Research On Thermo-Mechanical Characteristics Of Energy Piles In Clay Soil Under Cyclic Temperature

Energy pile is the combination of buried pipe and pile foundation in ground source heat pump system.Embed the heat exchange tubes in the pile fou ndation.The heat exchange liquid circulates in the pile foundation through the pipeline instead of the traditional buried pipe technology.The pile foundation and the upper surface heat exchange system together form the energy pile system.The energy pile can not only be used as a load-bearing member to bear the upper load,but also as a medium for heat exchange between the superst ructure and the shallow ground.In line with China's policies on energy conser vation and pollution reduction.In recent years,more and more scholars have started research on energy piles.But most of the research on the thermal respon se of energy piles.Due to the heat exchange with the soil,the energy pile will cause expansion and contraction and cause some changes in the mechanical properties of the pile itself.However,few studies on the mechanical properties of energy piles in cohesive soil.Therefore,in this paper,through the indoor model test and finite element numerical simulation analysis method,the stress characteristics of the energy pile in the clay foundation are studied.The main work content and conclusions of this article are as follows:(1)Temperature field distribution: The higher the temperature of the circulating fluid in the energy pile system.The faster the soil temperature spreads.The temperature propagation is centered on the heat exchange tube and diffuses outward in a circle.The distribution of temperature changes along the depth of the soil increases first and then decreases during the heating process.During the cooling process,thetemperature change value decreases along the depth of the soil and then increases.(2)Settlement of pile top: As the temperature rises and falls,the pile top position moves up or down.When the top of the pile is at zero load,the top is free to expand and contract.The pile top displacement gradually increases with increasing temperature,gradually decreases with decreasing temperature.Additional settlement at the top of the pile at the end of one cycle;When there is a load on the top of the pile,the initial displacement of the pile top is negative.As the temperature rises,the position of the pile top cannot be freely extended.Therefore,the change in displacement at the top of the pile is less than that at zero load.In the cooling stage,the pile top produced a larger change in pile top displacement than at zero load.(3)Pile end compressive stress: At zero load at the top,the higher the tempe rature,the greater the compressive stress generated at the pile end.As the nu mber of temperature cycles increases,the maximum compressive stress gradually decreases.When there is a load on the top,the pile end produces a greater compressive stress than when there is no load.Since the compressive stress caused by the load on the pile top is greater than the compressive stress caused by temperature,the compressive stress change caused by temperature change under load is less than the compressive stress change at zero load.(4)Pile body stress: When the load on the top of the pile is zero,the tensile and compressive stress is generated in the pile due to the temperature rise and fall.The stress increases first and then decreases along the pile body from the top to the end of the pile.The maximum stress is near the lower half of the pile.When there is a load on the top of the pile.The maximum stress of the pile body is at the top of the pile.From the pile top to the pile end,the pile body stress gradually decreases.(5)Side friction resistance: At zero load on the top of the pile,the higher the temperature,the greater the side frictional resistance of the pile.During heating and cooling,the pile body produces negative frictional resistance in the upper and lower parts.When there is a load on the top of the pile,the sidef rictional resistance of the pile body increases as the load increases.Due to the load,there is basically no negative frictional resistance on the side of the pile.Temperature changes have little effect on the distribution of frictional resistance on the side of the pile.(6)The experimental model was numerically simulated by COMSOL Multiphy sics.Considering the thermo-mechanical characteristics of energy piles under zero load and working load under the action of cyclic temperature field.Compare simulation results with model test results.It is found that the numerical sim ulation results are consistent with the test results.This verifies the correctness and feasibility of the built numerical model.And provide guidance for the pop ularization and application of energy piles in engineering.