Model Tests And Numerical Simulation Study On Thermo-mechanical Behavior Of Static Drill Rooted Energy Pile

Shallow geothermal energy is a kind of clean energy with broad application prospects,and vigorously promoting the use of shallow geothermal energy in the building field is highly compatible with China’s energy conservation and emission reduction policy.The use of static drill rooted energy piles as a medium for heat exchange between the building and the underground geotechnical body can save floor space and construction costs compared to ground source heat pump systems.Traditional energy piles often use bored piles or prefabricated pipe piles as heat exchange carriers,static drill rooted energy piles have the advantages of high bearing capacity and good stability due to their special working method.In this paper,based on indoor model tests and COMSOL Multiphysics numerical model,we study the thermo-mechanical behavior of static drill rooted energy piles under long-term(20times)temperature cycling conditions to provide a reference for engineering applications of static drill rooted energy piles,and the main conclusions drawn are as follows:(1)The temperature of the pile body is not uniformly distributed during the temperature cycle.During heating,the temperature of the pile body is distributed in the form of "high in the middle and low at the ends" along the depth direction,while during cooling,the opposite is true.The temperature of the pile body has a cumulative effect as the number of cycles increases and gradually stabilizes after 15 temperature cycles.The change in soil temperature around the pile has a certain lag compared to the pile body temperature,and the radius of influence of the pile-soil temperature field is about 2.5D(D is the diameter of the borehole).(2)Under long-term temperature cycling,the pile top displacement gradually accumulates as the number of cycles increases and stabilizes after 17 temperature cycles.The cumulative rising displacement of the pile top at the end of 20 thermal cycles is 0.063mm(0.72‰D);under the cold cycle condition,the displacement of the pile top shows settlement,and the cumulative settlement of the pile top after 20 cold cycles is 0.47mm(5.4‰D).About 80% of the pile top displacement during a single heating/cooling session occurs in the first 50% of the time.(3)The observed strain in the axial direction of the pile is distributed in the form of "large in the middle and small at the ends" in the hot-cycle condition,and gradually decreases in the depth direction in the cold-cycle condition,and the distribution of the observed strain in the axial direction is closely related to the constraint conditions of the pile.The distribution pattern of axially observed strains along the depth of the transition layer of cemented soil is basically the same as that of precast piles.(4)When subjected to temperature load,the pile body cannot deform freely due to the existence of restraint conditions,and axial additional temperature stress will be generated inside the pile body.At the end of each temperature cycle,there is residual additional temperature stress in the pile body,which accumulates with the increase of the number of cycles,and the accumulated increment gradually decreases and stabilizes after 14 temperature cycles.When the static drill rooted energy piles are deformed by the temperature load,the bottom expansion at the pile tip cemented soil can strongly restrain.(5)The temperature cycling action will change the original load transfer mode of the energy pile,the additional lateral frictional resistance generated by the pile body is negative at the upper part and positive at the lower part under the thermal cycling condition,while the opposite is true under the cold cycling condition.The strength of the restraint conditions at both ends of the pile greatly influences the distribution of the additional lateral frictional resistance and the change in the location of the displacement zero point.(6)Based on the model test and numerical model,the ultimate bearing capacity of the energy pile monopile increases after long-term thermal cycling and decreases after long-term cold cycling.Static drill rooted energy piles have obvious advantages over ordinary energy piles in terms of bearing capacity,safety and stability.