Geotechnical aspects of foundation energy piles – PiNRJ

The energy piles (also called “heat exchanger piles”) are the foundation piles which are used as heat exchangers. The piles foundation is used to erect structure on an underground with poor load bearing properties. A system of heat exchanger pipes is embedded in the piles allowing the exchanges of thermal energy between the ground and the building via a fluid circulating in the pipes. This system combining with a heat exchanger allows extracting heat from the soil in winter and re-injecting back heat to the soil in summer. Nowadays, this technique has already been used in more than 300 installations in Europe. In France, the geothermal energy of very low temperature (<30° C) and shallow boreholes (less than 100 m depth) has a role less important than other geothermal techniques; the technique of energy pile has never been used. Actually, the main obstacle to the industrial development of this technology is the lack of reliable knowledge concerning the thermal effects on the structural behaviour of the foundation. In the design of existing installations, the geotechnical (structural) aspects and the thermal performance of the heat exchanger piles are usually considered separately. Few works have been done to investigate the geotechnical aspects of the heat exchanger piles. Nevertheless, some in situ tests performed recently in London (UK) and Lausanne (CH) have shown that the geotechnical capacity of the pile was significantly affected by the heating/cooling cycles. The present proposal aims at providing a further understanding on the geotechnical behaviour of energy piles foundation subjected to thermal cycles. Various approaches will be used to deal with this general objective. First, a small-scale experiment will be developed to perform energy pile tests in laboratory. That consists of a small-scale pile embedded in the middle of a cylindrical container filled with the soil being studied. After the installation of the system, temperature-controlled fluid will be circulated along the pile to simulate the heating/cooling phases of the energy pile. The following parameters will be monitored during the experiments: temperature of the soil surrounding the pile, heave or settlement of the top of pile and of the soil surface, changes on the stress distribution along the pile. The experiment will be performed with three types of soil (sand, silt, and clay) and at various boundary conditions. Beside the small-scale experiments, numerical modelling by the finite element method will be used to simulate the behaviour of a single energy pile. First, a thermo-poro-mechanical model will be formulated to describe the behaviour of the surrounding soil under thermal changes. Second, a soil-pile interface model will be extended to account for temperature effects. The two models, implemented into a finite element numerical code, will allow simulating the soil-pile interaction subjected to thermal cycles. Finally, the small-scale experiments performed in the proposed project and the in situ tests performed previously in Lausanne and London will be simulated using the developed finite elements tool. That will allow a deeper interpretation of the observed results. The third approach consists of the homogenization technique to predict the behaviour of the whole energy piles foundation system subjected to thermal changes. The experimental results and the numerical modelling of a single energy pile will be used to analyse the behaviour of the Representative Elementary Volume (REV) of the reinforced soil. This analysis will allow then the development of a homogenized model for the analysis of soils reinforced with energy piles (based on the periodic distribution of the piles), taking into account the thermo-hydro-mechanical couplings between the piles foundation and the soil mass. This model will be finally incorporated into a finite elements numerical code and would be useful for designing energy piles foundation.