Thermomechanical Behavior Study Of Energy Pile By Load Transfer Method

Load transfer method is an effective calculation method in the pile foundation capacity analysis,which can adapt the corresponding load transfer function according to the properties of soil,and it has the advantages of conceptual clarity and applicability,thus it is widely used in the energy pile analysis.An ordinary pile foundation will only generate displacements in one direction under mechanical loads.When applying the load transfer method,one can assumes the displacements at the pile base and then iteratively find the displacement from the bottom to the top sequentially.However,in the case of energy pile,as a result of thermal load,the pile will expand or contract around a point,generating displacement in two directions,which is referred to as the null point.Therefore,when the load transfer method is applied in energy pile domain,the positions of null point usually are assumed first,then the pile displacements on either side of the null point can be calculated by iterations separately,when the assumed null point fail to match the actual ones,then the null point positions are required to be re-assumed.In order to simplify the procedure of load transfer method for energy pile,this paper presents a new method which can directly calculate the thermomechanical response by constructing a boundary shape function(BSF)that satisfies the boundary conditions automatically,without assuming the null point location.The calculation method was validated by comparison with the in-situ test data.Then,the effects of pile end restraint,soil stiffness,thermal and mechanical loads on the mechanical response of the energy pile were discussed with the help of the calculation method mentioned above,and the pattern of null point movement were summarized in detail.Finally,taking the influence of temperature profile on the T-z curve parameter into account with the aid of the finite-length cylindrical heat source model solution,a calibration of the method for temperature was performed.The main findings are as follows:(1)The variation in the null point position laterally reflects the bearing characteristics of energy piles.When energy piles are subjected to changes in the boundary conditions imposed by the pile tip and the pile sides,the position of the null point undergoes a certain distance of displacement.However,the constraints at the pile tip play a stronger controlling role in determining the null point position.Increasing the constraints at the pile head or pile tip causes the null point to shift towards the direction of increased constraints,leading to an increase in the maximum thermal-induced stress.Conversely,reducing the constraints imposed by the surrounding soil results in a more uniform distribution of thermal-induced stress along the pile.This leads to a decrease in the maximum thermal-induced stress and an increase in the minimum temperature-induced stress,with the null point slightly shifting towards the side with stronger constraints at the pile end.(2)The mechanical response of energy piles is also influenced by variations in thermal load and initial mechanical load.When both loads increase,the thermal-induced stress in energy piles also increases.When only the thermal load is increased,the null point will slightly shift towards the side of the pile end with stronger constraints.However,when only the initial mechanical load is increased,the null point will move upwards to maintain the overall force equilibrium of the pile body.(3)The temperature influence range of energy piles is closely related to the operational time,with a larger influence range observed for longer operational durations.Within the normal operating time range(e.g.,30-60 days),the variation in temperature along the depth direction is relatively uniform,with significant changes occurring mainly at the pile head and pile tip locations.(4)Considering the uneven temperature distribution along the pile-soil interface,modifications are made to the load transfer function at the pile sides,resulting in minimal changes in pile body displacement.However,the thermal-induced stress shows an increased magnitude at the null point and a decreased magnitude near the pile head,particularly for higher thermal loads.Nonetheless,in general,the influence of temperature correction on the mechanical response of energy piles is negligible and can be disregarded in most cases.In the given study example,where the temperature rose by 16.37℃,the maximum thermal-induced stress increased by only 0.1% after the correction.