| Energy pile is a kind of building energy saving technology combining the traditional ground source heat pump and the building pile foundation.It has been widely used because it overcomes the disadvantages of the traditional ground source heat pump which covers a large area and high initial investment.In practical engineering,energy piles are mostly arranged in the form of pile group,in this way thermal interference phenomenon will occur when temperature load is applied,which leads to the difference between thermo-mechanical characteristics of energy pile group and single pile condition.At present,there are many experimental and theoretical studies on single energy pile,but relatively few researches on energy pile group,which are mostly qualitative analysis and lack of analysis on the working performance of energy pile group under different factors.Therefore,in this paper,an energy pile group model test bench and a numerical model of the energy pile group were built.The method of combining model experiment and numerical simulation was used to study the heat transfer performance and mechanical behavior of the energy pile group.In the aspect of the experiment research,an energy pile group experiment bench was built,and the thermo-mechanical behavior of energy pile group under different temperature loads,different number of energy piles and different connection forms of buried pipes were studied experimentally.The results show that the average heat exchange rate per unit pile depth of single energy pile and energy pile group are 62.75 W/m and 35.59 W/m,respectively.Meanwhile,the residual pile body temperature of energy pile group is larger,which increases the variation range of the pile head displacement,the pile tip pressure and the pile side friction.The higher the inlet water temperature,the greater the average heat exchange rate per unit pile depth.For example,the average heat exchange rate per unit pile depth under the inlet temperature of 35℃ and 38℃ are 15.5%and 30.5%higher than that under the 32℃ condition.However,the larger temperature load also produces the higher residual temperature of the pile and soil around the pile,leading to the larger displacement of the pile top,which is not conducive to natural recovery,and the axial force of the pile and the pile side friction resistance will also increase.Reducing the number of energy piles in the pile foundation is beneficial to the increase of heat transfer,but it will weaken the effect of pile group effect and lead to the increase of pile axial force.Although the energy pile has little influence on the thermo-mechanical characteristics of traditional concrete pile,it will produce differential settlement,and the load-bearing imbalance needs to be considered.As for the connection mode of embedded pipes,compared with the parallel connection mode,the connection mode of embedded pipes in energy group pile in series can improve the heat transfer of energy pile,reduce the temperature rise of energy pile and the soil,reduce the axial force and friction of pile body,but will cause the rise and fall of different pile head,resulting in the imbalance of bearing field.In numerical simulation,based on the single pile model of energy pile,a numerical model of energy pile group was established,and the effects of soil type,pile spacing,pile foundation material,pile arrangement and cross-season operation on the working performance of energy pile group were discussed.The results show that the heat exchange rate per unit pile depth of corner pile,side pile and center pile in energy pile group decrease by 29.44%,43.90%and 61.12%,respectively,compared with single pile.At the same time,the temperature rise of the central pile is the largest,leading to the largest pile body displacement.After 15 days of operation,the pile body displacement of the central pile is 1.83 mm.The axial force of any pile in energy pile group system is much less than that of single pile system.When the soil is rock soil,the heat transfer efficiency of energy pile is the highest.When the soil is sand soil,not only the heat transfer performance is poor,but also the pile body displacement is large.Increasing pile spacing is beneficial to obtain greater heat transfer efficiency and reduce the temperature between pile and soil,thus reducing the pile body displacement,but it will weaken the effect of pile group effect and lead to the increase of pile body axial force.Increasing the thermal conductivity of concrete can improve the heat transfer capacity of energy pile group in a short time,but because of the influence of thermal interference,the decay rate of heat transfer is accelerated.Under the same temperature load,different concrete compression modulus has little influence on the thermo-mechanical behavior of energy pile group.The heat transfer capacity of energy pile group system can be improved by adopting the cross-row mode,and the variation of pile body displacement and axial force can be reduced.The annual heating capacity of energy pile group is greater than the cooling capacity,and the influence range of soil heat was about 7 m.After one year of operation,the system as a whole expanded upward.This paper aims to explore the influence of different factors on the thermo-mechanical behavior of energy pile group and provide theoretical basis for the engineering design of energy pile group. |
