Study On Bearing Capacity Of Energy Piles Under The Coupling Action Of Load And Temperature

Energy pile is a new type of ground source heat pump which is formed by burying the buried pipe of the traditional ground source heat pump system into the pile foundation of building.The energy pile has the functions of pile foundation and ground source heat pump.Therefore,in addition to examining its heat transfer performance as a ground source heat pump,examining its bearing capacity as a pile foundation is also necessary.Considering the difference of soil properties at different layers,this paper studies the mechanical properties of energy piles and discusses their bearing capacity response mechanism under the combined action of temperature and load.The following research work has been completed:（1）The field test of the bearing capacity of the energy pile was carried out in the shallow geothermal energy scientific experimental site of Fengxian Bay.6 kinds of inlet temperatures were set for 6 energy piles respectively,and the three stage tests of graded loading and unloading,thermal response,thermal coupling and displacement situation were carried out..（2）The heat-fluid-structure coupling simulation of the energy pile heat transfer process in the first-phase pile foundation project of the Fengxian Bay Shallow Geothermal Energy Science Experiment Field was carried out by using the Fluent fluid mechanics calculation software.The soil layer is treated in layers,and the influence of temperature on its heat transfer coefficient,specific heat capacity and other parameters is considered.The results show that after 20 hours of heat exchange process for a single energy pile,the temperature of the pile tends to be uniform,and the temperature field is basically stable after 50 hours,and the influence range of the temperature field can reach more than twice the diameter of the pile.The accuracy of the simulation is demonstrated by comparison with field test results.（3）After setting the soil parameters under the influence of temperature,the numerical model of the energy pile in the shallow geothermal energy scientific experimental field was established by Abaqus,and the coupled simulation at 30 °C and 10 °C was carried out.The results show that the axial force of the pile increases when heating at 30°C and graded loading work together;when the cooling at 10°C acts together with the load,and the load level is less than 1200 k N,the axial force at the lower part of the pile is the tensile force.In the soil with a larger friction angle,the friction resistance of the pile side is more fully exerted.The higher the soil strength at the bottom of pile,the more significant the reinforcement effect of the pile side friction near the the bottom of pile is.The results of numerical simulation are in good agreement with the results of field test and the correctness of the simulation was proved.（4）The effects of different inlet temperatures,pile diameters,and intermittent working conditions on the bearing capacity of energy piles were studied using the above model.The results show that the change of the pile side friction resistance caused by temperature is mainly located at both ends of the pile,and the top of the pile is more significant.The larger the diameter of the energy pile,the larger the additional axial force caused by temperature.Under the same pile top load,larger diameter energy piles have less settlement.Compared to the continuous operation condition,the temperature additional axial force of the energy pile under the intermittent operation condition is smaller,but the axial force in the middle of the pile body will fluctuate periodically.Under intermittent conditions,the smaller the operation-stop ratio,the smaller the cumulative displacement of the pile top,but the larger the periodic fluctuation.（5）On the basis of single pile finite element simulation,the finite element simulation research of pile group bearing capacity is carried out.A finite element numerical model of the three piles under the bearing platform was established and the effect of the pile group spacing on the load carrying capacity of the pile group was investigated.The results show that the axial force caused by the temperature also expands with the expansion of the pile spacing.The settlement of pile groups is significantly higher than that of single piles,whether heated,cooled or loaded only.The settlement of the pile groups tends to increase as the pile spacing decreases.