Research On Bearing Characteristics Of Deep Buried Pipe Type Energy Pile Under Thermal-mechanical Coupling

The new type of energy pile has flexible structure and can design the buried depth of deep well at the bottom of the pile according to the heat exchange demand,which overcomes the shortcomings of buried pipe energy pile in the pile and traditional ground source heat pump.The heat exchange pipe passes through the pile body and enters the deep well at the bottom of the pile,reaching the optimum efficiency depth below 100 m of the ground surface.Based on the research on heat transfer,structural response characteristics and bearing deformation characteristics of energy pile,this paper establishes a numerical model of deep buried pipe-type energy pile based on ANSYS Workbench,and carries out temperature field characteristics analysis and single-pile bearing characteristics analysis of deep buried pipe-type energy pile under temperature load and thermal-mechanical coupling.(1)Under cooling and heating conditions,there is a small sudden change in the temperature field at the junction between the pile body and deep well,and the temperature around the deep well in the pile body is higher and the temperature field range is wider,which indicates that the heat exchange effect of the part of the pile body is better than that of the deep well.(2)When temperature load is applied only,the cumulative settlement of pile body decreases under cooling condition and increases under heating condition.In both cases,the axial temperature additional stress is large in the middle and small at both ends,and it does not exceed the tensile strength and compressive strength of C30 concrete.The numerical simulation and field in-situ test basically conform to the variation rule of pile shaft friction under the same conditions,which verifies the rationality of the numerical model.(3)When thermal-mechanical coupling is applied,the pile body shrinks and deforms under the action of temperature reduction coupling,and the shaft friction of the lower and middle parts of the pile body is larger,which results in significant change of accumulated axial force generated in the lower and middle parts of the pile body.Under the action of temperature-rise coupling,the pile body is heated to expand,the soil restraint around the upper part of the pile is weak,and the side friction resistance is relatively small,and the side friction resistance of the pile body after thermal expansion decreases.The axial force of the middle and lower part of the pile body increases.(4)During loading,the settlement value decreases when refrigerating and increases when heating.It can be seen that the heat exchange process of deep buried pipe type energy pile has obvious influence on the settlement of pile top,which should be considered in the design and construction of energy pile.When the load on the top of the pile approaches 4800 k N,the corresponding axial force of the pile increases nonlinearly.When the load on the top of the pile is applied to 4800 k N,the distribution law of axial stress of the pile body along the depth of the pile changes and basically decreases gradually.(5)As loading progresses,the range of non-linear interaction between pile and soil caused by load gradually extends downwards along the pile depth,the position of the pile body reaching the ultimate side friction resistance gradually moves downwards,and the change of axial force and side friction resistance of pile body caused by temperature rise coupling gradually decreases,which indicates that the temperature effect in load transfer characteristics of deep buried energy pile is gradually weakening.When loaded to 2400 k N(50% limit load),the effect of pile top load gradually increases and becomes dominant,which will reduce the scope and magnitude of negative friction resistance.