Thermally induced group effects among energy piles

Abstract

The behaviour of conventional pile groups (e.g. closely spaced) that are subjected to mechanical loads has been shown to be different from the behaviour of single isolated piles. The so-called 'group effects' are responsible for this behaviour and must be considered for an optimal design of pile foundations. In recent years, energy piles have shown potential to work as both structural supports and geothermal heat exchangers, and thus are subjected to both mechanical and thermal loads. An increasing amount of research has investigated the previously unexplored impact of thermal loads on the thermo-mechanical behaviour of energy piles. However, no field data over typical timescales of practical geothermal applications have been available to analyse the development and impact of thermally induced group effects between energy piles (e.g. closely spaced) on their thermo-mechanical behaviour. To investigate this problem, a full-scale in situ test of a group of energy piles and coupled three-dimensional thermomechanical finite-element analyses were performed and are presented in this paper. This work demonstrates that significant thermally induced group effects characterise closely spaced energy piles. Attention must be devoted to these effects throughout the design process (e.g. geotechnical, structural and energy) of energy piles because they play an important role in the serviceability performance of these foundations.

The equivalent pier method for energy pile groups

“…There are two crucial dimensionless parameters that characterise the response of energy pile groups subjected to thermal and mechanical loads, assuming that the loads and all other material properties of the groups are the same (Rotta Loria & Laloui, 2016a, 2016b: the pile-soil stiffness ratio, Λ = E EP /G soil , where E EP is the Young's modulus of the piles composing the group and G soil is the shear modulus of the soil (…”

“…Equation (8b) accounts for the impact of a linear thermal expansion coefficient of the soil in excess compared to that of the piles on the deformability problem by considering superposition of the representative areas involved. The phenomena described by equations (8a) and (8b) have been recently observed to characterise the deformation behaviour of energy pile groups subjected to thermal loads (Rotta Loria & Laloui, 2016a, 2016b.…”

“…They have also been recently observed in innovative applications of energy piles subjected to both mechanical and thermal loads when serving as structural supports and geothermal heat exchangers for civil structures and infrastructures (e.g. Di Donna et al, 2016;Rotta Loria & Laloui, 2016b).…”

“…end-bearing piles) (Rotta Loria et al, 2015b) and the dependence of the deformation behaviour of the group on the conditions in the soil away (i.e. far field) from the single energy piles composing it (Rotta Loria & Laloui, 2016b). To address the displacement response of closely spaced energy pile groups subjected to thermal loads, considering the behaviour of a solid block constituted by the energy piles and the soil surrounding them may thus be suitable.…”

Coupled consolidation and heat flow analysis of layered soils surrounding cylindrical heat sources using a precise integration technique

“…Bai presented an analytical solution for the thermal consolidation of a saturated, porous, and hollow cylinder with infinite length. Dupray et al and Di Donna and Laloui discussed the behaviour of energy piles foundations during heating‐cooling cycles by the finite element method (FEM), and then Rotta Loria and Laloui numerically analysed the development and impact of thermally induced group effects among energy piles on their thermo‐mechanical (TM) coupling responses. Selvadurai and Najari presented the experiment results where THM processes were initiated in an intact cylindrical sample of limestone containing a central cylindrical fluid‐filled cavity.…”