A method for assessing non-uniformity of thermally induced stresses in the cross-section of energy piles

Abstract

The non-uniform distribution of thermally induced stresses within energy piles presents a significant challenge when assessing their thermo-mechanical behavior. This study proposes an innovative approach to assess the non-uniformity of thermally induced stresses in energy piles by employing a novel parameter known as the concentration coefficient of thermally induced stress (Kth). Kth serves a dual role: firstly, it quantifies the non-uniformity of thermally induced stresses in the cross-section of energy piles, and secondly, it enhances the accuracy of determining the average thermally induced stress and axial force in energy piles when combined with field testing. Sensitivity analyses were performed using both the Taguchi method and the full factorial test method to investigate the influence of various factors on Kth. The descending order of impact weights for each influencing factor on the non-uniformity of thermally induced stress is as follows: pile-soil stiffness ratio, pile configuration, soil's Poisson's ratio, soil-pile thermal expansion coefficient ratio and slenderness ratio, with pile-soil stiffness ratio, pile configuration and soil Poisson's ratio combined effects take up over 90% of the observed variations in Kth. Notably, when the soil stiffness is low, Kth can exceed a value of thirteen, indicating an extreme non-uniformity in the thermally induced stress distribution across the cross-section. Additionally, a lower number of heat exchanger pipes is associated with a more non-uniform thermally induced stress distribution, but the non-uniformity tends to stabilize as the number of U-shaped heat exchanger pipes exceeds three.