An Experimental Investigation on Performance of a Model Geothermal Pile in Sand

Open Access
Author:
Kramer, Cory Adam
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 07, 2013
Committee Members:
  • Prasenjit Basu, Thesis Advisor
  • Tong Qiu, Thesis Advisor
  • Shelley Marie Stoffels, Thesis Advisor
Keywords:
  • geothermal
  • pile
  • sand
  • experimental
  • laboratory
  • energy
  • soil
Abstract:
The use of geothermal piles as foundation elements of residential and office buildings is an innovative and sustainable method of energy conservation. Heat exchange through piles may have significant impact on their geotechnical performance due to thermally induced mechanical stresses and additional settlements caused by constrained thermal strains. Thermal behavior of geothermal piles are often assessed based on available idealized heat transfer models which rely on over-simplified assumptions and neglect critical operational parameters such as variable heat flux and circulation rate of heat carrier fluid. The physics and mechanics of geothermal piles subjected to thermal and mechanical loadings need to be properly understood in order for the development of an efficient design methodology. An experimental setup is developed as part of this research to investigate performance of a model geothermal pile under a series of thermal and mechanical loading. Such controlled laboratory experiments in a soil chamber allow isolation of different variables and their effects on system performance. Temperature was measured at several locations within the pile-soil system to identify the nature of heat transfer through the model heat exchanger pile and to quantify energy output, which is a function of temperature gained from or rejected to the soil, from the pile. Axial load-displacement behavior of the model pile was studied through a series of mechanical load tests performed under different thermal conditions. Load was measured at the pile head using a load cell, and pile head and base displacements were measured using a pair of Linear Variable Differential Transformers (LVDTs). Such displacement measurements were critical in studying how the load-displacement behavior changed with thermal loads. Radial heat transfer was observed along most part of the pile length. Circulation velocity of the heat carrier fluid and thermal conductivity of soil were found to be important parameters that affect heat transfer through a geothermal pile. Results obtained from this study may further be used to validate numerical and analytical models that predict heat transfer through vertical heat exchangers. Although some changes in mechanical load-displacement behavior due to thermal loading were observed, the role of thermal loading in changing mechanical capacity of the model geothermal pile could not be precisely quantified. Nevertheless, the observed changes in pile load-displacement behavior warrants further research that would quantify strains (and thus stresses) at different pile cross-sections. Simulation of different in situ stress levels in the soil tank should be an important aspect to consider in such a future work.