An Efficient Pile‐Soil Interaction Model for Thermo‐Mechanical Response of Energy Piles in Sand

Abstract

Energy piles are analyzed using the load‐transfer method or numerical methods. While the equivalent soil spring‐based load‐transfer method is fast and easy to use, the equivalent soil spring constants often incorporate empiricism in the analysis. In contrast, numerical methods are more accurate and represent the pile‐soil interaction properly, but they are computationally expensive. This study presents a semi‐analytical framework for a single energy pile embedded in elastoplastic soil, developed using the principles of virtual work and variational calculus. The pile‐soil system is considered as a continuum in the present analysis, and the interaction between the pile and the soil is represented by a judiciously chosen compatible soil displacement function related to the pile displacement. The formulation employs an incremental procedure, current state variables are updated from the previous increment. Soil stiffness is consistently updated based on stress evaluation, performed through the cutting plane stress integration scheme. The elastic soil response before yield is considered to be nonlinear, which is represented through shear modulus degradation based on the strain developed in the soil. The Drucker–Prager constitutive model is adopted to capture the plastic behavior of soil. The temperature change in soil that is caused by the temperature change of the energy pile is calculated using the heat distribution model for a finite length heat source. An iterative solution algorithm with the one‐dimensional finite difference method is used to solve the governing differential equations for pile and soil displacements. The accuracy of the proposed method is validated against experimental results under mechanical, thermal, and combined loading. The method is used to investigate the response of end‐bearing and semi‐floating energy piles subjected to mechanical and monotonic or cyclic thermal loads. The effect of cyclic thermal load on the null point in an energy pile is also discussed. Additionally, the study highlights the inherent capability of the present model to capture the nonlinear pile‐soil interaction through several soil resistance‐pile displacement plots. The present model can be used to determine the axial displacement and stress in an energy pile under different loads for the purpose of structural design of the pile.