Page 299 - Analysis and Design of Energy Geostructures

Thermomechanical behaviour of single energy piles 275

Figure 6.2 Temperature 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.

flow velocity result in greater temperature variations in energy piles. Variations of the
boundary conditions at the surface can markedly influence the temperature field char-
acterising energy piles (Bidarmaghz et al., 2016), especially when the piles are charac-
terised by a limited length.

Maximum average temperature variations of ΔT 5 30 C and minimum average

temperature variations of ΔT 52 15 C can be expected along energy piles for typi-
cal operating conditions. An example of the temperature variations along the depth z
of a single energy pile free to move vertically at its head is shown in Fig. 6.2 with ref-
erence to the results presented by Laloui et al. (2003a). In the considered case study,
the heating of the energy pile for a time of t 5 12 days resulted in an average tempera-

ture variation along its length of ΔT 5 22 C.

6.5 Thermally induced vertical strain variations
Thermal loads applied to energy piles generate expansive pile strains upon heating and
contractive pile strains upon cooling. These strains are usually not uniform with depth.
Their evolution depends on the end-restraint provided by the presence of the ground
and the superstructure. The stiffer the ground surrounding energy piles, the lower the