Experimental Study On THM Coupling Process Of Unsaturated Bentonite

The development and utilization of nuclear energy are accompanied by the generation of nuclear waste.At present,the internationally recognized treatment method is deep geological disposal,which uses geological body to permanently isolate nuclear waste from the ecological environment.As the filling material between the nuclear waste container and geological body,bentonite is an important factor to ensure the function and stability of deep geological disposal reservoir.In the practical application process,the buffer material will experience the process of hot water force.Unsaturated bentonite as buffer backfill material in the process of work at the same time by the nuclear waste in the process of decay heat and groundwater infiltration,the soil heat and moisture transfer and stress can occur within the growth,the process involves polyphase body and how the change of the physical fields,for further research of unsaturated soil under the action of temperature and water temperature,moisture and stress change rule and the process,In this paper,the hydrothermal coupling tests of unsaturated bentonite under temperature and hydraulic action were carried out.On the basis of heat transfer theory,seepage theory and mechanical model of unsaturated soil,the phenomenon and law of the test are analyzed.Combined with the physical model of the test,the finite element software is used to verify the test.The main research contents and conclusions include:(1)In view of the physical environment of buffer backfill material in the actual application process and the presence of construction joints,a one-dimensional unsaturated soil hydrothermal coupling test device was developed and tests were carried out under three different conditions.The test device can study on the heat transfer of soil,seepage,hygroscopic and expansion of steam migration,thermal expansion,problems such as,using the sensor for real-time detection internal points of soil temperature,moisture content and soil pressure volume quantities,will test results were analyzed,and get the unsaturated bentonite in the process of test response law of various physical quantities over time.(2)Based on the theory of unsaturated soil and the hydrothermal coupling model,the finite element software was used to verify the existing tests.The evolution laws of temperature,moisture and stress of bentonite under given boundary conditions were analyzed.The results of numerical calculation were compared with those of test.It was found that the numerical calculation results were consistent with those of test results,which verified the reliability of the model.(3)The temperature transfer,water transport and stress growth in soil are mutually coupled.The temperature is mainly transmitted in the form of heat conduction in the soil,and the seepage velocity in the soil is small,and the heat carried by the heat convection term is small.The water migration includes the migration of liquid water and water vapor,and the stress growth includes hygroscopic expansion and thermal expansion,of which the thermal expansion has a small contribution to the stress.(4)The initial dry density,initial moisture content and insulation layer thickness of soil have significant effects on the development process of temperature,moisture and stress.In terms of heat transfer,the higher the dry density is,the faster the heat transfer speed will be.The higher the steady-state temperature of each point in the soil body,the thicker the insulation layer,and the higher the steady-state temperature value will be,and the improvement effect is more obvious than that of the dry density.In terms of water migration,the higher the dry density,the slower the migration speed of liquid water and water vapor,and the higher the initial water content,the more water vapor migration.In terms of stress development,the higher the dry density is,the greater the maximum expansive force of soil energy will be,which has an exponential relationship with the dry density.The development of expansive force is mainly affected by the distribution of water field in soil.