Geothermal structures: experimental insights into stress redistribution in 2 × 2 pile groups under asymmetrical thermal loading

Abstract

Introduction. As energy piles are increasingly utilized for sustainable energy solutions, understanding how thermal loading affects stress distribution within pile groups becomes essential for optimizing their design and functionality. The research aims to elucidate the mechanisms of stress transfer and the resultant effects on pile group behaviour.

Materials and methods. A 1g physical modelling approach was used to investigate the thermo-mechanical behaviour of 2 × 2 pile groups under asymmetrical thermal loading. Three separate tests were conducted, each featuring a group with 1, 2, or 3 energy piles subjected to cyclic thermal variations. The model employed closed-end aluminum pipes for the piles and dry, fine-grained silty sand for the ground. During thermal cycling, pile-head displacements, axial forces and bending moments along the piles, soil pressure changes beneath the pile tip, and temperature distribution around the group are monitored.

Results. The study demonstrates that thermal cycling has a substantial impact on load distribution among energy piles, with load shares rising during heating phases and falling during cooling phases. This results in an irreversible increase in load share due to soil compaction beneath the pile tips. Additionally, the contribution of the pile tip to the estimated head load increases with each heating-cooling cycle, underscoring the effects of thermal softening at the soil-pile interface.

Conclusions. Experimental observations suggest that the classic Boussinesq method may underestimate soil pressure beneath the pile tip during heating phases, potentially due to the soil’s plastic behaviour.