Thermomechanical Effects Induced by Energy Piles Operation in a Small Piled Raft

In this work, a series of fully coupled 3—d thermomechanical FE (Finite Elements) analyses have been carried out to investigate the mechanical and thermal interaction effects induced in a small piled raft equipped with energy piles, during the operation of an air conditioning system based on GSHP (Ground Source Heat Pumps). In particular, attention has been focused on: ) the axial load redistribution among the different piles of the raft as a result of differential hermal dilations occurring in the pile and the soil during the transient heat conduction process, and ii) the thermal interaction effects which might affect the heat exchange process when multiple energy piles are placed at short distances within the same piled raft. The results of the numerical simulations, in qualitative agreement with the limited experimental observations from full—scale tests on energy piles currently available in the literature, show that significant (positive and negative) axial load changes can be experienced by both thermally active and non— active piles of the raft. The load redistribution effects among the piles of the raft reaches their peak in a very early stage of the thermal conduction process, when the differences in temperature among active and non—active piles are largest, and then reduce steadily with time up to steady—state conditions. For the thermal properties of the soils and the raft geometries considered in this study, the peak in axial load variations are observed within about 30 days after the beginning of the thermal stage. This time span is well inside the normal operation times of GSHP systems, of the order of a few months. As for the thermal efficiency of the system, the numerical results show that, in the same time span, the thermal interaction effects between the different heat exchangers in the raft is egligible, but that significant reduction in the specific heat flux towards and from the soil can be observed for larger operation times.