Energy piles are an innovative and environmental friendly way of using renewable energy by combining geothermal heat exchange and structural foundation support. As a result of their unique roles, they are exposed to daily and seasonal temperature variations during their lifetime. Temperatures in the pile and in the surrounding soil fluctuate during the day in between operation and stoppage times resulting in short term temperature changes. Furthermore, there is a seasonal increase in temperatures after episodes of heat injection during summer followed by seasonal temperature reductions during heat extraction in winter. These temperature changes may cause axial displacements, additional axial stresses and changes in the shaft resistance with a daily and seasonal cyclic nature along their lengths. Yet, the primary role of energy piles which is the structural support, should not be jeopardized by the effects of these cyclic temperature changes. The main objectives of this work are from a fundamental perspective to understand the long-term behaviour of energy pile groups subjected to cyclic thermo-mechanical load, for which very limited knowledge remains available to date. The response of soils and soil-concrete interfaces to extensive applied thermal cycles still remains a major challenge and may contribute to the development of reliable, long-term predictions of the behaviour and performance of energy pile groups. From a practical perspective, the current interest in the energy geostructure technology requires the development of simplified, yet reliable analysis and design tools.

Energy piles are one of the innovative and environmental-friendly technologies where piles that are already required for structural support are used for exploiting the near surface geothermal energy for efficient heating and cooling of buildings, with the inclusion of circulation pipes. The dual nature of energy piles eliminates the additional drilling costs compared to traditional boreholes. However, it also leads to unprecedented challenges related to thermally induced effects on pile and soil behaviour, as well as structure-pile-soil interaction. Up to date, the extent of these effects has not been fully understood to this date, which results in uneconomical design of such foundations, preventing their wider use. Currently, there is a lack of rational and practical tools for evaluating the interactions and couplings that occur in a group of energy piles with a slab lying on the soil, which contribute to the response of the foundation.

Thermo-active diaphragm walls, also known as energy walls (EWs), are geotechnical structures typically used for multi-floored basements, cut-and-cover tunnels and underground car parks. The thermal activation of walls is made by inserting pipes attached to the reinforcing cage. Such civil structures usually present the top part of the wall exposed to the soil on one side and to the air void on the other side, while the bottom part is embedded in the soil on both sides. Due to the large surface exposed to the soil, these structures show a great potential for geothermal activation. EWs represent a modern and innovative solution to provide heating/cooling to buildings.

This research project aims at tackling energy walls from either a thermal and a mechanical point of view. The goals are to understand thermal performance and propose design guidelines, as well as quantify thermally-induced mechanical effects and propose a design methodology. This project is developed in the framework of the Marie-Curie Innovative Training Network project TERRE and accounts for the collaboration with the industrial partner Nobatek (France).

An in-situ, thermo-mechanical, testing campaign is being performed at one of the under construction stations of CEVA railway (Geneva). The station name is Lancy-Bachet and it is equipped with energy walls and slabs. The goals of such experimental campaign are to understand the thermal potential of the installation, to quantify thermally-induced mechanical actions on the wall structure and to define future scenarios for geothermal exploitation of this thermo-active underground train station. This project is developed in collaboration with Services Industriels de Genève and BG Consulting Engineers.