NSF Geothermal Foundations Project

Project Overview 
This project (NSF CMMI-0928159) developed an understanding of soil-structure interaction mechanisms in energy piles through a combination of element scale testing, centrifuge modeling, numerical simulations, and field testing. Thermal oedometer tests helped understand the permanent strains encountered during cyclic heating and cooling for different initial stress states (Vega et al. 2012; Vega and McCartney 2014) and temperature-controlled true-triaxial tests helped understand the impact of stress-induced anisotropy on the thermal expansion of soils in different directions (Coccia et al. 2011; Coccia and McCartney 2012). Centrifuge testing revealed the role of radial thermal expansion on the side shear resistance of soils (McCartney and Rosenberg 2011), as well as the impact of end constraint boundary conditions on the distributions of stresses and strains in energy piles (Stewart and McCartney 2012; Khosravi et al. 2012; Stewart and McCartney 2013; Goode and McCartney 2014a, 2014b; Goode et al. 2014; Goode and McCartney 2014). The numerical models developed and validated in this study include heat transfer analyses (Rouissi et al. 2012), a load transfer analysis for prediction of thermally induced axial stress/strain predictions (McCartney et al. 2014), as well as a poro-thermo-elasto-plastic finite element model for evaluation of the impact of heat and water flow processes on energy piles (Wang et al. 2012, 2014a, 2014b). This project also supported the instrumentation and monitoring of a pair of energy piles in a building in Denver, CO (McCartney and Murphy 2012; Murphy and McCartney 2014). The intellectual merit of the project consisted of the new centrifuge modeling approach that provided data to validate new soil-structure interaction tools and verify data collected from instrumented field sites, and the broader impacts include outreach presentations and involvement of undergraduate students and minorities. A total of 1 PhD, 5 MS, and 4 undergraduate students worked on this project. This study led to a keynote paper (McCartney 2011) and a book chapter (McCartney 2013).