Unified Constitutive Model Of Thermal Hardening And Thermal Softening Behavior Of Geothermal Materials And Its Applications

Some projects,such as application of energy piles,storage of nuclear waste and development of geothermal resources,are need to consider the effect of temperature on the mechanical properties of soil.From the results of a large number of temperature controlled triaxial tests,it can be known that shear strength of different type saturated soils may be improved(thermally hardened)or weakened(thermally softened)after heating.Therefore,this paper proposes a thermo-elastoplastic soil model that can simultaneously reflect these two different experimental phenomena.Next,the proposed constitutive model is secondary developed.Finally,the model is used to carry out some thermo-hydro-mechanical coupling numerical analysises of some geothermal heat storage problems.The main research contents include:(1)A new thermo-elastoplastic constitutive model is obtained by introducing temperature evolution formula of preconsolidation pressure and critical state parameter into the yield function of Subloading Modified Cambridge Model.The model uses a joint flow rule,and the plasticity expression is re-derived through a coordination equation.A calculation program that can simulate some triaxial tests is written by Fortran,and the new model is placed in it.The rationality of this model is verified through some temperature controlled triaxial drainage and undrained shear tests.In addition,the effects of some parameters in the new model on the shear strength after heating are discussed.The research results show that the simulation results of this new model fit stress-strain relationship in the test well and can accurately reflect thermal hardening or thermal softening of different type soils after heating.It can be known from discussion of several examples that emergence of heat hardening is due to the greater proportion of temperature influence on the critical state parameter,and vice versa as thermal softening.The larger the initial overconsolidation ratio and the parameter which can control development of overconsolidation ratio is or the smaller the rebound index is,the more apparent the thermal softening of soil becomes.(2)ABAQUS can achieve the purpose of secondary development by writing a subroutine.This paper uses explicit second-order algorithm to perform stress integration,and completes the writing of the new model subroutine.The verification of the subroutine is mainly divided into two parts,one is to simulate some unit tests,and the other is to calculate multiple units.The results show that the results of several reference examples are consistent with subroutine calculation results,and this subroutine is written correctly.(3)ABAQUS is used to simulate some field and indoor model tests of energy pile,among which soil model is this model that is mentioned in this paper.Some effects of heating and cooling cycles on the pile mechanical properties under different pile top loads and different types of foundation soil are further studied.The results show that the simulation results predict pile-soil temperature distribution,pile strain,and axial force of pile body.Increasing the overconsolidation ratio,permeability coefficient,and rebound index of foundation soil,the final pile top settlement after ten temperature cycles will decrease.The effects of over-consolidation ratio on the additional side friction resistance and additional axial force of pile is small.However,decreasing the permeability coefficient or increasing the rebound index will decrease the additional side friction resistance and the additional axial force.(4)The Mont Terri Underground Laboratory conducted a field exothermic test of underground geotechnical layers to investigate some effects of heat release from the nuclear waste storage room on the surrounding soil.This experiment is numerically simulated.Based on this,two calculation examples are discussed,and the influence of critical state development control parameter g on the pore pressure,stress and effective stress path near the heat source is studied.The results indicate that the calculation results reasonably reflect the changes of temperature,pore pressure and strain at various points in the soil over time during the test.The pore pressure and Mises stress value of g greater than 0 are greater than g equal to 0 at different times,and this gap is continuously expanding during the continuous heating process.The effective stress paths of these two examples also produce different trajectories.