during
the initial conditions, following the short-term and long-term
thermal heating and cooling periods. The test results indicate
that whilst the pile shaft of an energy pile undergoes thermal
heating, the pile concrete slowly expands and the ultimate shaft
resistance increases. However, as the energy pile was cooled
following the heating period, the pile concrete slowly contracted
back to its initial conditions, the ultimate shaft resistance
decreased and returned closely to its initial conditions. Figure 3
also shows that the shaft resistance gained during the heating
period was lost during the cooling period. However, ultimate
shaft resistance did not decrease following the heating or
cooling periods compared to its initial conditions.
5 CONCLUSIONS
Heat exchanger or energy piles have the potential to reduce
energy demand in built structures and tackle the ever
challenging climate debate. Energy piles are increasingly used
in various parts of the world today, and the benefits, experiences
and opportunities gained from these experiences can be adapted
and applied to the local conditions. The energy pile testing
works carried out at Monash University shows pile shaft
resistance gained strength during thermal heating loads where
the pile is founded in unsaturated, very dense sand. However,
further research is required to understand the pile shaft
behaviour in different soil conditions as well as assessing
thermal properties of the energy pile ground heat exchanger and
the surrounding soils in field conditions.
6 ACKNOWLEDGEMENTS
This study was part of a larger research program on geothermal
energy piles funded by the Victorian Government Sustainability
Fund, Vibropile Pty. Ltd., Golder Associates Pty. Ltd.,
GenesisNow, MIRVAC and GeoExchange Australia Pty. Ltd.
Their support is gratefully acknowledged.
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