Research On Non-isothermal Effects On Physical And Mechanical Properties Of Saturated Soils

Shallow geothermal energy is a kind of renewable energy.Ground source heat pump can transfer the heat in buildings and soil to achieve the purpose of refrigeration or heating,which is an environmentally friendly clean renewable energy technology.The energy pile can organically combine the building pile foundation with the ground source heat pump,that is,to achieve the purpose of bearing the building.It can also effectively use the shallow geothermal energy to store abundant geothermal resources,provide heating,hot water and air conditioning for the building,and also reduce the additional investment cost of ground source heat pump.However,there are still some problems to be solved in the application of energy piles.For example,the temperature cycle will lead to the temperature change of the pile-soil cross section and the surrounding soil,and whether the resulting settlement or bearing capacity change will affect the safety of the building is the key issue in the application of energy pile technology.Therefore,only by studying the non-isothermal effect of soil physical and mechanical properties and clarifying the mechanism of soil THM coupling can the safe operation of energy piles be ensured.In terms of the test,the non-isothermal bound water content test and non-isothermal permeability test were carried out by using the self-made bound water permeability coefficient test device.The bound water content test was mainly carried out on the sand particles mixed with clay particles molding soil at different temperatures.The influence of sand particle size and clay particle content on the bound water content was studied by controlling the sand particle size and clay particle content.The test results show that the increase in temperature will lead to a significant decrease in the bound water content,while the bound water content of molded soil is higher than that of dispersed particles.In the range from fine sand with similar water content to coarse sand,the particle size does not significantly change the bound water content of clay-containing molding soil.In the range of 5 % – 20 % of clay content,the clay content and the bound water content of soil are approximately linear.The permeability test found that under the condition of variable temperature,when only free water was considered to participate in permeability,the theoretical value of permeability coefficient would overestimate the permeability of sand and underestimate the permeability of clay.In the compression / consolidation test of dry sand and saturated clay sand under cyclic temperature,it is found that the volume shrinkage of dry sand and saturated clay sand is caused by temperature cycle,and the low density dry sand is more likely to produce significant thermal shrinkage.With the increase of pressure or temperature cycle amplitude,the accumulation of thermal deformation will also increase.It is believed that the thermal plasticity of dry sand is mainly caused by the randomness and heterogeneity of particle system structure,while the thermal plasticity of saturated clay-containing sand is more related to the change of bound water state and the difference of thermal expansion between sand and clay.In theory,a permeability coefficient model considering the temperature effect of bound water is established,and the model is used to predict the test results.The prediction results show that when the bound water content of soil is high,it will be more accurate.The basic thermodynamic principle of saturated soil is summarized,and the TM coupling constitutive model of saturated soil based on this is given.This model was used to study the effects of stress state,water content,temperature cycle amplitude and cycle number on TM coupling behavior of saturated silty clay.The simulation results of existing tests show that the model can well reflect the consolidation phenomenon of clay under nonisothermal conditions,and the confining pressure,initial density,thermal load amplitude and cycle number have a great influence on the thermal deformation and pore pressure of soil.