Energy Pile _ thermal axial stress

Hi all,  I am modelling the energy pile behaviours under different thermal operations ( by setting the different thermal boundary conditions to pile boundaries to see the temperature-induced volume change of pile and soil). For the initial installation stage, I used Plastic + steady-state groundwater flow + steady-state thermal flow; for the stages of the thermal operation, I used Fully coupled flow-deformation + Use temperatures from the previous phase. According to experimental results, I can see that the temperature distribution and pile head displacements under thermal operations are both responsible. Still, the only issue is my thermal axial stress (Cartesian effective stress) at the pile head near the ground surface had a very large tension ( it needs to be 0 kN/m^2 ). I hope to ask for any ideas or suggestions to double-check my function inputs.