Axial and Radial Thermal Responses of a Field-Scale Energy Pile under Monotonic and Cyclic Temperature Changes

Abstract

The axial and radial thermal responses of a field-scale energy pile installed in dense sand and subjected to monotonic and cyclic temperatures are examined. It is found that the axial thermal strains in the pile are more restricted to thermal expansion/contraction compared to radial thermal strains. The radial thermal strains are close to that of a pile expanding/contracting freely, indicating minimal resistance from the surrounding soil in the radial direction. As a result, very low magnitudes of radial thermal stresses developed in the pile compared to axial thermal stresses. The pile-soil radial contact stresses estimated from cavity expansion analysis are up to 12 kPa for a pile temperature change of 22.5 o C and are likely negligible for the range of commonly-encountered operating temperatures of energy piles installed in dense sand. During cyclic heating and cooling, unstable changes in axial and radial thermal strains were observed initially during initial cycles indicating a ratcheting response. The changes in strains became more stable over further cycles, without significant changes in side friction, pile-soil contact stresses, or strength of the dense sand.

Effects of Cyclic Temperature Variations on Thermal Response of an Energy Pile under a Residential Building

“…Energy piles can be subjected to cyclic changes in temperature associated with long-term seasonal ground-source heat pump (GSHP) operation (Brandl, 2006;McCartney and Murphy, 2017) and daily intermittent operations of the GSHP (Faizal et al, 2016;2018). The ground temperatures during daily intermittent operations of the GSHP may recover naturally during non-operating times or could be recharged forcefully using optimized hybrid systems that utilize cooling towers or solar collectors for maintaining a balance of ground temperatures and improving geothermal energy utilization (Yi et al, 2008;Wood et al, 2010).…”

“…Of these studies, only a few were conducted on energy piles installed under building loads (Brandl, 2006;Laloui et al, 2006;McCartney and Murphy, 2012;Mimouni and Laloui, 2015;McCartney and Murphy, 2017). Also, the evaluation of the long-term impacts of daily cyclic temperature changes on the thermal response of energy piles in hybrid systems are minimal (Faizal et al, 2016;2018). The frequent reversals in pile temperatures in hybrid systems compared to normal systems would maintain the pile and ground temperatures closer to undisturbed initial temperatures since the pile and ground temperature changes will always be recovered when the heating and cooling cycles are switched (Yi et al, 2008;Wood et al, 2010).…”

“…Evaluation of radial thermal reactions of energy piles at field scale is scarce (Laloui et al, 2006;Amis et al, 2008;Mimouni, 2014;Mimouni and Laloui, 2015;Wang et al, 2015;Wang, 2017;Faizal et al, 2018), often with conflicting conclusions. For example, the field studies reported by Wang et al (2015) and Faizal et al (2018) indicated that radial thermal responses were not significant in comparison to axial thermal responses of the pile, while Mimouni and Laloui (2015) studies suggested that radial thermal responses might be significant. The differences in conclusions from these studies might be due to differences in soil properties encountered on site, and variations in pile construction techniques and pile geometries.…”

“…Small-scale physical model studies with thermal cycles on energy piles (Kalantidou et al, 2012;Stewart and McCartney, 2014;Yavari et al, 2014Yavari et al, , 2016aWang et al, 2017;Nguyen et al, 2017) have indicated that the thermally induced axial settlement of the pile is reversible for pile head loads corresponding to as low as 20% of the ultimate pile capacity, but becomes irreversible for higher pile head loads closer to the ultimate pile capacity. The field tests conducted by Faizal et al (2018) indicated that the axial and radial thermal responses of an unrestrained energy pile embedded in dense sand followed linear reversible paths for heating and cooling cycles, suggesting that both the pile and the soil did not undergo significant thermally induced deformations. As highlighted in the smallscale physical model studies reported by several investigators (Kalantidou et al, 2012;Stewart and McCartney, 2014;Yavari et al, 2014Yavari et al, , 2016aWang et al, 2017;Nguyen et al, 2017), it is conceivable that building loads could lead to irreversible axial and radial thermal responses along with associated deformations of the pile and the surrounding soil during cyclic temperature changes.…”

Coupled consolidation and heat flow analysis of layered soils surrounding cylindrical heat sources using a precise integration technique

“…Coupled consolidation and heat flow problems for saturated soils undergoing temperature change are frequently encountered in the fields of civil, energy, and environmental engineering. A variety of engineering applications, eg, energy pile systems, deep geological radioactive waste disposals, and high temperature pipelines, have gained considerable attentions . In particular, numerous research efforts have been concentrated on the heat transfer analysis for these problems in the fields of energy and environmental engineering .…”