Axial Stress Distribution in Geothermal Energy Pile Group in Sand

Abstract

The integration of geothermal heat exchange techniques in structural pile foundation is gaining acceptance to cater the heating and cooling requirements of a building. The purpose of the present study is to analyze the response of energy pile foundations subjected to heating and cooling cycle through numerical investigations. For this purpose, a highlight of finite element method is used to simulate energy pile group. The interaction between the piles in a group is analyzed during thermal cycle and constant mechanical loading of the piles. The pile response is considered to be linear elastic. The soil behavior is reproduced using a constitutive model CASM, based on concepts of critical-state soil mechanics. The CASM model has been implemented in finite element software Abaqus through user-defined material subroutines. In the present study, the axial stress distribution in the piles in a group of six piles is analyzed. It is observed from the results that the thermal piles are subjected to heating-induced excess load in addition to the axial mechanical load from the superstructure. However, for nonthermal piles, unloading of the piles happens during heating of the thermal piles. Heating of the thermal piles causes an increase in the axial stress in the thermal piles and decrease in axial stress in the nonthermal piles. However, the cooling of the piles causes decrease in axial stress in thermal piles and an increase in axial stress in nonthermal piles. At the end of cooling, there is a setup of additional axial stress in the thermal piles as compared to the nonthermal piles. The analysis results conclude that the thermally induced axial stress should be taken into consideration for the geotechnical design of energy piles.