| Geothermal energy is a kind of green,clean and sustainable energy.The development of energy pile technology provides new ideas for using geothermal energy to alleviate the energy crisis.Compared with the traditional cast-in-place pile as a heat exchanger,the static drill rooted energy pile has the advantages of no squeezing soil,no mud discharge,high bearing capacity,and good durability of the heat exchange system.In this paper,the method of combining the self-designed model test and ABAQUS numerical simulation analysis is used to study the bearing characteristics of static drill rooted energy pile under thermo-mechanical coupling,which provides evaluation and reference value for the popularized use of static drill rooted energy piles as heat exchangers.The main conclusions are as follows:(1)After heating and cooling,the temperature of the pile body varies unevenly along the depth,and its temperature distribution law is related to the way the heat exchange tubes are buried.The degree of the influence of the heat source on the temperature of the surrounding soil gradually decreases as the distance from the center of the pile increases,and the influence radius is about 3 to 4 times the pile diameter.And the heat transfer performance of the built-in metal heat exchange tube model pile is better than that of the outer-wrap plastic heat exchange tube model pile.(2)Under the action of multiple cooling and heating cycles,the cumulative settlement of the pile top and the surrounding soil surface of the static drill rooted energy pile will occur,and it has an increasing trend with the increase of pile top load and the number of heating and cooling cycles.Within the temperature range of-15℃～20℃,the increase percentage of the bearing capacity of the foundation pile is in the range of-2.69%～+8.33%.(3)Under the effect of heating and cooling cycles,both precast pile and cemented soil have additional thermal stresses,and their distribution is closely related to temperature changes,pile top loads and constraints,etc.The additional axial compressive stress caused by the temperature rise of the precast pile and the additional axial tensile stress caused by the temperature decrease tend to be small at both ends and large in the middle along the depth distribution.The distribution of the additional axial stress in cemented soil is opposite to that of the precast pile.When the load on the top of the pile is small,tensile stress appears locally in the precast pile under cooling conditions.However,within the temperature variation range of the normal operation of the energy pile,the strength damage of the precast pile and the cemented soil caused by the additional thermal stress has not been found.(4)The variation of the lateral friction resistance of the static drill rooted energy pile shows that the upper part decreases and the lower part increases under the heating condition,while the cooling condition shows the upper part increases and the lower part decreases.When the load on the top of the pile is small,a small range of negative friction resistance appears in the upper part by heating and lower part by cooling,which indicates that the safety factor of bearing capacity should not be too high when designing energy piles.The location of null point of the pile displacement change is related to the load on the top of the pile and the constraints.(5)The way of embedding the heat exchange tubes mainly affects the heat transfer performance of the static drill rooted energy pile,and the bearing characteristics are closely related to the factors such as pile type,temperature change value,number of cooling and heating cycles,soil layer properties,pile top load and constraints.In the actual engineering design,it is necessary to determine a reasonable heat exchange fluid temperature change range and bearing capacity design safety factor according to specific working conditions to ensure the safety of long-term work of the static drill rooted energy pile. |
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