Date of Award

Spring 1-1-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

John S. McCartney

Second Advisor

Dobroslav Znidarcic

Third Advisor

Karen S. Henry

Abstract

This study focuses on the thermo-mechanical and thermal behavior of full-scale energy foundations installed as part of two buildings recently constructed in Colorado. The soil stratigraphy at each of the sites differed, but both foundations were expected to function as primarily end-bearing elements with a tip socketed into rock. The heat exchanger configurations were also different amongst the foundations at both sites, permitting evaluation of the role of heat exchange. A common thread for both energy foundation case histories was the monitoring of the temperature and axial strain within the foundations during heat exchange operations.

The first case study involves an evaluation of the long-term thermo-mechanical response of two full-scale energy foundations installed at the new Denver Housing Authority (DHA) Senior Living Facility at 1099 Osage St. in Denver, Colorado. Due to the construction schedule for this project, the thermal properties of the foundations and surrounding subsurface could not be assessed using thermal response tests. However, instrumentation was incorporated into the foundations to assess their long-term heat exchange response as well as the thermo-mechanical strains, stresses, and displacements that occurred during construction and operation of the ground-source heat pump system. The temperature changes within the foundations during heating and cooling operations over a period of approximately 600 days ranged from 9 to 32 °C, respectively. The thermal axial stresses in the foundations were calculated from the measured strains, and ranged from 3.1 MPa during heating to -1.0 MPa during cooling. These values are within reasonable limits for reinforced concrete structures. The maximum thermal axial stress was observed near the toe of both foundations, which is consistent with trends expected for end- bearing toe boundary conditions. The greatest thermal axial strains were observed near the top of the foundations (upward expansion during heating). The mobilized thermal expansion

coefficients inferred from the instrumentation confirm that side shear stresses provide resistance to thermally induced movements, as the measured strains are less than the theoretical thermal expansion and contraction of the reinforced concrete. The thermal axial displacements indicate that the head of the foundation moves up by -0.8 mm relative to the toe during heating to 32 °C (cooling of the building) and downward by 0.3 mm relative to the toe during cooling to 9 °C (heating of the building).

The second case study evaluated the thermal and thermo-mechanical properties of eight full-scale energy foundations constructed as part of a new building at the U.S. Air Force Academy (USAFA). The foundations were constructed as part of this project, using lessons learned from the other case history in Denver. The foundations were designed so that the impact of 5 different heat exchanger configurations on the thermal response of the foundations could be assessed through evaluation of the temperatures of the heat exchanger fluids entering and exiting the foundations. The thermal response tests were analyzed using the infinite line source analysis to determine the apparent system thermal conductivity of four foundations. The heat exchange per unit meter was also assessed as a secondary measure of the thermal response of the foundations as the assumptions of the line source method are not fully satisfied. The thermal response tests were also compared to evaluate the impact of the run-out length from the heat pump to the location of the foundations, and to assess the role of different geometrical configurations of the heat exchangers within the foundations. The values of system thermal conductivity ranged from 1.7 to 2.3 W/mK, which are consistent with previously published values for energy foundations and sandstone. These correspond to heat exchange per unit meter of 64.5 to 108.5 W/m, which is within the range of values

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