Study On The Bearing Capacity Of Heat Exchanger Piles Considering Pile-soil Interaction

Heat exchanger piles are new foundation piles with a combination of building pile foundations and heat transfer tubes,which have the advantages of energy-saving and environmental protection,economical and efficient,sustained steadiness and so on.The research of heat exchanger piles includes two key problems of transfer efficiency and thermal characteristics.At present,there has been a lot of research at home and abroad,which is concentrated on heat transfer efficiency.Meanwhile,the relevant results of thermal characteristics are rare,which has not given enough attention.In this paper,large scale model tests and finite element analysis of heat exchanger piles under cyclic temperature load are carried out,studying on the bearing capacity,pile-soil settlement and heat-fluid-solid response.(1)By static loading test under different temperatures load,studying on the bearing capacity of heat exchanger piles.Analyzing the measured temperature,pore pressure,pile top settlement,soil surface settlement,axial force and load-settlement curves,the results show that:(1)Simple heating helps improve pile bearing capacity;The bearing capacity of heat exchanger piles is more than ordinary piles' after cyclic temperature load,and with the increase of cycle times,the bearing capacity of single pile gradually increases.This is because the soil occurs thermal hardening during heating which is equivalent to foundation treatment,and increase the soil strength.(2)Top of the pile under the vertical pressure,pile axial force decreases along the depth.This is because the pile-soil interface has friction.(3)The settlement of pile and soil are significantly changed and the plastic deformation is perduced by the temperature up and down.This is because the soil occurs thermal hardening and cause consolidation settlement during temperature cycling.(2)At the condition of room temperature,the test pile is grading applied constant load,then applying temperature cycling.The normal operating conditions of the heat exchange pile is studied under the temperature cyeling condition.Test results show that:(1)At the same time,the temperature of piles decreases with the increase of the pile depth.Heating causes excess pore water pressure in soil,from the pile side closer,the higher of soil pore-pressure,and the time required to reach the peak value of pore-pressure is alsoshorter.(2)Under long-term temperature cycling,the plastic deformation is accumulated by the temperature variation,but the deformation decreases.(3)Heating causes the pile to swell and appear multiple negative friction resistance along pile shaft,meanwhile its value and distribution range decreases under the increase of pile top load.(4)The axial strain of pile shaft are significantly changed by the temperature up and down,but changing small near the pile top.This is because pile top position is exposed to the outside air where the temperature is low.(3)By using Abaqus finite element software,based on model tests as prototype,a finite element model of heat exchanger pile is built.The reliability of the model is verified by comparing with the model test results.Then the influence of different temperature cycling and cycle times on the bearing capacity of single pile and the normal operating conditions of the heat exchange pile is analyzed.Results show:(1)The temperature effect helps improve pile bearing capacity.The laws are consistent with model test.(2)The settlement of pile top and soil surface are significantly changed and the plastic deformation is perduced by the temperature cycling.The laws are consistent with model test.(3)When the heat exchange pile is under the heating condition,pile will produce a relatively large temperature stress.And the maximum additional compressive stress is located in the lower-middle position of the pile body,this is because the heating causes the pile and soil to swell and the lower-middle part of the pile is relatively strong constraint.(4)When the heat exchange pile is under the heating condition,with the increase of cycle times,compressive stress of pile shaft decreases,and the decreasing maximum is located in the pile top and pile bottom of the pile body.This is because the soil gradually consolidates,meanwhile its strength increases,which leads to the increase of pile-soil friction.