Evaluation of thermo-mechanical behaviour of composite energy piles during heating/cooling operations

• Detailed numerically thermo-mechanical analysis of energy pile foundations.
• Evaluation of extra thermal stresses generated in pile structure during heating/cooling operations.
• Analysis of mobilised shaft friction of energy piles in thermal operations.
• Evaluation of variations in bearing capacity of energy piles in both structural and geotechnical dimensions.

Energy pile foundations can function as dual-purpose structures, i.e. as supports to transfer loads from building to ground and as energy production systems. Use of ground heat exchangers (GHE) for energy production in energy piles can result in temperature variations in the pile shaft, in turn affecting the thermo-mechanical behaviour of pile in structural and geotechnical terms. Despite large numbers of energy piles now being installed, there is still little reliable information and experience about the thermo-mechanical behaviour of these structures under different thermal loadings. This study calculated the structural and geotechnical resistance of a composite energy pile foundation fitted with GHEs using numerical finite element simulations. Pile and surrounding soil were assumed to behave within a linear thermo-elastic range, assuming perfect contact at the soil–pile interface. The results showed that when the pile is used for cooling the building, the pile shaft is in expansion mode and additional compressive stresses are generated in the pile shaft. These stresses are typically around 20% of the ultimate compressive strength of typical concrete (30 MPa). Consequently, it is recommended that in a design context, the structural bearing capacity of energy piles needs to be reduced due to the additional thermal stresses. The results also showed that when end-bearing piles were used, the temperature-induced mobilised shaft friction did not have a significant effect on the geotechnical failure of these piles.