Thermo-Mechanical Behavior of Energy Piles: Full-Scale Field Testing and Numerical Modeling

Research output: ThesisDoctoral Thesis

Abstract

Energy piles are deep foundation elements designed to utilize near-surface geothermal energy, while at the same time serve as foundations for buildings. The use of energy piles for geothermal heat exchange has been steadily increasing during the last decade, yet there are still pending questions on their thermo-mechanical behavior. The change in temperature along energy piles, resulting from their employment in heat exchange operations, causes axial displacements, thermally induced axial stresses and changes in mobilized shaft resistance which may have possible effects on their behavior. In order to investigate these effects, an extensive field test program, including conventional pile load tests and application of heating-cooling cycles was conducted on three energy piles during a period of six weeks. Temperature changes were applied to the test piles with and without maintained mechanical loads to investigate the effects of structural loads on energy piles. Moreover, the lengths of the test piles were determined to represent different end-restraining conditions at the toe. Various sensors were installed to monitor the strain and temperature changes along the test piles. Axial stress and shaft resistance profiles inferred from the field test data along with the
driven conclusions are presented herein for all three test piles. It is inferred from the field test results that changes in temperature results in thermally induced compressive or tensile axial stresses along energy piles, the magnitude of which increases with higher restrictions such as structural load on top or higher toe resistance. Moreover, lower change in shaft resistance is observed with increasing restrictions along the energy piles. In addition to the design, deployment, and execution of the field test, a thermo-mechanical cyclic numerical model was developed as a part of this research. In this numerical model, load-transfer approach was coupled with the Masing’s Rule in order to simulate the two-way cyclic axial displacement of energy piles during temperature changes. The numerical model was validated using the field test results for cyclic thermal load and thermo-mechanical load applications. It is concluded that the use of load-transfer approach coupled with the Masing’s Rule is capable of simulating the cyclic thermo-mechanical behavior of energy piles.
Original languageEnglish
QualificationPh.D.
Awarding Institution
  • Virginia Tech
Award date9 Sep 2016
Publisher
Publication statusPublished - 9 Sep 2016

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