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. / Ph. D. / Global energy demands are increasing rapidly, along with depleting natural resources. Of equal importance, the consumption of fossil fuels pose a great threat to the environment. Hence, there is an urgent need to find alternative energy resources, such as near surface geothermal energy. Energy piles are one of the ways of exploiting near surface geothermal energy. In this system, the piles that are already required for structural support are equipped with geothermal loops, for heat exchange operations. With the use of energy piles, the heat energy can be extracted from the ground to heat the buildings during winter. Similarly, the heat energy can be withdrawn into the ground, in order to cool the buildings during summer. Energy piles provide an environmental friendly way of heating and cooling of the buildings. However, there are several effects of the heat exchange operations on the behavior of energy piles. During winter, because of heat extraction, the temperature of the energy pile decreases, which causes the tendency of contraction of the pile. On the other hand, during summer, the heat injection into the ground increases the temperature of the energy piles, which results in a tendency of elongation of the energy pile. Depending on the level of restriction from the surrounding soil or the building on top, some of the expansion or contraction tendency of the energy piles actually take place, which results in axial displacements and changes in shaft resistance. The restricted part of the contraction or expansion causes axial stresses along the piles. The primary role of the piles, which is structural support, should not be jeopardized by these effects of heat exchange operations. In this doctoral research, the effects of temperature change on the behavior of energy piles are investigated. For the experimental investigation, a full-scale field test on three energy piles was performed, where temperature changes were applied to the test piles, to evaluate their effects. In addition, a numerical model was developed, and it is validated by using the field test results. This numerical model can be used for different soil profiles, pile characteristics and temperature changes, in order to estimate the behavior of various scenarios of energy piles during their design.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/82438 |
| Date | 09 September 2016 |
| Creators | Sutman, Melis |
| Contributors | Civil and Environmental Engineering, Laloui, Lyesse, Olgun, Celal Guney, Dove, Joseph E., Filz, George M., Mauldon, Matthew |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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