Study On Thermo-mechanical Behaviour Of Static Drill Rooted Geothermal Energy Piles With External Double U-tubes

Static drill rooted geothermal energy pile(SDRGEP)is a new type of geothermal energy pile which combines static drill rooted pile with ground source heat pump(GSHP)technology.Heat exchanger pipes of the SDRGEP are placed in cement soil outside the pile to effectively avoid the damage of heat exchanger pipes and facilitate construction.It has broad application prospects.At present,the geothermal energy pile is seldom used in deep soft soil foundation.Its theoretical research and in-situ test are seldom carried out.The research on the thermo-mechanical behaviour of the geothermal energy pile with the SDRGEP as carrier is basically blank.Therefore,this paper takes the static drill rooted geothermal energy pile with external double U-tubes as the object of study,to study its heat exchange performance and thermos-mechanical behavior through in-situ test,theoretical analysis and numerical calculation,so as to further promote its popularization and application.Firstly,in order to study the heat exchange performance and thermo-mechanical behaviour of the new geothermal energy pile,the ground comprehensive thermal conductivity was analyzed respectively by using linear heat source,hollow cylindrical heat source and solid cylindrical heat source model,which based on field thermal response test and its heat transfer mechanism.Besides,three theoretical models were compared with each other.At the same time,the relation between the initial time of three theoretical models and ground comprehensive thermal conductivity λ are discussed.It is suggested that the solid cylindrical heat source model is used to identify the ground comprehensive thermal conductivity of the SDRGEP.Secondly,a three-dimensional heat transfer numerical model was established and validated.The temperature distribution of the SDRGEP was obtained by using this model.Compared with the in-situ test values,it was found that the heat transfer of geothermal energy piles was mainly along the radial direction.In the early stage of the test,transient heat transfer occurs in the borehole,and the temperature change was obvious.The radius of temperature influence was less than 0.5 m.In the later stage of the test,the heat transfer in the borehole was near steady state,and the radius of temperature influence was less than 1 m.The longitudinal temperature of pile body first increased and then decreased with depth,showing a slightly higher middle and a slightly lower two ends.The temperature of pile top and bottom was lower than that of the middle of pile,and the temperature of pile body reached its maximum near the middle of pile.In addition,the influence of tube spacing and its thermal conductivity,fluid velocity and thermal conductivity of cement-soil around the pile on the heat transfer performance of the SDRGEP was analyzed by using the numerical model above.The results show that the heat transfer performance of geothermal energy pile can be improved by increasing the distance between heat exchanger tubes,the thermal conductivity coefficient of heat exchanger tubes,the flow velocity of heat exchanger fluid and the thermal conductivity of cement-soil around the pile.However,when the distance between heat exchanger tubes is larger than 0.25 m,the flow velocity of heat exchanger fluid reaches turbulent state,the thermal conductivity of cement-soil is higher than the ground comprehensive thermal conductivity,improving these parameters above has little contribution to the development of the geothermal energy pile heat transfer performance.The heat transfer performance of the pile was affected by the ground comprehensive thermal conductivity,and the change was linear.In opposite terms,the thermal conductivity of PHC pile had little influence on its heat transfer performance.Based on the experimental research,it was found that the free thermal strain distribution of pile body was roughly the same as the temperature distribution of pile body.It increased and then decreased as the depth increasing,the free thermal strain distribution was slightly higher in the middle and lower at both ends.The observed strain increment was proportional to the temperature increment,and both changed synchronously.The measured strain of the pile increased gradually from the zero point of displacement to both ends.After the pile body heated up,due to the existence of the constraint around the pile,the pile generates the constraint strain and the additional axial compressive stress.With the increase of the pile temperature,the additional stress of the pile increased gradually.In general,the distribution of the additional stress of the pile body was contrary to the observed strain,showing a tendency of large in the middle and small at both ends.When the pile body rose 6℃ as the whole,the maximum additional compression stress was occurred in the middle and lower position of the pile body,to 1.7 MPa.There was a linear relationship between the additional stress and temperature increment along the depth of pile.In the whole process of thermal response test,the pile was in thermoelastic state.Based on the field test and the published paper,the thermal-mechanical response rules of geothermal energy piles under partial constraints were proposed which considers the existence of weak layer at the pile top or homogeneous soil layer.At the same time,it shown that different pile types and construction techniques have influence on the thermal-mechanical properties of geothermal energy piles.By using three approaches,including field test,theoretical analysis and numerical simulation,the heat exchange performance and thermo-mechanical behaviour of the SDRGEP in Ningbo deep soft soil area was investigated,which provides a reference for further research on the design method of geothermal energy pile.