Centrifuge modelling of displacement and replacement energy piles constructed in saturated sand: a comparative study

Energy piles serve as supporting structures and heat-exchange elements. They can provide thermal comfort much more efficiently than traditional air-sourced systems as the ground offers high thermal conductivity and a stable temperature below a certain depth. Energy piles are commonly installed as bored piles (replacement); however, driven (displacement) energy piles are also used in practice. A direct comparison of the performance of these two different types of energy piles subjected to thermal cycles is rarely explored not fully understood. In this study, two centrifuge energy model piles, one wished-in-place at one gravity (i.e. at low stress, simulating bored pile) and the other pile was jacked in at elevated gravity were constructed in saturated Toyoura sand. After construction, they were subjected to five heating and cooling cycles (7-37 degrees C) under a constant working load. Cumulative settlement with a ratcheting pattern was observed for the 'bored' energy pile after five thermal cycles. In contrast, a slight heave was recorded for the jacked-in energy pile. The observed heave can be attributed to the densification effect and particle crushing of soil when the pile was jacked in, reducing or even eliminating thermal-induced contraction of sand and hence decreasing the reduction of horizontal stress during thermal cycles.