Page 300 - Analysis and Design of Energy Geostructures

276 Analysis and Design of Energy Geostructures

Figure 6.3 Vertical strain variations along an energy pile throughout a heating passive cooling
cycle. Redrawn after Laloui, L., Moreni, M., Vulliet, L., 2003a. Comportement d'un pieu bi-fonction, fon-
dation et échangeur de chaleur. Can. Geotech. J. 40 (2), 388 402.

observed thermally induced strains. The same applies for the structural element poten-
tially present at the head of energy piles.
Typical values of average expansive vertical strain variations caused by the tempera-

ture variations between ΔT 5 15 C and 30 C of approximately Δε z 52 100 to
2250 με can be observed in energy piles. The opposite holds for cooling thermal
loads applied to energy piles [see, e.g. Bourne-Webb et al. (2009)]. An example of
thermally induced strains along a single energy pile free to move vertically at its head
is shown in Fig. 6.3 with reference to the results presented by Laloui et al. (2003a).In
the considered case study, an average temperature variation along the energy pile of

ΔT 5 22 C resulted in average expansive vertical strains of Δε z 52 180 με.
When energy piles are (at least theoretically) free to move vertically at their head,
the thermally induced vertical strain variations approach those under free expansion
conditions towards the pile head (Bourne-Webb et al., 2009; Mimouni and Laloui,
2013). An example of this phenomenon is shown in Fig. 6.4, which shows a compari-
son between the observed thermally induced vertical strain variation and the strain
associated with free thermal expansion conditions along the energy pile tested by
Laloui et al. (2003a). These findings are based on the linear thermal expansion coeffi-
cient of the pile, α EP , and the average temperature variation applied along the pile
length, ΔT (i.e. Δε 52 α EP ΔT ).
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