Thermo-mechanical radial expansion of heat exchanger piles and possible effects on contact pressures at pile–soil interface

Abstract

This letter shows that the increase of heat exchanger pile capacity in response to heating, observed in several small-scale laboratory studies cannot be directly attributed to the increase of contact pressure at the soil/pile interface. The main thermo-hydro-mechanical processes that influence the capacity and behavior of heat exchanger piles include thermal hardening of the soil, thermally induced water flow, excess pore pressure development and volume changes upon thermal consolidation. Due to the lack of understanding of the behaviour around the soil-pile interface, thermo-mechanical interactions between the heat exchanger pile and the ground are not taken into account appropriately in energy foundation design. However, in situ and reduced-scale experiments provide evidence about temperature-induced changes in pile capacity, presumably as a result of the altered stress state around the test pile. A Finite Element analysis was conducted to quantitatively assess the radial stresses and strains undergone by a heated pile embedded in deformable soil. The study indicates that radial contact pressures typically increase less than 15 kPa, which cannot fully explain the increase of shaft resistance observed in heating tests. Further analyses are underway to characterize the mechanisms that govern pile loaddisplacement behavior and limit state.

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

“…Evaluation of the radial thermal response will also provide confirmation of the role of pile-soil 5 interface stresses due to radial thermal expansion of energy piles on their ultimate capacity, which has been proposed as a possible mechanism contributing to the side shear resistance of centrifuge-scale energy piles in compacted silt along with changes in effective stress in unsaturated soils associated with thermally induced drying (McCartney and Rosenberg, 2011;Goode and McCartney, 2015). Preliminary numerical and analytical studies using cavity expansion analyses by Olgun et al (2014) and Zhou et al (2016) indicate that no significant changes in pile-soil interface stresses are expected from the radial thermal expansion of the pile. However, these studies have not been validated against field scale studies.…”

“…Frequent temperature reversals may induce different magnitudes of thermal loads in the pile and at the pile-soil interface compared to monotonic 4 temperature changes. Depending on the soil surrounding the energy pile, thermal cycles could cause fatigue-like processes which could intensify deformations of the pile and the surrounding soil (Suryatriyastuti et al 2013;Olgun et al, 2014;Pasten and Santamarina 2014). Although investigated using numerical simulations for energy piles in idealized soil layers, this cyclic thermal mechanism is not well understood at a field scale.…”

Thermomechanical Behavior of Energy Pile Embedded in Sandy Soil

“…However, the temperature change of pile foundations during heating and cooling may lead to some irreversible volumetric strain for the surrounding soils [4,5]. It would also cause the changes in the toe resistance and shaft friction along the pile, which would affect the serviceability and stability of pile foundations [6][7][8].…”

Numerical Investigation on the Thermo-mechanical Behavior of a Quadratic Cross Section Pile Heat Exchanger

“…These temperature changes can affect the stress state at the pile-soil interface and the shear strength of the soil that affects the tip resistance (Olgun et al, 2014). Recent studies on the impact of thermal loading at pile-soil interface indicate that the bearing capacity of the pile is not significantly affected (Suguang et al, 2014, Di Donna, 2014, GSHP Association, 2012, Mimouni, 2014, Olgun et al, 2014.…”

“…These temperature changes can affect the stress state at the pile-soil interface and the shear strength of the soil that affects the tip resistance (Olgun et al, 2014). Recent studies on the impact of thermal loading at pile-soil interface indicate that the bearing capacity of the pile is not significantly affected (Suguang et al, 2014, Di Donna, 2014, GSHP Association, 2012, Mimouni, 2014, Olgun et al, 2014. Xiao et al (2014) and Di Donna (2014) have analyzed monotonic temperature variations in the range from 6°C to 50°C-60°C and have concluded that higher temperatures increase the strength of the clay-concrete contact and that the sand-concrete interface is not affected by the monotonic temperature changes.…”