Page 296 - Analysis and Design of Energy Geostructures

272 Analysis and Design of Energy Geostructures

To address the aforementioned aspects, idealisations and assumptions are presented
first: in this context, the objective is to propose a summary of the assumptions made
to interpret the response of energy piles subjected to mechanical and thermal loads.
Second, the classification of single energy piles is treated: the objective of this part is to

summarise a characterisation of the types of single energy piles. Third, the temperature
variations in energy piles are discussed: in this context the purpose is to expand on the
thermal field characterising energy piles. Next, the thermally induced vertical and radial
strains characterising energy piles are treated: in this framework the purpose is to dis-
cuss the influence of thermal loads on the deformation of energy piles. Afterward, the

thermally and mechanically induced variations in vertical displacement, shear stress and vertical
stress characterising energy piles are discussed: the purpose of this part is to expand on
the variation of the considered variables along energy piles and to highlight crucial dif-
ferences between the influence of thermal loads compared to mechanical loads. Then,
the variations in degree of freedom are considered: in this context the purpose is to com-
ment on the response of energy piles depending on the restraint provided by the

ground and the superstructure characterising such foundations. Finally, questions and
problems are proposed: the purpose of this part is to fix and test the understanding of
the subjects covered in this chapter by addressing a number of exercises.

6.2 Idealisations and assumptions
As previously highlighted in Part B of this book, the materials constituting energy
geostructures, soils and rocks are nonhomogeneous. However, in many cases, an effec-
tive analysis approach can resort to the continuum medium idealisation while consid-
ering the materials isotropic and homogeneous. This approach is again considered for
the forthcoming interpretation of the thermomechanical behaviour of energy piles
(cf. Fig. 6.1A).
While in practice piles are not often exactly characterised by a cylindrical shape
and the surrounding soil layers are not fully horizontal, assuming the considered con-
ditions markedly simplifies the analysis of pile response. Accordingly, these hypotheses
are accounted for the following developments (cf. Fig. 6.1B).
Thermal and mechanical loads involve nonuniform variations of the temperature,
stress, strain and displacement fields within (and around) energy piles (Abdelaziz and
Ozudogru, 2016; Caulk et al., 2016; Rotta Loria and Laloui, 2017). Despite a fine
analysis of the response of energy piles would require consideration of the actual non-
uniform nature of the considered fields, in the following it is assumed that the
reported variations in temperature, stress, strain and displacement are uniform and rep-
resentative of the energy pile response (cf. Fig. 6.1C). The validity of this hypothesis
increases at successive stages of the geothermal operation of energy piles and for more
uniform pipe configurations within the cross-section of energy piles.