Analysis of Friction Induced Thermo-Mechanical Stresses on a Heat Exchanger Pile in Isothermal Soil

Abstract

In most analytical and numerical models of heat exchanger piles, strain incompatibilities between the soil and the pile are neglected, and axial stresses imposed by temperature changes within the pile are attributed to the thermal elongation and shortening of the pile. These models incorporate thermo-hydro-mechanical couplings in the soil and within the pile foundation, but usually neglect thermomechanical couplings between the two media. Previous studies assume that the stress changes imposed by temperature variations in a heat exchanger pile are mainly due to the constrained thermal elongation and shortening of the pile. Also, several recent approaches utilize spring models that focus only on the soil-pile interface in modeling temperature-induced stresses in a heat exchanger pile and implicitly ignore the effect of the full displacement field on soil-pile interaction. By contrast, in this paper, interface elements are introduced in a numerical model of a heat exchanger pile, analyzed in axisymmetric and stationary conditions. The pile is subjected to a uniform temperature increase, with free top and fixed top conditions in elastic and elasto-plastic soil profiles. Simulation results show that the constrained vertical elongation is the most detrimental factor for pile foundation performance. However it is worth noticing that while mechanical constraints (e.g., fixed top and/or fixed bottom) impose maximum stress increases at the ends of the pile, interface effects result in maximum stresses around the mid-length of the pile. This preliminary study indicates that soil-pile friction does not increase pile internal stresses to the point where it would be necessary to over-dimension the foundation pile for heat exchanger use. Furthermore, one cannot expect a significant gain in foundation performance due to the improvement of soil-pile frictional resistance as a result of increased lateral stresses at soil-pile contact. Additional numerical analyses are ongoing, in order to investigate the role of the degree of fixity induced by the building on the heat exchanger pile, and to extend these preliminary analyses to transient operational modes and cyclic thermo-mechanical loading of the heat exchanger pile.

Non-uniform thermal strains and stresses in energy piles

“…The Young's modulus and the thermal expansion coefficient of the tested pile were 40 GPa and 8·5 me/°C respectively. The shaft friction along the pile shaft was modelled using elastic perfectly plastic interface elements with stiffness as presented by Ozudogru et al (2015). As presented in Figures 4(b) and 4(c), the axial thermal strains and loads along the tested pile estimated using the proposed hybrid model agree with those approximated using the full 3D finite-element model and also with the results of the field test.…”

“…The second step considers validating the hybrid thermomechanical modelling technique against the full 3D finite-element models used by Ozudogru et al (2015) to mimic the results of a thermomechanical field load test on a free-head heat sink pile (Amatya et al, 2012). In this field test, an energy pile -55 cm Non-uniform thermal strains and stresses in energy piles Abdelaziz and Ozudogru dia.…”

Numerical modelling of thermo-active piles in London Clay

“…Ouyang et al (2011) and Bailie (2013) used the load-transfer method which simulates thermal effects as a mechanical expansion of the beam representing the pile. The finite-element method was adopted by Ozudogru et al (2015), where the focus was on the modelling of the pile-soil interface without simulating the heat transfer in the soil, as well as Yavari et al (2014b) and Yavari et al (2015), who performed uncoupled analyses where the soil was modelled as linear elastic-perfectly plastic. Recently, Abdelaziz and Ozudogru (2016) combined the finite-element method, to simulate the heat transfer in the pile and the soil, with the load transfer approach, to approximate the thermal stresses.…”

Introduction to the Special Issue of Geotechnical and Geological Engineering Entitled: “Thermo-Hydro-Mechanical Behavior of Soils and Energy Geostructures”

“…Murphy and McCartney (2014) studied the thermal and thermo-mechanical response of two energy piles during operation of a heat pump used for heating and cooling an 8-story building. Ozudogru et al (2014b) used a load transfer model to evaluate the role of side shear stresses in the axial stress distribution in energy piles. Chakraborty and Saggu (2014) used the results from a field study on energy piles to calibrate a thermo-elastic finite element analysis, and extrapolated the findings to study the cyclic response of energy piles.…”

“…These challenges include the roles of cyclic heating and cooling, which was revealed in the work of Chakraborty and Saggu (2014) and Murphy and McCartney (2014). As testing of full-scale energy piles can be expensive, the roles of physical modeling and numerical modeling of these systems needs to be expanded in future years to understand basic mechanisms of energy piles (Kramer et al 2014;Ozudogru et al 2014b;Chakraborty and Saggu 2014;Wang et al 2014;Yu et al 2015). These challenges also include the role of unsaturated conditions, which can have significant impacts on the heat exchange behavior of heat exchangers (Dong et al 2014;Likos 2014;Wu et al 2014) as well as the thermo-mechanical behavior of energy piles and other thermo-active foundations (Houston et al 2014;Wang et al 2014).…”