Research On Multi-field Coupled Constitutive Model For Soils Based On Thermodynamic Theory Of Granular Matter

The study of multi-field coupled model of geomaterials is the theoretical basis and key technology of some practical engineering problems,such as engineering and geological disaster prevention,nuclear waste storage,geothermal or natural gas resources exploitation,etc.With the development of engineering construction,geotechnical engineering is becoming more and more difficult,which is facing severe challenges.And there is an urgent need for the support of multi-field coupling theory and analysis technology of geomaterials,in order to improve the quality of geotechnical engineering,save investment in engineering construction,improve the efficiency of resource exploitation and enhance disaster prevention and mitigation capabilities.In addition,as a special kind of soil,the engineering properties of gassy soils are easily disturbed by the external environment.Exploring the evolution law of the properties of gassy soils before and after disturbance can establish a reasonable theoretical system to reveal its catastrophic mechanism,so as to provide guarantee for the exploitation and utilization of natural gas and marine engineering infrastructure.Based on the thermodynamic theory of granular matter,a multi-field coupled thermodynamic model framework for unsaturated soils is established in this paper.Under the framework of the model,the application of the model is analyzed for saturated clay,fine-grained gassy soil,saturated sand and gassy sand respectively.The main research results include:(1)Based on granular solid hydrodynamics theory(GSH)and mixture theory,extending the GSH modeling method to the three-phase mixture system of unsaturated soil,considering the energy dissipation process at granular level caused by temperature and saturation changes,and combined with the modified soil-water characteristic curve(SWCC)model considering deformation and temperature effects,this paper presents a thermo-hydro-mechanical(THM)coupled model for unsaturated clay soils.The model introduces concepts of granular entropy and granular temperature.The dissipative structure composition and the migration coefficient relationship of thermodynamic system can be determined theoretically.Based on the thermodynamic differential equation,conservation equations and entropy increase equations of unsaturated soils,thermodynamic identities are established,and the constitutive relation of inelastic deformation is obtained by thermodynamic identities.And the relationship between dissipation mechanism of unsaturated soils and macro physical and mechanical behavior is established by migration coefficients and energy function model.Compared with the existing experimental results,it is proved that the model has the ability to describe the hydro-mechanical and thermo-hydro-mechanical coupling characteristics of unsaturated soils,including effect of dry-wet cycle on consolidation characteristics,the consolidation and shear properties at different temperatures and suction,and the evolution of thermal volume strain under non-isothermal conditions.(2)Based on the basic framework of the proposed multi-field coupled model for unsaturated soil,a constitutive model describing the thermo-mechanical coupling characteristics of saturated clay is developed by degrading the model to the condition of saturated soils.Laboratory tests were conducted to investigate the thermal consolidation behavior of saturated silty clay soils using an axial thermal consolidation test apparatus suitable for saturated hollow cylindrical specimens.The effects of over consolidation ratio(OCR)on thermal consolidation characteristics with different heating-cooling paths and confining pressures are discussed.The evolution law of the pore water pressure caused by undrained heating/cooling and volume strain caused by drainage consolidation with time are analyzed.By comparing the thermal consolidation test results and the thermal response test results induced by cyclic temperature load with the model simulation results,the ability of the proposed model to describe the thermal consolidation characteristics of saturated clay is verified.(3)On the basis of the theoretical framework of the multi-field coupled model for unsaturated soils proposed in this paper,combined with the ideal gas state equation,considering the effect of gas on the plastic deformation of soil skeleton,a thermodynamic model of fine-grained gassy soil is proposed,and its effective stress is analyzed.According to the conceptual model of soil containing large gas bubbles proposed by Wheeler(1988a),fine-grained gassy soil is considered to be composed of big gas bubbles and saturated soil matrix.In the model,the gas pressure is affected by total stress and pore water pressure,and it is linear with total stress,while the saturated soil matrix is controlled by "quasi-effective stress".By comparing the simulation results with the test results,the model's ability to describe the compression and undrained shear properties of finegrained gassy soil is verified.What's more,the effect of temperature on fine-grained gassy soil considering the various responses for different drainage conditions and overconsolidation ratios is discussed and simulated by the proposed model,including compression,undrained shear properties and temperature-induced changes in pore water pressure and thermal volume strain.(4)Based on the theoretical framework of the proposed multi-field coupled model of unsaturated soils,and the dilatancy equation with state parameters,a thermodynamic constitutive model describing the dilatancy of saturated sands is proposed.The form of the model is relatively simple,which can describe the influence of relative density and effective confining pressure on the strength and deformation characteristics of saturated sand during shear through a set of model parameters.Comparing the simulation results with the results of isotropic compression,triaxial undrained and drained shear tests of saturated sands,the ability of the model to describe the compression and shear properties of saturated sands is verified.For gassy sand,it is considered that the existence of bubbles only increases the compressibility and does not affect the deformation of soil skeleton.Based on the proposed thermodynamic model of saturated sand,and the relationship between pore gas pressure and pore gas volume established by Hilf(1948),a thermodynamic constitutive model for gassy sand is developed.Through the simulation of triaxial undrained shear test results of remolded sand with different saturation,the ability of the model to describe undrained shear properties of gassy sand is verified.