A simplified coupled method for geotechnical analysis of geothermal energy pile with single U-shaped tube heat exchanger

Abstract

Geothermal energy pile (GEP) is an environmentally friendly energy source that utilizes heat energy present in the shallow earth surface to decrease building energy consumption. However, thermal stresses of pile shafts due to the temperature changes during the operation of GEP may cause distress to the supported structures e.g. buildings, and there is currently a lack of established calculation methods for the geotechnical design of GEP. This paper presents a simplified coupled method for geotechnical analysis of GEP with a single U-shaped tube heat exchanger. Primarily, leveraging the finite line heat source model and the superposition principle, we propose a segmentation superposition heat transfer model tailored for GEP featuring a singular U-shaped tube heat exchanger. This model adeptly computes the temperature variations along the pile shaft. Secondly, a hyperbolic curve is chosen as the load-transfer function, and the unloading and reloading characteristics of the relationship between the shaft friction/shaft displacement during temperature cycling are simulated by Massing’s criterion. Building upon the aforementioned, taking into account the uneven temperature rise of the pile and the influence of soil shear deformation, a coupled calculation method for the working characteristics of energy piles under thermal–mechanical conditions has been established. The method is validated on the basis of in-situ measurements of the loads and deformations experienced by heat exchanger test piles and a model test on the thermal–mechanical behavior of GEP with a single U-shaped tube heat exchanger.