Exploration On Thermal And Mechanical Characteristics Of Energy Pile Group

Energy piles combine the underground heat exchangers with the traditional piles foundation to serve the heat pump.It has been developed and applied at home and abroad because of its low initial investment and good heat transfer performance.Energy piles,which are not only the foundation of buildings,but also the heat exchangers of ground source heat pumps.The friction between piles and soil is changed after heated and cooled,which affects the bearing and settlement characteristics of piles foundation.At the same time,different from single pile and traditional pile groups,the thermal mechanical properties of energy piles are more complex due to the influences of thermal barrier and pile group effect.In this paper,the three-dimensional dynamic numerical simulation model of energy pile groups are established.The reliability of the established model is verified by the field test results.Based on the analysis of the simulation results,different thermal-mechanical characteristics parameters of energy piles are given.Furthermore,the thermal-mechanical response characteristics of energy piles under different load levels and the pile groups effect coefficient are analyzed.A correction factor is proposed to make the pile groups effect coefficient of traditional piles foundation suitable for energy piles.Finally,combined with the actual situation,the thermal-mechanical response characteristics of energy piles under continuous operation and intermittent operation are discussed.The results show that,the thermal interference effect will be effectively reduced by increasing the pile space.When the pile spacing is 3 times of the pile diameter,there is obvious thermal interference between the piles,but this effect can be ignored when it is expanded to 5 times.Increasing the pile length can increase the heat exchanger’s area and have higher heat exchange capacity.But pile length has little effect on the pile temperature.As energy piles heated,the pile stress will increase obviously.The overall distribution of additional temperature load is approximately uniform.The displacement of the energy pile body under thermal and mechanical loads is settlement.Due to the restraint effect of the pile cap at the top of pile groups,the displacement difference of each pile near the top of pile groups is very small,and gradually presents difference along the depth of pile groups.Up to the bottom of pile groups,the displacement of corner pile is the largest,that of side pile is the second,and the center pile is the smallest.The side friction is negative near the top of the pile,then reaches zero along the pile depth and the peak at the bottom of the pile.The influence of pile spacing and pile diameter on the side friction is significantly greater than that of pile length.Temperature load and structure load will increase the pile stress,and the influence of temperature load is greater.The influence of the structural load on the pile side friction is much greater than that of the temperature load.The inlet water temperature of the heat exchanger has little influence on both the overall pile side friction and the peak value at the bottom of the pile.The temperature and stress of the pile increase with the increasement of the water temperature at the inlet of the heat exchange pipe.The energy piles will be expanded after heated,and the bearing capacity of pile foundation calculated based on a certain amount of settlement will be larger.According to the conditions of this paper,a temperature correction factor of 0.0037 is proposed,which makes the modified pile group effect coefficient based on traditional pile foundation suitable for energy piles.The displacement of pile top and the outlet water temperature fluctuates periodically in intermittent operation,and accumulates obviously after three heating periods.The side friction of intermittent operation is always greater than that of continuous operation in the upper part of pile,but less than that in the lower part.