Study On Thermo-mechanical Behavior And Load Transfer Mechanism Of Single Energy Pile In Loess Foundation

Energy pile is a new type of geothermal energy utilization technology,which realizes the heat exchange between building and foundation soil through the circulation tube built into the pile body,bearing the upper load and satisfying the building energy supply at the same time.Compared with the traditional building pile foundation,the energy pile operates with heat expansion or cooling contraction,which makes the pilesoil extrusion and shear interaction more complicated,and the long-term heating and cooling cycle may reduce the safety and usability of the pile foundation.This paper studies the thermo-mechanical characteristics of energy pile under two conditions of unimmersed and immersed loess foundation by indoor model test,and defines the range of variation of pile shaft resistance and end resistance of energy piles affected by temperature changes based on theoretical method,which provides reference for the design and application of energy piles.The main conclusions obtained are as follows.(1)Firstly,the thermal-mechanical coupling model test of energy pile in unimmersed loess foundation under multiple cooling and heating cycles was carried out.The heat exchange rate of energy pile,the temperature variation law of pile body and soil around the pile were studied,the distribution law of the internal force along the depth of the pile body is analyzed,and the cumulative settlement changes of the energy pile top and surrounding soil under multiple thermal cycles were obtained.The research results show that with the cooling and heating cycle,the temperature of the pile body,1D and 2D soil from the pile body show hyperbolic,linear and parabolic trends respectively.The maximum axial force in the heating stage appears in the middle of the pile body,while the lower part of the pile body is pulled in the cooling stage,and the friction resistance of the pile body within the range of 200mm-1000 mm is positive;the cumulative settlement curve of pile top and surrounding soil are all serrated.(2)Secondly,the thermal-mechanical coupling model test of energy pile in immersed loess foundation under multiple cooling and heating cycles was carried out,and the influence of soil immersion on the thermal response of energy pile is studied.The results show that compared with the unsoaked conditions,the heat exchange efficiency in the wet range is increased,the temperature change of the soil around the pile is reduced,the soil around the pile moves upward in each cycle heating and cooling stage,the change of soil constraint around the pile makes the distribution of internal force of pile body complex,and the cumulative displacement of pile top also changes in serration,but the settlement value becomes larger.(3)Finally,based on the existing field test results,the variation range of the shaft friction resistance influence factor λ and the end resistance influence factor δ of friction type and end-bearing friction type energy piles in silt and clay is defined,and the λvalues of the energy piles in Loess Foundation under the two working conditions are given.The range of the coordination coefficient η in the calculation formula of the shaft friction resistance influence factor is back calculated,and the calculation formula of the change of δ with temperature is obtained.