Energy Pile In Layered Foundation Soil Under Long-term Temperature Cycle Research On Bearing Capacity

Energy pile is a pile foundation buried pipe technology,combined with shallow low temperature geothermal energy,to bear the upper building load and play the role of ground source heat pump heat exchanger.Regarding the bearing performance of energy piles,the long-term temperature cycling is mainly numerical simulation and most of the studies on the soil around the pile are mainly for a single soil layer,and the research on the cyclic shear damage to the soil around the pile caused by the rise and shrinkage of the energy pile due to the temperature load is limited to the application of five temperature loads.Therefore,in this paper,by combining the indoor model test method and finite element numerical simulation,clay and sand are used for the pile perimeter soil to study the change of pile top loading,different temperature loading,long-term temperature cycling process,the pile top vertical displacement and other indicators,the specific conclusions are as follows:(1)Temperature effect: Increasing or decreasing the inlet temperature will have a certain enhancement on the heat transfer performance of the energy pile.The temperature change of the pile is greater than the temperature change of the soil,and the closer the distance to the pile,the greater the temperature change of the soil,and the temperature change of the soil is not obvious due to the thermal resistance of the soil causing 2D soil temperature change beyond the pile.(2)Earth pressure at pile tip: The earth pressure at pile tip is positive in the warming stage cooling stage is the opposite,with the increase in the number of cycles is gradually accumulated trend,which is slightly smaller change in the value when cooling,the size of the earth pressure is determined by the temperature load,when the pile top load is larger its size is determined by the pile top load.(3)Pile body stress: The pile body stress increases with depth and then decreases,the temperature increases to produce compressive stress and decreases to produce tensile stress,and the maximum value appears in the middle and lower part of the pile body,and the maximum stress gradually accumulates with the increase of the number of cycles,in which the maximum tensile stress(0.435MPa)accounts for30.42% of the design value of C30 concrete tensile strength(1.43MPa),so in the actual project Therefore,the tensile stress caused by long-term temperature cycling in the actual project should be considered.(4)Pile shaft friction: The shaft friction resistance of the pile arises from the displacement of the pile-soil contact surface,and the zero point of displacement occurs in the middle and lower part of the pile body,with the increase of the number of cycles the soil around the pile due to the disappearance of the pore water pressure leads to a greater degree of consolidation,which makes the lateral friction resistance gradually increase,but due to thermal expansion and contraction causes the pile-soil interface to produce cyclic shear damage many times resulting in a decrease.(5)Vertical displacement of soil around pile: Vertical displacement of soil around the pile gradually accumulates with the increase of the number of cycles,and the settlement is the largest within 3 temperature cycles,and the settlement is smaller after more than 15 temperature cycles,in which the effect produced by long-term cold and hot loading is the most obvious,and the settlement of soil reaches0.0085 mm after the end of the 3rd cycle,accounting for the settlement of soil at the end of 20 temperature cycles(0.0118 mm)of 72.03%.(6)Vertical displacement of pile top: The vertical displacement of the pile top accumulates with the increase of the number of cycles.The most intuitive effect is produced by the long-term cold load,and the final pile top settlement is 1.3 mm after20 temperature cycles,which is about 1.55% D(D is the pile diameter).The settlement produced after the first 3 temperature cycles accounted for 69.23% of the total settlement,and stabilized after 17 temperature cycles.(7)Ultimate bearing capacity of pile: The ultimate bearing capacity of the pile was not reduced but increased in 52 days after long-term thermal cycling and thermal cycling,among which 52 days after long-term thermal cycling was the most obvious;long-term hot and cold cycling and cold cycling were reduced,among which long-term cold cycling was the most obvious,reduced by 16.67%,so the ultimate bearing capacity involving long-term cold cycling leading to pile reduction should be fully considered.