Centrifuge Modeling of Strain Distributions in Energy Foundations

In the recent push for sustainable engineering design, integration of ground-source heat exchange systems into foundations (energy foundations) have emerged as an energy efficient solution to reduce the cost of heating and cooling systems for buildings. Although full-scale energy foundations have been used in several buildings throughout the world, quantification of the effects of temperature changes on their mechanical response has not be systematically investigated. The objective of this study is to evaluate the impact of the mechanical boundary conditions and cyclic heating and cooling on the thermo-mechanical response of energy foundations. The approach used in this study to reach the objectives is to evaluate the loading, heating and cooling of scale-model energy foundations within a soil layer in a geotechnical centrifuge. Centrifuge physical modeling facilitates the characterization of carefully controlled soil-foundation systems with dense instrumentation arrays, and permits parametric evaluations of variables which may affect the behavior of energy foundations. The boundary conditions considered in this study are an “end-bearing” condition, where the tip (bottom) of the foundation is resting on a rigid base and a constant load is applied to the foundation head (top), and a “semi-floating” condition where the tip of the foundation is resting on a layer of soil and a constant load is applied to the foundation head. Both foundation boundary conditions were evaluated in a layer of unsaturated silt compacted around the model foundation. Instrumentation was embedded in the foundation as well as into the surrounding soil iv mass to characterize soil-structure interaction and heat flow phenomena during heating and cooling cycles. Specifically, variables monitoring during testing include changes in axial strain and temperature in the foundation, movement of the foundation head, movement of the soil surface and changes in temperature and volumetric water content of the soil at different depths and radial locations. The results obtained in this study agree well with strain distribution data collected from fullscale energy foundations. The results from this study will be useful for validation and calibration thermo-mechanical soil-structure interaction models. The effects of the foundation boundary conditions were substantial in the sense that the magnitude of thermal strains was larger for the end-bearing foundation than for the semi-floating foundation. In addition, the location of the maximum strain along the length of the foundation depended on the foundation boundary conditions. The maximum strain was located near the top of the foundation for the semi-floating foundation and near the base of the foundation for the end-bearing foundation. Heating and cooling cycles led to cumulative decreases in compressive strains in both foundations after cooling in each cycle. This was attributed to changes in soil stiffness due to thermally induced drying of the unsaturated soil surrounding the foundation.