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The response of energy foundations under thermo-mechanical loading
La réponse des fondations thermo actifs sous chargement thermo mécanique
Bodas Freitas T.M., Cruz Silva F., Bourne-Webb P.J.
Instituto Superior Técnico, Lisboa, Portugal
ABSTRACT: The need to establish a sound basis for the understanding of the behaviour of geo-structures that are utilised for energy
exchange within ground source heat pump systems has received increasing attention in recent years and a number of physical and
numerical modelling studies of such systems have been undertaken. This paper details the results of a preliminary numerical study of
the response of a foundation pile under steady-state heating, which when compared to the results of published observations, raises
some interesting questions regarding the models of behaviour proposed in the literature and areas requiring further study in the future.
RÉSUMÉ : La nécessité d'établir une base solide pour la compréhension du comportement des géo-structures qui sont utilisés pour
l'échange d'énergie dans les systèmes de pompe géothermique a reçu une attention croissante ces dernières années et un certain
nombre d'études de modélisation physique et numérique de ces systèmes ayant été entrepris. Cet article détaille les résultats d'une
étude numérique préliminaire de la réponse d'une fondation sur pieux sous chauffage stationnaire q
ui
, lorsqu'il est comparé aux
résultats de certaines observations publiées, soulève des questions intéressantes concernant les modèles de comportement proposés
dans la littérature et les zones nécessitant une étude plus approfondie dans le futur.
KEYWORDS: pile, foundations, ground source energy, thermo-mechanical loading.
1 INTRODUCTION
The use of ground energy systems that employ heat-exchange
loops within trenches and boreholes is well established and the
technology is recognised as a key component for future
sustainable energy use (Mackay, 2009).
While still a small component of the ground energy market,
the use of civil engineering structures that are in contact with
the ground (geo-structures) to replace the more convention heat-
exchange methods is creating great interest. Bearing piles have
been used for this purpose since the mid-1980s and more
recently other elements have been used, e.g. retaining walls &
tunnel linings.
Energy geo-structures and in particular, bearing piles are
now often used in Austria, Germany and the UK, and there is
increasing interest in their potential in many countries including
the USA, Japan and China. However, the uptake of these
alternative means for facilitating heat-exchange with the ground
has been impeded by a lack of technical evidence regarding the
impact of the thermal cycles on the serviceability and safety
performance of the geo-structures.
This paper presents the results of a set of numerical analyses
that were undertaken to evaluate the mechanisms of response of
piles used for heat-exchange. First, to complete this
introduction, observations of the thermo-mechanical (TM)
response of piles and clay soil are reviewed. Then the basis for
the analyses and the predictions that were obtained are
presented, and the implications of the results are discussed.
Finally, some ideas for future research in this field are
suggested.
1.1
Energy geo-structures
Very few field or laboratory studies, where the TM response of
energy geo-structures has been systematically observed, have
been published. Energy geo-structures include load bearing
piles, piled and diaphragm walls, and tunnels. To-date,
published TM studies have involved only pile foundations
(Brandl, 2006; Laloui et al. 2006; Bourne-Webb et al. 2009;
McCartney & Rosenberg 2010).
The mechanisms of response seen in the pile tests appear to
be broadly consistent and can be described in a simple
schematic way (Amatya et al. 2012; Bourne-Webb, et al. 2013).
Underlying this descriptive framework is the implicit
assumption that the pile expands and contracts relative to the
surrounding soil when heated and cooled, respectively. Thus,
when heated the axial strain/forces in the pile become more
compressive and when cooled less compressive (potentially
even tensile pile axial response is seen), Fig. 1.
Associated with the pile axial response described above, the
response at the pile-soil interface is also affected with the
changes in mobilised pile shaft friction (shear stress) opposing
the expansion and contraction of the pile, Fig. 1. These changes
in pile axial response and mobilised pile-soil interface friction
will occur on a daily and seasonal basis as the heating/cooling
energy demand of the structure, that the ground energy system
serves, varies.
1.2
Thermo-mechanical characterization of clay
The effect of temperature changes on the behaviour of soil is of
interest in a number of fields including the sequestration of
nuclear waste, buried high voltage electricity cables, buried
pipelines, and increasingly energy geo-structures.
