Application of Geoexchange Experience to Geothermal Foundations

Geothermal heat exchange (geoexchange) systems have been widely used to reduce energy requirements for cooling and heating of buildings. Details on the design, installation, and post-construction monitoring of conventional borehole-type geoexchange systems in North Dakota are presented in this paper. These case histories, along with others from the technical literature, are reviewed with the goal of providing insight into geotechnical and thermal issues that may be encountered if similar geoexchange systems were incorporated into deep building foundations. Field monitoring observations reflect the need to accurately estimate the thermal demand of the building and the thermal capacity of the ground so that the geothermal heat exchangers are properly sized to avoid sub-freezing foundation temperatures in the winter. Rules-of-thumb that are currently used to size geothermal heat exchange systems in North Dakota are presented. These rules can be used to make preliminary estimates of the quantity of geothermal foundations needed to meet the thermal and geotechnical requirements of a building. The results of a design example emphasize the importance of considering thermo-mechanical effects on foundation performance in the development of LRFD resistance factors for these structural elements.