Research On Mechanical Characteristics Of Energy Piles With Different Buried Pipes Under Thermo-mechanical Coupling

With the promotion of green economy in the integration of the world economy,energy saving and environmental protection have been recognized by the public,and the application of energy pile ground source heat pump system in geotechnical engineering field is increasing.The energy pile is built by embedding heat exchange pipe in the pile foundation,and the circulating liquid flows through the pipeline of heat transfer in the pile foundation,which causes the temperature of the pile body to rise and fall,a new type of the system of energy pile is composed of energy pile foundation and water flow through the upper surface heat transfer system.As a load-bearing member,the energy pile acts as a medium of heat exchange,which can be used to heat or cool the building through the shallow geothermal energy,this is in line with China’s energy conservation,pollution reduction and sustainable development policies.As the temperature of the circulating liquid increases or decreases,the temperature of the pile body will also increase or decrease in the process of the transfer in the pile,resulting in some thermal stress in the pile body.The thermal stress will not only change the stability of the pile,but also affect the settlement of the pile,which will endanger the safety of the whole structure of the building.In this paper,through the indoor model test and finite element software ABAQUS,the thermodynamic characteristics of the energy piles with different buried pipes in clay foundation under the action of load or not are studied,the variation laws of soil temperature around pile,pile body stress,pile body friction,pile tip pressure stress and vertical displacement of pile top are obtained,the main conclusions are as follows:(1)Variation of soil temperature field around pile: under the same temperature field,the longer the pipe of heat exchange pipe,the pipeline of heat transfer speed of soil is.The heat is gradually transferred from the heat exchange pipe inside the pile to the outside of the pile.When the temperature rises,the temperature of soil is highest near the pile body,and decreases gradually away from the pile body,and the temperature of soil increases first and then decreases vertically.(2)The variation of axial force of pile body: the stress curve of pile body under different types of buried pipe has the same change trend,when zero load action,the stress of the pile first increases and then decreases along the depth,at this time,the maximum value of pile body stress produced by the energy pile appears in the lower part of the pile body,because the top is restrained under the load action,the pile body stress produced first increases along the depth and then decreases,and the increment near the top of the pile body is larger.(3)Variation of pile side friction: The pile body expands or contracts with the change of temperature,the pile body generates side friction through the contact with the surrounding soil,when the top of the pile is not loaded,heating or cooling the circulating liquid in the pipeline of heat transfer,the upper part and the lower part of the pile body produce negative side friction or positive side friction respectively,and the side friction produced by the pile body is in the same direction with the load restriction at the top,and the temperature has little influence on the pile body.(4)Vertical displacement variation of pile top: The temperature rise and fall causes the pile body to have the upward displacement variation and downward displacement variation respectively,when the pile is heated,the value of expansion is less than that of zero load,and the maximum displacement of pile top decreases with time,but the maximum displacement of pile top increases with zero load.(5)Change of compressive stress at pile tip: with or without load,the upward or downward movement of pile produces compressive stress on pile-tip soils,and the maximum compressive stress decreases gradually with the increase of cold and heat cycles.The compressive stress of pile tip under load is greater than that under zero load.(6)The variation of different buried pipe forms in the energy pile: The test results show that different types of buried pipe also affect the thermodynamic characteristics of the energy pile,especially the spiral(P =5cm)type buried pipe pile,spiral(P =10cm)type,the type of double U and single U type pile from large to small,because spiral(P =5cm)type pile produces the most stress and settlement when it is heated and cooled,therefore,the spiral(P=5cm)type buried pipe has the greatest influence on the safety of the building structure.(7)Numerical simulation: In this paper,the finite element software ABAQUS is used to simulate the behavior of energy piles in clay foundation,considering the effect of zero load and load on the top of pile,several cycles of cooling and heating are carried out.Finally,the results of numerical simulation are compared with those of the model test in the laboratory,it is found that the results of the two methods are consistent,which verifies the feasibility and accuracy of the numerical simulation.(8)Economic analysis: comparing the solar-energy pile heat storage system with the ground-source heat pump heat exchanger and the air-conditioning heating system,it is found that it can not only reduce the initial investment cost and the maintenance cost during the operation period,through the analysis of heat exchange per unit well depth and comprehensive evaluation indexes,it is concluded that the spiral buried tube heat exchanger has the highest heat exchange capacity but also the highest cost,secondly,double u-type and single u-type ground source heat pump with double u-type buried pipe can be used as the priority in practical application because of its low heat transfer capacity.