Date of Award
Master of Science (MS)
Ground-source heat pumps (GSHPs) are a well-established approach to provide energy-efficient heating and cooling to buildings by exchanging heat with the subsurface soil or rock, which has a relatively constant temperature compared to that of the outside air. Although heating and cooling of the ground may lead to thermally-induced volume change in the soil surrounding the heat exchanger, this is typically ignored in the analysis and design of these systems. This is not the case when heat exchangers are embedded within the foundations of buildings to form "energy foundations". In this case, temperature fluctuations within the foundation during typical heat exchange operations may lead to permanent thermal-induced volume changes in the soil which may affect the performance of the foundation and overlying structure. The objective of this study is to understand the impact of cyclic heating and cooling on the thermally induced volume change of soils, as a first step in understanding the expected long-term deformation behavior of energy foundations. Although thermo-elasto-plastic theories of thermal consolidation in the literature indicate that permanent, plastic volume change should only occur on the first heating cycle, some data in the literature indicates additional permanent volume changes may be encountered on subsequent cycles.
This study is focused on the thermal volume change behavior of compacted, saturated silt, under different stress states. Specifically, a temperature-regulated oedometer with back-pressure control is used to measure the temperature-axial strain behavior of silt under normally consolidated to heavily-overconsolidated stress states. During the first cycle of heating, the silt specimens were observed to have thermal volume changes that were similar to those reported in the technical literature, with the normally consolidated specimen showing contraction and the overconsolidated specimens showing expansion during heating. The specimens all showed contraction during cooling as expected. However, on subsequent heating and cooling cycles, all of the specimens were observed to show a small amount of cumulative contraction. This behavior contradicts the theories of thermal volume change in soils in the literature which indicate that the volume change on subsequent heating and cooling cycles should be elastic. This cumulative contraction was observed in the results even though a stiffer response was expected due to differential radial expansion of the oedometer ring during heating and cooling. Further, thermally induced pore water pressures were observed on each heating cycle, with the magnitude of the change in pore water pressure decreasing after each cycle. Although the results from this study provide data that may be used to improve the theories for thermo-elasto-plastic thermal volume change mechanisms in soils, testing of other soil types is necessary for these conclusions to apply more broadly.
Vega, Alexander, "Impact of Temperature Cycles on the Thermal Consolidation of Saturated Silt Under Different Stress States" (2012). Civil Engineering Graduate Theses & Dissertations. 246.