Experimental And Numerical Evaluations Of Stress-Strain Behavior Of Saturated Sand Under Thermo-Mechanical Coupling Process

Exploitation of the shallow geothermal energy is now an interesting topic with the rapid development of renewable energy and geothermal heat pump technologies.However,the theory of the shallow geothermal energy falls far behind its application.In particular,the mechanism of soil under the thermal-mechanical coupling remains unclear.Base on the temperature-controlled triaxial experiment and numerical simulation,the deformation and strength behaviors of hollow-cylinder saturated sand specimen due to temperature change were performed systematically.Moreover,the effects of mean effective stress,initial relative density and loading-unloading conditions were also emphatically considered.The DEM method considering temperature change was preliminarily discussed.The main scopes and results of this study are given as follows:?1?The effects of thermal expansion of the apparatus on the thermal axial and volumetric strains of specimens were calculated by a series of heating and cooling calibration tests on metallic specimen at different mean effective stresses.And then,under the frame of thermomechanical theory,a method to calculate the actual thermal axial and volumetric strains of saturated sand specimen were developed by considering the thermal expansion of the apparatus.Test results observed that the axial strain of the metallic specimen was nearly reversible during the heating and cooling phases,which indicated that the influence of thermal expansion of the apparatus on the actual thermal axial strain of specimen were little enough to be neglected.In addition,the volume change of the metallic specimen during the temperature cycle could be classified into two components,which were reversible and irreversible.This irreversible component which was the change in volume of the apparatus due to temperature change could not be neglected and could be used to correct the volumetric strain of the saturated sand specimen during heating.?2?An investigation of the thermal axial and volumetric strains,peak state,normal consolidated state and critical state behaviors of dense saturated sand specimens were involved through a series of temperature-controlled,hollow cylinder triaxial tests under different mean effective stress and temperature conditions.Test results observed that the thermal axial and volumetric strains of dense,saturated sand specimen were negative,reflecting expansion,and it would increase linearly with increasing temperature.While the mean effective stress were not observed to have a major effect on the thermal axial and volumetric strains.The order of magnitude of the thermal expansion coefficient of the sand specimen was the same as that of sand skeleton,which indicated that these two coefficients were roughly the same scale and the thermal expansion of sand skeleton had the main effect of the thermal volumetric strain of saturated sand specimen during heating.With increasing mean effective stress,this coefficient of sand specimen had no obvious change.Otherwise,as the temperature increased,the deviatoric stress at peak state decreased linearly,the pore water pressure at peak state increased linearly,while the tangent moduli had no major change.As the mean effective stress increased,the deviatoric stress and pore water pressure at peak state increased.With an increase in temperature,the intercept and slope of the critical state line were constants,which indicated that the critical state lines at different temperatures were unique.?3?The deformation and strength behaviors of saturated sand specimens were investigated by a series of temperature-controlled,hollow cylinder triaxial tests under different initial relative density and temperature conditions for a constant mean effective stress.Test results observed that the thermal axial and volumetric strains were negative,reflecting expansion,and an increase in temperature would lead to both of them increase in a linear trend.An increase in initial relative density of sand specimens would lead to a linear increase in thermal axial strain,while a linear decrease in thermal volumetric strain.The thermal expansion coefficient of sand specimen depended on its initial relative density,and was between that of pore water and sand skeleton.As initial relative density went up,the thermal expansion coefficient of sand specimen would decreased linearly to the same scale of that of sand skeleton.Then,the thermal volumetric strain of sand specimen would be caused from the common work of pore water and sand skeleton when it was loose to the alone work of sand skeleton when it was dense.With increasing temperature,the deviatoric stress decreased,while the pore water pressure increased.?4?A series of temperature-controlled,hollow cylinder triaxial compression tests were conducted to perform the water expelled,axial and volumetric strains behaviors of the loose and dense saturated sand specimens under different mechanical loading,mechanical unloading,and drained heating conditions.Test results indicated that during the mechanical loading phase,water would expelled from the sand specimen,and the axial and volumetric strains would be positive,reflected compression.During the mechanical unloading phase,water would flow into the sand specimen,and the axial and volumetric strains would be negative,reflected expansion.During the drained heating phase,water expelled from the sand specimen,but the axial and volumetric strains of the sand specimen would be negative,reflected expansion,because the compression deformation caused by the water expelled was smaller than the expansion deformation caused by the thermal expansion of pore water and sand skeleton.?5?A series of numerical simulations using PFC^3D were developed to indicated that effect of temperature on the particle radius,thermal axial and volumetric strains of hollow cylinder specimen under different mean particle size,initial relative density,mean effective stress and temperature conditions.Numerical results observed that the mean particle radius would increase with an increase in temperature and the change of that was dependent with temperature and initial particle size.Moreover,the thermal axial and volumetric strains of model specimen were negative,reflected expansion,and would increase with increasing temperature in a linear trend.