Combined lOading Of energy Piles – COOP

WP1 - Real-scale experiments Two experimental energy piles (0.42 m in diameter and 12.0 m in length), which have been installed in the campus of Ecole des Ponts ParisTech, will be used. These piles are equipped with PE tubes for the circulation of the heat transfer fluid, PT 100 temperature sensors and optical fibbers sensors. Additionally, temperature sensors were installed in the soil, around the piles. The piles will be initially subjected to axial static compressive load (10, 20, or 40% of the ultimate axial load) and then in the subsequent stage, horizontal static load will be incrementally applied (30, 50, or 70% of the ultimate horizontal load), while the axial load is kept constant. At each level of horizontal load, to simulate the actual operating condition of energy piles, ten thermal cycles with temperature variation of -/+ 10 °C will be repeatedly applied to the piles while the mechanical loads are maintained constant. WP2 : Centrifuge Modelling Thermal cycling loading will be applied inside the piles while the mechanical loads (horizontal and vertical) remain constant. The accumulation of lateral displacement (the ratcheting effect) will be monitored. Several couples of thermal and mechanical loadings will be studied. A parametric study will be conducted on key aspects governing the energy pile response subjected to lateral loading: i) the pile slenderness ratio, ii) the soil saturation and iii) the soil density. WP3 : Finite Element Modelling The experimental results obtained in methods 1 and 2 will be further analysed using 3D Finite Element modelling using CESAR LCPC. Advanced constitutive laws (accounting for kinematic hardening elastoplasticity and THM couplings), for the materials and the interfaces between them, will be used to properly model the experiments. Once validated, the numerical model will then be used to simulate the full scale experiments. In a second stage, a systematic study will be carried out to analyse the influence of several key parameters (both geometric and material parameters) on the overall response of thermal piles subjected to combined mechanical loading. As an outcome of this WP, a THM package for 3D analyses of energy piles will be available. Furthermore, the final parametric study will provide data to WP4 and support the development of practical design tools for a more reliable design of these piles.

Report on setup and preliminary tests; Experimental database of tests; Report on test analysis (papers submitted to journals); Report on the design of the thermo-mechanical loading set-up and the small scale model of energy pile; Experimental database of centrifuge tests; Numerical model to simulate the behaviour of full-scale energy pile is validated and calibrated Report showing analyses of the full-scale and centrifuge experiments from the numerical simulation and data obtained from the parametric study Mathematical model (key elements for energy pile modelling under combined mechanical and thermal loading) - toward code (algorithmics) and software Scientific manuals (user-guide and tutorial) - Technical Report (comparison with other numerical methods) (submission of journal paper)

The growing energy needs linked to the expansion of urban areas, coupled with the environmental challenges facing our countries lead to the development of new energy technologies. In particular, since the 1980s, a new geothermal method has been developed: energy geostructures, consisting in fixing heat exchanger pipes to the reinforcement cages of geotechnical structures like foundations to extract/inject the heat from/into the ground with the purpose of meeting the building heating and/or cooling demands. However, the synergy between thermal and mechanical loads makes their design more complex and challenging.
Currently, insight from previous studies about the response of energy piles subjected to thermal and vertical loads as well as their energy performance is accessible but the knowledge capitalised from previous studies is fragmented and it does not yet account for the piles’ combined loading. Moreover, EP installation in France is still held back by the uncertainty of their thermomechanical behaviour despite all the economic and ecological advantages of these technologies. Among these uncertainties, one of the biggest still unsolved problems is the adaptation of design methods under combined lateral and axial loads coupling to volumetric thermal loading acting on surrounding ground and along the pile.
The aim of the COOP project is to investigate the coupled effect of combined mechanical and thermal loads on the global behaviour of the energy piles, and to build an overall failure envelope that will allow engineers to design energy piles within safety limits. Both real scale and physical model experimental campaigns as well as numerical modelling will be performed within this scope. This project also aims to provide a better understanding of the behaviour of energy piles in complex configurations, and it is expected to unlock the apprehension among some of the stakeholders, especially in France where the development of this technology is still limited