3379
Thermo-Mechanical Behavior of Energy Foundations
Comportement thermo-mécanique des pieux énergétiques
McCartney J.S., Murphy J.S., Stewart M.A.
University of Colorado Boulder
ABSTRACT: This paper focuses on the impact of the upper boundary condition on the thermo-mechanical response of end-bearin g
energy foundations during heating. To support this discussion, results from tests performed on a centrifuge-scale energy foundation
and a full-scale energy foundation beneath an 8-story building in Denver, Colorado are compared. Although the soil profiles differ in
both tests, the centrifuge-scale foundation involved heating during a load-controlled (free displacement) scenario, while the full-scale
foundation involved heating during the constraint associated with a real building. The stress distribution in the centrifuge test showed
greater stresses near the toe of the foundation than near the head of the foundation, while those in the full-scale foundation were closer
to being uniform along the length of the foundation. The soil in the centrifuge-scale test was unsaturated, compacted silt with uniform
strength, while the soil in the field included unsaturated urban fill with relatively low side shear resistance underlain by claystone.
This indicates that the constraint of the reinforced grade beams within the building foundation led to the higher thermally-induced
stresses within the full-scale energy foundation.
RÉSUMÉ : Cet article met l'accent sur le rôle de la condition imposée à la limite supérieure pour la réponse thermo-mécanique des
pieux énergétiques travaillant en pointe pendant le chauffage. Pour cette étude, les résultats de tests effectués sur des systèmes de
fondations en centrifugeuse et à échelle réelle pour un immeuble de 8 étages à Denver sont comparés. Les profils de sols diffèrent
dans les deux essais : la fondation utilisée dans la centrifugeuse nécessite de chauffer sous chargement contrôlé avec déplacement
libre, tandis que la fondation à échelle réelle implique un chauffage sous la contrainte associée au bâtiment réel. La distribution des
contraintes dans le test de centrifugation a montré des contraintes plus fortes à la base de la fondation que près de la tête du pieu,
tandis que ceux de la fondation à grande échelle étaient plus uniformes sur toute la longueur de la fondation. Le sol de l’essai en
centrifugeuse était saturé et constitué de limon compacté avec une résistance uniforme, tandis que le sol in situ était un remblai urbain
de faible résistance au cisaillement surmontant une couche d'argilite. Ceci indique qu’il est nécessaire de prendre en compte les
contraintes induites thermiquement dans les pieux énergétiques.
KEYWORDS:Energy foundations, soil-structure interaction, centrifuge physical modeling.
1 INTRODUCTION
Energy foundations are drilled shafts that incorporate ground-
source heat exchange elements, which can be used to transfer
heat to or from the ground to a building (Brandl 2006; Laloui et
al. 2006; McCartney 2011). Ground-source heat exchange
(GSHE) systems exploit the relatively constant temperature of
the ground to improve the efficiency of heat pump systems for
heating and cooling of buildings. Traditional GSHE systems
typically require a network of boreholes installed outside of the
building footprint, which can be cost-prohibitive (Hughes
2008). To counter this problem, heat exchange elements can be
incorporated into deep foundation elements during construction
to minimize GSHE installation cost. Although energy
foundations may not provide all the energy required to heat and
cool residential or commercial buildings, they may provide
sufficient heat exchange to supplement a conventional system
for little extra cost.Studies on full-scale foundations have
established the efficiency of heat extraction and thermal
properties of energy foundations (Ooka et al. 2007; Wood et al.
2009; Adam and Markiewicz 2009; Ozudogru et al. 2012).
Although important information has been collected regarding
the thermo-mechanical behavior of energy foundations during
heating and cooling, there are still questions to be answered.
Several experimental studies have been performed in the
laboratory using centrifuge-scale models of energy foundations
which identified mechanisms of soil-structure interaction in
energy foundations (McCartney et al. 2010; McCartney and
Rosenberg 2011; Stewart and McCartney 2012). Further,
several full-scale energy foundations have been installed
throughout Europe and Asia, including two well-documented
thermo-mechanical tests on full-scale foundations published to
date; in Switzerland (Laloui et al. 2006), and in the UK
(Bourne-Webb et al. 2009; Amatya et al. 2012). In these studies,
proof load tests along with heating/cooling tests were used to
evaluate the thermo-mechanical stress-strain response in the
foundations. Data from these tests were used to develop soil-
structure interaction design tools (Knellwolf et al. 2011). This
paper addresses an important topic identified by Knellwolf et al.
(2011), specifically the impact of the head boundary conditions
on the distribution in thermally-induced axial stresses in energy
foundations. This topic is investigated by comparing strain
gauge data from two energy foundations having different head
constraints (load-control and actual building constraint).
2 BACKGROUND
As a structural element is heated and cooled, thermally induced
axial strains are superimposed onto already present mechanical
strains. Thermal strains are induced in energy foundations due
to thermoelasticity, although a combination of end bearing, side
shear resistance, and head stiffness may provide constraint to
the foundation, leading to the development of thermally induced
stresses. A load transfer analysis may be used to represent these
different features. A schematic of a load transfer analysis
developed by Plaseied (2012) based on the work of Knellwolf et
al. 2011) is shown in Figure 1. Different from a mechanical load
