A full-scale experimental site with four energy test piles was built on the campus of the Swiss Federal Institute of Technology in Lausanne, Switzerland. This site was used to investigate interaction effects within a group of energy piles. First, the ground constraints were evaluated by testing the piles without any structure on top. Next, each pile was individually tested once the overlying structure was built, which provided information on the structural constraints and allowed the quantification of the pile-structure-pile interactions. Finally, the four test piles were simultaneously heated to quantify the group effects. A thermal response test was performed on one pile and the thermohydraulic response of the soil between the piles was monitored with piezometers and thermistors. Load redistributions may occur in mixed foundations, i.e., with conventional and energy piles, because of differential displacements. Conversely, heating an entire foundation increases the individual pile displacements, but reduces the differential displacements and consequently the pile thermal efforts. Radial strains may also have a significant impact on the axial thermomechanical response of the piles in stiff soil layers. Heat barely propagated farther than a couple of metres with no significant pore-water pressure variation observed.Key words: energy pile, full-scale in situ experimental site, group effect, thermomechanical response, thermal response test. Résumé :Un site d'essais à échelle réelle comportant quatre pieux énergétiques expérimentaux a été construit sur le campus de l'École polytechnique fédérale de Lausanne, en Suisse. Ce site a servi à étudier les effets d'interactions au sein d'un ensemble de pieux énergétiques. Dans un premier temps, on a évalué les contraintes du sol en soumettant les pieux à des essais, en l'absence de structure au-dessus de ces derniers. Puis, une fois cette structure construite, chaque pieu a été mis à l'essai, ce qui a permis d'obtenir des données sur les contraintes structurales en présence et de quantifier les interactions pieu-structure-pieu. Enfin, on a chauffé les quatre pieux expérimentaux afin de quantifier les effets de groupe. Un test de réponse thermique a été effectué sur un pieu et la réponse thermohydraulique du sol situé entre les pieux a été mesurée en continu à l'aide de piézomètres et de thermistances. Des redistributions des charges peuvent avoir lieu dans les fondations mixtes, c.-à -d., dans le cas de pieux conventionnels ou énergétiques, en raison des déplacements différentiels. Inversement, si une fondation est entièrement chauffée, les déplacements de chacun des pieux augmentent alors que les déplacements différentiels, et donc les efforts thermiques à l'intérieur des pieux, diminuent. Les contraintes radiales peuvent aussi beaucoup influer sur la réponse axiale thermomécanique des pieux dans les couches de sol rigides. La chaleur se propage à peine sur quelques mètres au-delà des pieux et aucune variation importante de la pression de l'eau interstitielle n'...

Using pile foundations as heat exchangers with the ground provides an efficient and reliable energy source for the heating and cooling of buildings. However, thermal expansion or contraction of the concrete brings new challenges to the design of such structures. The present study investigates the impact of temperature variation on the mobilised bearing capacities of geothermal piles. The mechanisms driving the variations and redistribution of mobilised bearing forces along geothermal piles are identified using Thermo-Pile software. The EPFL and Lambeth College test piles are modelled and analysed as real-scale experiments. Three simple representative cases are used to investigate the impact of over-sizing geothermal piles on their serviceability. It is found that the mechanisms responsible for the variations and redistribution of mobilised bearing forces along the piles are unlikely to cause geotechnical failure, even if the ultimate bearing force of a pile is reached. Furthermore, over-sizing geothermal piles compared to conventional piles can have a negative impact on their serviceability.

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This paper presents a review of current design tools used for thermo-active geotechnical systems, along with validation efforts. The capabilities of available analytical methods used for the thermal and thermo-mechanical design of these systems are evaluated and shortcomings of the existing methods are identified. Although the analytical methods permit accurate prediction of the thermal stress and strain response of thermo-active piles from readilyavailable soil and concrete properties, current shortcomings consist of the ability of the methods to simulate cyclic heating and cooling effects, transient pore water pressure generation and dissipation, and the effects of radial stress changes. Recommendations are provided on how to properly address the current design requirements and the efforts to overcome shortcomings with the development of constitutive relationships from further full-scale and laboratory-scale experimental studies on thermo-active piles. Furthermore, the need for the development of both simplified analytical tools and advanced finite element models is emphasized. In addition, the existing analytical tools should be validated using field data from recently available case studies of thermoactive piles in varying soil deposits. An urgent need for an extensive design guide for energy geostructures was identified. The guidelines should be targeted towards practitioners and include field observations and measurements, as well as laboratory and numerical studies.

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