Fully Coupled THM Analysis And Its Application In Energy Pile Engineering

Energy pile is a new type of ground source heat pump technology with both load and heat transfer functions.In the process of operation,energy piles are subjected to both mechanical load and thermal load,which will produce a prominent coupled thermo-mechanical response,and the coupled thermo-hydro-mechanical(THM)feature will also emerge in the soil adjacent to the pile shafts.In addition,the mechanical properties of soil,pile–soil interaction and long-term thermal cycling will affect the performance of energy piles and the behavior of soil around piles,and then affect the long-term stability of superstructures and the utilization efficacy of geothermal energy.However,most of the existing numerical simulation studies of energy piles only consider the thermo–mechanical coupling and simple constitutive relations of soils,which cannot reflect the coupled THM effect and the relevant mechanical properties of soil.Therefore,based on the coupled THM theory,a fully coupled finite element analysis is developed,which considers the thermo–elasto–plastic constitutive relationship of soil and the pile–soil nonlinear contact effect,and is applied to the thermo–mechanical analysis of the energy pile under long-term unbalanced cyclic thermal loads.The main work and conclusions of this thesis are summarized as follows:(1)Based on the conservation equations of momentum,mass and energy,the fully coupled THM governing equations for saturated porous media are derived,and the variations of physical properties of the fluid with temperature are considered.The results of numerical simulation agree well with the analytical solutions of the three typical coupled THM problems,which verifies the correctness and validity of the derived equations and its numerical implementation,and highlights the necessity of considering the fully coupled THM effect.(2)The state-dependent Mohr–Coulomb model and the unified hardening(UH)model considering temperature effects are implemented in COMSOL software.The comparison with experimental data indicates that the methods can accurately simulate the mechanical behavior of soil.Note that the state-dependent Mohr–Coulomb model is suitable for sandy soil,with the emphasis on its state dependency,while the UH model considering the temperature effects is suitable for cohesive soil,mainly focusing on its thermoplasticity.(3)The typical in-situ tests are simulated by considering the fully coupled THM equations,the variation of fluid properties with temperature,the constitutive relationship of soil and the pile–soil contact conditions.The simulation results show that the fully coupled numerical model developed in this study can well reflect the complex thermo–mechanical behavior of the energy pile.(4)The axisymmetric finite element numerical analysis of the energy pile considering unbalanced cyclic thermal load is developed.The results show that the temperature,displacement,pore pressure and stress of the energy pile and the soil around the pile all exhibit cyclic responses with seasonal heating and cooling.In the region with unbalanced energy demand,the temperature and displacement of the energy pile and the soil around the pile show similar but opposite cumulative changes,but there are few cumulative changes of axial stress in the pile and shear stress at the pile–soil interface.