Date of Award

Spring 1-1-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

John Scott. McCartney

Second Advisor

Dobroslav Znidarcic

Third Advisor

Richard Regueiro

Abstract

Energy foundations, or deep foundations which incorporate heat exchangers, are a novel approach to improve the energy efficiency of building heat pump systems. This study focuses on the development of a new load-transfer analysis to assess the thermo-mechanical response of energy foundations. This new analysis builds upon well-established soil-structure interaction algorithms for mechanical loading to incorporate the effects of restrained thermal expansion of the energy foundation during changes in temperature. Novel aspects include a method to evaluate the impact of increased lateral stresses on the ultimate side shear resistance due to radial expansion of the foundation, a method to track the direction of mobilized side shear stresses during thermal expansion, and a method to consider consolidation of the soil at the foundation toe during thermal expansion on the ultimate end bearing.

A parametric evaluation of the new model was performed to assess the thermo-mechanical response of different representative boundary conditions for energy foundation, including the stiffness of the soil at the toe of the foundation and the stiffness response of the overlying structure. As expected, the results indicate that the higher the temperature change, the higher the magnitudes of thermally induced axial stress and strain along the length of the foundation. The main impacts of the boundary conditions are reflected in the nonlinearity of the distribution of the thermal axial stresses and strains and the location of the maximum stress and strain (the null point). Also, it was observed that the response of energy foundation is directly related to the soil properties, primarily the friction angle (which affects the side shear resistance) and the c/p ratio (which affects the end bearing).

Axial thermal stress distribution data from a centrifuge-scale model energy foundation were used to validate the results from the load-transfer model. In the centrifuge test, an end-bearing foundation restrained by a building load was heated in stages from 20 to 40 °C. The maximum axial thermal stress was observed in the lower half of foundation. After selecting an appropriate value for the stiffness of the loading system applying the prototype building load, the results of the load transfer analysis were found to represent the data well. The observations from the centrifuge model and the load transfer analysis were consistent with those obtained for full-scale load tests on energy foundations in the technical literature. The new load transfer analysis provides a useful design tool to evaluate stress and strain distributions in energy foundations.

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