The thermal response of an energy pile that is part of a pair of energy piles spaced at 3.5 m, was examined experimentally and numerically. The field tests included: (1) heating of the energy pile alone; (2) heating of both energy piles simultaneously, and (3) heating of the other energy pile while the considered energy pile was not heated. Parametric studies of the validated numerical model was performed to understand the effects of varying soil thermal conductivity, thermal expansion coefficient, and elastic modulus on the thermal response of the considered energy pile. The numerical results confirmed the field results that radial thermal stresses were insignificant compared to axial thermal stresses. The impact of the soil elastic modulus was more significant on the thermal stresses of the energy pile compared to the effects of soil thermal conductivity and thermal expansion coefficient. The thermal stresses of the considered energy pile were not significantly affected when both energy piles were heated simultaneously, even though ground temperature changes between the energy piles were more significant due to thermal interaction. Only minor thermal effects on the non-thermal pile were observed during heating of one of the energy piles for different soil properties.
Despite the widespread research on energy piles, there remain critical knowledge gaps in the cross-sectional thermal responses of concrete energy piles. This paper implements a unique research approach by developing and validating a numerical model with cross-sectional temperatures and strains measured in a field-scale energy pile (diameter = 0.6 m and length = 10 m), strengthening the reliability of modelling for energy piles. The numerical model was further used to investigate the influences of inlet fluid temperature, soil thermal conductivity, soil elastic modulus, soil thermal expansion coefficient, and the presence of a nearby energy pile at a centre-to-centre distance of 3.5 m on the cross-sectional thermal responses of an energy pile. These investigations demonstrate the practical significance of the above parameters on the cross-sectional thermal responses of energy piles. The results show that the temperature and stresses were largest at the centre of the pile and reduced with increasing radial distance to the pile's edge, with differences up to 4°C and 2.2 MPa, respectively, between the centre and the edge. A comparison of the cross-sectional results with existing stress estimation methods,in the cross-section of the piles, commonly based on average cross-sectional temperature and temperature measured at a single spot, reveal that existing methods lead to an overdesign of 2 MPa. Therefore, the actual temperature and stress variations in the planar cross-section of energy piles should be accounted for in the design of energy piles.
Abstract. The thermo-mechanical behavior of energy piles has been studied extensively in recent years. In the present study, a numerical model was adapted to study the effect of various parameters (e.g. heating/cooling temperature, head loading condition and soil stiffness) on the thermo-mechanical behavior of an energy pile installed in unsaturated sandstone. The results from the simulations were compared with measurements from a thermal response test on a prototype energy pile installed beneath a 1-story building at the US Air Force Academy (USAFA) in Colorado Springs, CO. A good agreement was achieved between the results obtained from the prototype and the numerical models. A parametric evaluation were also carried out which indicated the significance of the stiffness of the unsaturated sandstone and pile's head loading condition on stress-strain response of the energy pile during heating/cooling cycles.
An axial load-transfer analysis is presented in this study that incorporates empirical models for 19 estimating the side shear resistance and end bearing capacity in rock along with associated 20 normalized stress-displacement curves. The analysis was calibrated using results field experiments 21 involving monotonic heating of three 15.2 m-long energy piles in sandstone. Analyses of the field 22 experiments indicates that poor cleanout of the excavations led to an end restraint smaller than that 23 expected for a clean excavation in sandstone. Specifically, end bearing parameters representative 24 of cohesionless sand were necessary to match the load-transfer analysis to the field experiment 25 results. Parametric evaluations of the analysis demonstrate the importance of using appropriate 26 rock-or soil-specific empirical models when estimating the side shear resistance and end bearing 27 capacity of energy piles. The end bearing capacity and side shear resistance in rock are greater 28 than in soils, leading to more restraint and greater thermal axial stresses. The stiffer side shear 29 restraint in rock was also found to lead to a less nonlinear distribution in thermal axial stress.
Energy piles have great potential for improving the heating and cooling performance of new buildings. However, their axial and radial thermo-mechanical behaviour due to thermal interaction between different energy piles through the surrounding soil is not well understood. This paper combines results from field experiments and numerical simulations on two bored energy piles with a centre-to-centre spacing of 3.5 m to investigate how energy piles interact under balanced and imbalanced daily temperature cycles and a range of monotonic thermal loads. One of the two energy piles' axial and radial thermo-mechanical responses were investigated during single and dual pile operation. Cyclic temperature variations of the piles induced lower soil temperature changes and pile thermal stresses than monotonic temperature variations. The balanced cyclic temperatures induced lower thermal effects in the pile and the soil than imbalanced cyclic temperatures. Significant soil temperature changes were recorded between the piles when the two piles were heated to 40°C and cooled to 0°C. However, the pile thermal stresses were similar for single and dual pile operations, indicating that thermal interaction between the piles through the surrounding soil had negligible effects on pile behaviour for the setting investigated in this paper. The piles radial thermal stresses were negligible compared to the axial thermal stresses for all studied cases. Overall, the results from this study provide validated insights into the situations where thermal interaction should be considered in design.
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