Thermal Response Of Energy Pile Considering The Influence Of Temperature On The Stratum Properties In Shanghai

Chinese government departments vigorously advocate the use of green energy and reduce the application of fossil energy.At present,with the proposal of"carbon peaking and carbon neutrality",energy piles have great development prospects in the fields of geothermal and green buildings.When the energy pile bears the upper load,it also needs to provide heat exchange for the upper structure.The temperature change leads to the redistribution of the stress in the soil.When the bearing capacity of the ground layer is not enough to bear the external load of the upper part,it will be damaged.Therefore,the influence of the temperature stress between the pile and the soil on the bearing capacity of the pile foundation may become a hidden danger in the engineering construction,which puts forward higher requirements for the design and installation of energy piles.Based on this,this paper considers the influence of temperature on the thermal properties and mechanical parameters of the formation,and studies the heat transfer performance and mechanical properties of the energy pile during operation.The main research contents and results are as follows:（1）The thermal consolidation test was carried out by using a temperature-controlled multi-functional triaxial instrument to study the（4）gray silty clay,（5）₁ gray clay,and（5）₂ gray silty clay in Fengxian District,Shanghai under different confining pressures（50,100,200,300 k Pa）and temperature（10,21,30,37℃）were analyzed,and the effects of confining pressure and temperature on the dissipation of pore water pressure and the change of pore water discharge were analyzed.The test found that:pore pressure dissipation and pore water discharge experienced three stages:fast,slow and stable;the degree of pore pressure dissipation and drainage increased with the increase of confining pressure and temperature;with the increase of temperature,pore pressure dissipation and drainage reached stability The time required for the stage is reduced.（2）The thermal properties of the samples were tested with a thermal constant analyzer,and the effects of temperature（10,21,30,37℃）on the thermal properties were discussed.The study found that the thermal conductivity and thermal diffusivity of soil are proportional to the temperature.As the temperature increases,the thermal conductivity and thermal diffusivity increase;when the temperature rises from 21℃to37℃,the thermal conductivity The diffusion coefficient is increased by 126.3%.With the increase of temperature,the specific heat capacity gradually decreased.When the temperature increased from 21℃to 37℃,the specific heat capacity decreased by a maximum of 48.4%.The test results were fitted to obtain the functional relationship between thermal conductivity,specific heat capacity,thermal diffusivity and temperature.（3）Direct shear tests under different vertical pressures were carried out on samples from various layers in Fengxian District,Shanghai,and the influence of changing the interface shear material on the mechanical properties of the interface was analyzed.At the same time,by changing the test temperature of the pile-soil interface,the influence of temperature on the shear stress,cohesion and friction angle at the interface is discussed.The test shows that when the temperature of the pile-soil interface increases,the interface shear stress and friction angle increase,and the cohesion first decreases and then increases.（4）Combined with the test platform built by the Fengxian Bay Shallow Geothermal Energy Science Test Site,the field test of the buried pipe of the pile base was carried out to study the change of the heat transfer efficiency of the energy pile,the temperature of the pile body and the soil body around the pile,the axial force of the pile body and the settlement of the pile top.law.The test found that the heat transfer along the meter of the energy pile corresponding to the inlet temperature of 37,33,10,and 5°C was 146 W/m,126 W/m,32.3 W/m and 61 W/m,respectively.The axial force of the body is proportional to the load on the top of the pile;when the load reaches 1600k N,the maximum lateral friction resistance is about 40 k N,which is located at 12 m in the middle of the pile body;The cumulative displacements are 4.12 mm,5.48 mm and6.23 mm respectively,that is,the cumulative amount of settlement in the heating condition decreases,and the cumulative amount of settlement in the cooling condition increases.（5）Due to the limited field test data,it is difficult to obtain the overall change law.Therefore,this paper uses the COMSOL Multiphysics software to establish a three-dimensional model of the energy pile and verify the model.On the basis of the above laboratory tests,according to the field test conditions,the fitting formula of the effect of temperature on formation parameters is brought into the model,and the heat transfer and stress characteristics of energy piles under the influence of temperature on formation parameters are further analyzed.