Heat exchanger pile: effect of the thermal solicitations on its mechanical properties

Laloui, Lyesse and Moreni, Matteo and Fromentin, Antoine and Pahud, Daniel and Steinmann, Gilbert (1999) Heat exchanger pile: effect of the thermal solicitations on its mechanical properties. Bulletin d'Hydrogéologie, 17 (1). pp. 331-340. ISSN 1421-4946

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A heat exchanger pile is a pile foundation equipped with a channel system, so that a heat carrier fluid can be circulated in order to exchange heat with the surrounding. Coupled with a heat pump for heating and/or cooling purposes, the advantage of such a system is its ability to use the shallow geothermal resources (in fact solar energy) without too high cost. One question which remains to be answered with such a technique is the static pile behaviour (and of its interface with the ground) under thermal solicitations. In other words, do the thermal constraints imposed by the heat pump have an influence on the fundamental role of a pile, which is to sustain a construction ? This paper will present results of an experimental test under real conditions. An heat exchanger test pile (25 m length, 880 mm diameter) has been equipped with different sensors in order to observe its behaviour under both mechanical (load of the building) and thermal constraints. At each step of the building construction, a thermal solicitation (several days with a power up to 9 kW) is imposed to the pile; its deformations, additional constraints, changes of its mechanical characteristic and changes of the friction forces with the surrounding ground is then measured. The first results show an increase of the friction mobilisation with a temperature rise and an added thermal compressive stress in the pile, which should be taken into account when the pile is sized.

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