A new method for analyzing the thermomechanical behavior and the null point movement of energy piles

Abstract

During heating or cooling, an energy pile elongates or shortens around the null point, where the thermally induced displacement is zero. Previously proposed computing methods typically require an initial null point assumption and iterations before calculating the thermomechanical response. We proposed a new method to directly calculate the thermomechanical response of energy piles based on the boundary shape function and load transfer approach, and we verified its feasibility by comparing the results with a field test. Then, this work investigated the effects of the pile head and tip restraint conditions, soil restraint, and thermal and initial mechanical load magnitudes on the thermomechanical response of energy piles through a series of case studies, particularly focusing on the interaction between the aforementioned influencing factors and the null point location. The analysis demonstrates that the position of the null point is primarily determined by the end restraint boundary conditions of the pile; however, changing the soil restraint and thermal load will cause the null point to move, and the direction and distance of the movement are related to the symmetry between the two extremity restraint conditions. Finally, changing the initial mechanical load also shifts the null point position.