| The energy pile is a geothermal utilization device that is embedded in a heat exchange tube in a traditional concrete pile,and uses a heat exchange tube to extract clean,renewable and sustainable shallow geothermal energy to heat and cool the building.With the continuous increase,the development and utilization of new energy sources is becoming more and more important.Under this background,the application of energy pile technology that makes full use of shallow geothermal energy is increasing in geotechnical engineering.Heat transfer efficiency and thermodynamic response are the key to energy piles.Performance has become the focus of this paper.It mainly depends on the strength,deformation,thermal properties and temperature field distribution of the pile body and the surrounding soil.Microbial induced calcium carbonate precipitation(micp)technology is an emerging cross.Disciplinary technology,the process of which involves a series of biochemical and ionic chemical reactions,which are excellent in improving the strength,stiffness,liquefaction resistance and erosion resistance of geomaterials.However,micp technology requires the cultivation of microbial flora.The test conditions are more stringent,and the curing reaction procedure of the technology is more complicated.The curing effect is affected and restricted by many factors,and the implementation is difficult.Therefore,this paper The chemical-mediated calcium carbonate deposition mineralization technology is used instead of the micp technology to carry out the thermal property test.The chemical mineralization technology,that is,the chemical method to form calcium carbonate precipitate,can produce a cementation-removing calcium carbonate precipitate by optimizing the grouting process.In this paper,the chemical grouting method is used to carry out relevant experimental research.On the basis of studying the influence of chemical grouting on the mechanical properties of soil materials,the thermodynamic properties of grout-reinforced soil are studied,that is,the large-scale formation of soil pore calcium carbonate crystals The influence of the thermal conductivity of the soil around the pile.In addition,compared with microbial mineralization,the chemical mineralization reaction mechanism is simple,the processparameters involved are less,the raw materials are convenient to obtain,and the cost is low.Based on the method of chemically mediated calcium carbonate precipitation,a series of experiments were conducted to study the effect of chemically mediated calcium carbonate on the thermal properties of soil,in order to improve the heat exchange rate of soil and improve the soil-heat exchange rate and improve the mechanics of soil.Performance to improve the load bearing performance of the pile.Based on the actual data of the completed project,the ANSYS Workbench numerical analysis software is used to study the temperature field distribution of the energy pile and the thermo-mechanical response characteristics of the pile under both summer and winter conditions,and to find out its complex mechanism and Thermal-mechanical coupling characteristics.The research work involves: unconfined compressive strength,calcium carbonate content and dry density of the segmented soil column to analyze the chemical strengthening effect of the soil;and multiple test blocks are produced with the number of grouting and grouting as variables,revealing the note Effect of slurry frequency and grouting amount on soil compressive strength,dry density and distribution of calcium carbonate;multiple sets of test blocks were prepared with the number of grouting as variables,and the thermal conductivity was tested to clarify the crystal form of calcium carbonate produced by chemical mineralization.The influence of the thermal properties of the soil;the electron microscopy scan test of the test block,the reinforcement effect of the soil is fully evaluated from the microscopic scale,and the cause of the change of thermal properties is analyzed;the single pile model of the energy pile is established,and the single pile diameter under two working conditions is analyzed.The distribution of the temperature field in the vertical and vertical directions,and the thermal response(axial stress,pile side shear stress and axial displacement)of the pile under two working conditions;establishing an energy pile group model and analyzing the pile group under two working conditions The temperature field distribution law and thermal response,and the single pile simulation results are compared with the group pile simulation results.The main findings are as follows:(1)By measuring the calcium carbonate content,dry density and unconfined compressive strength(UCS)of the sand column sample,it is found that the calcium carbonate content in the pores of the sand column gradually increases with the increase of the number of chemical grouting,and the cementation The filling effect of calcium carbonate effectively improves the compactness of the sand sample and bonds the loose sand particles into a whole,thereby increasing the compressive strength of the sample.The compressive strength of the sample gradually decreases from the upper(grouting port)to the lower side of the test sand column.The average compressive strength of the upper part of the sample grouted 20 times is 0.56 MPa,and the average value of the lower compressive strength is 0.51 MPa.The content of calcium carbonate in the sand column gradually decreases from top to bottom,and the upper deceleration rate(0.17%/cm)is smaller than the lower deceleration rate(0.53%/cm).The dry density and unconfined compressive strength of the sand column sample are directly proportional to the calcium carbonate content in the pores.The calcium carbonate content in the sample increases by 1%,and the dry density and the unconfined compressive strength increase are respectively 4.05.%,17.96%.(2)The results of microstructure analysis by scanning electron microscopy showed that the calcium carbonate crystals distributed on the surface of sand particles were clustered in multiple particles and distributed in the contact parts of sand particles;the sand particles in spatial contact with each other were in two dimensions.There is no contact on the plane,and the calcium carbonate crystal produced here only plays a role of filling.When the calcium carbonate crystals continue to accumulate,the crystals that are not in contact are continuously enlarged,thereby contacting each other,and thereby playing a role of cementation.(3)The thermal conductivity of the poorly graded quartz sand and the variation of the specific heat capacity with the number of chemical grouting(16,18,20 times)were studied by using the test block grouting test.The results show that: 1)the thermal conductivity and specific heat capacity of the sample increase linearly with the increase of the number of grouting;2)the calcium carbonate crystal in the pore increases the specific heat capacity of the sand,and when grouting 20 times,Themaximum increase of thermal conductivity of the sample is 87.4%,and the maximum increase of specific heat capacity is 159%.3)The thermal conductivity of sand after grouting treatment is significantly higher than that of untreated sand,and increases with the increase of the number of treatments.The results of the study prove that the calcium carbonate grouting material can effectively improve the thermal conductivity of the soil,thereby effectively increasing the heat transfer efficiency of the energy pile.(4)The results of numerical simulation analysis show that after the heat exchange of parallel U-tubes is stable,the diffusion range of temperature in the pile foundation and soil gradually increases first,and then decreases gradually after reaching stability.The diffusion shape of the temperature in the single pile model is circular,and the radial diffusion shape at the pile foundation is approximately triangular,and is approximately circular as the radial distance increases.Thermal interference occurs between the branches of the double U-shaped tube at the 1m below the pile top.With the increase of depth,thermal interference occurs between the piles,and the pile depth is most obvious at 7m.(5)The thermal-force coupling effect of the energy pile makes the load transfer mechanism different from the conventional engineering pile.The thermal-force coupling changes the load transfer characteristics and bearing behavior of the energy pile under single load.The intervention of temperature will inevitably cause additional axial displacement of the pile due to thermal expansion and contraction.Under summer conditions,the additional axial displacement of the single pile will be 0.05 mm upward,and the additional axial displacement of the pile will be 0.68 mm.Under the working conditions,the single pile produces a downward additional axial displacement of 0.78 mm,and the group pile produces a downward additional axial displacement of 0.97 mm.The generation of additional axial displacement will cause uneven settlement,damage to the pile and damage to the foundation in the upper building. |
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