| With the continuous improvement of China’s economic strength and the continuous improvement of people’s living standards,China’s urbanization process is accelerating day by day,in which heat pipes play a huge role in maintaining the city’s operation as the "lifeline" of the city.Directly-buried thermal pipelines,as a new way of laying undergroun d pipes,have the advantages of less land occupation,fast construction,long life,and easy maintenance.They have been vigorously developed.However,the current relevant theoretical research and technical regulations have not fully kept up with the deve lopment trend of directly buried thermal pipelines.At the same time,explosion accidents of directly buried thermal pipelines have occurred from time to time in recent years,which has seriously affected people’s safety and production.At present,domesti c and foreign scholars’ research on direct buried thermal pipelines is mainly in frozen soil areas,and the research on the sand field environment where most the rmal pipelines are located is relatively rare,and the mechanical properties between pipes and soil are also studied in pipelines.It is mainly operated at room temperature,and there are relatively few studies involving temperature changes,and its main r esearch methods are also based on numerical simulation.In this paper,through the combination of on-site model test and finite element simulation method,the stress,soil settlement,soil temperature field and pore pressure of the directly buried thermal pipeline in the sand field at different temperatures and under different loads Research on the changes in the field has begun.(1)The thermodynamic characteristics of the pipe-soil under the operating conditions of direct buried thermal pipeline under dif ferent temperature were analyzed by the method of on-site model test.In the test,heat conduct ion oil was used as the heat medium in the pipe,and the temperature of the heat medium was set to 25 ° C →60 ° → 80 ° → 100 ° → 120 ° → 25 °.By measuring the stress of the pipe body under different temperatures,the temperature of the soil around the pipe,the pore pressure and the settlement value of the soil surface,the thermodynamic properties of the pipe soil under different temperature conditions are anal yzed.The test results show that:(1)The temperature of the directly buried thermal pipe is transfe rred to the soil around the pipe by heat conduction.The closer the soil is to the pipe,the faster the temperature rises and the higher the final temperature stability value.Due to the water seepage in the soil Affected,the temperature of the soil above the pipeline is slightly higher than the temperature of the soil on the side of the pipe and the bottom of the pipe under the same pipeline temperature.(2)The pore pressure of the soil will increase with the rise of temperature.When the temperature of the soil remains stable,the pore pressure gradually dissipates.The closer the soil is to the heat source,the faster the pore pressure rises.(3)When the pipeline is heated,the surface of the soil is slightly uplifted due to thermal expansion;during the operation of the pipeline,the displacement of the surface of the soil is mainly manifested as settlement.(4)The temperature rise will cause the strain of the pipe body to increase rapidly,in which the pipe bottom and the pipe side show compressive stress,and the pipe top shows tensile stress;after the temperature is stable,the strain will slowly decrease and remain stable;during the cooling process,The s tress mainly shows a decreasing trend,in which the stress at the top of the tube changes from tensile stress to compressive stress;the change in the circumferential strain of the pipe body is greater than the axial strain of the pipe body,so the circumf erential expansion failure is more likely to occur.(2)To analyze the coupling effect of load and temperature on the thermodynamic properties of the pipe and soil by carrying out model tests on the direct operation of directly buried thermal pipelines in the sand field under different loads.First raise the temperature of the tube body in the model box to 80 ℃ and keep it unchanged.After the parameters are stabilized,the loading test is started,and the load is graded and applied within the range of 1m2 on the surface of the soil,in order of 0 k Pa →40k Pa → 80 k Pa → 160 k Pa → 0k Pa,the temperature starts to drop after the unloading is completed,and the pipe body stress,soil temperature around the pipe,pore pressure and displacement changes are recorded at all times.The experimental results show that:(1)When a load is applied above the soil body,the surface of the soil body exhibits a larger settlement.As the load continues to increase,the settlement range also expands.Com Pared with the settlement of the soil surface during the loading process,the effect of temperature changes on the displacement of the soil is negligible.During the cooling process,the soil surface will rebound slightly.(2)During the heating process,the pore pressure showed a tendency to rise first and then dissipated;during the loading process,the pore pressure expanded rapidly and reached its apex within three hours,then the pore pressure gradually dissipated and remained stable within two days;The expansion of pore pressure is greater than that of the upper soil mass.At the same time,when loading 40-80-120 k Pa,the peak value of pore pressure can be approximately linear.(3)The increase of temperature will cause the pipeline stress to increase rapidly,and the load will also increase the strain of the pipe body,but the rate and increase are less than the effect of temperature on it;after the load is stable,the strain will gradually decrease and remain stable;Among them,the effect of load on the strain of the pipe body is relatively large at the top and bottom of the pipe,while the strain on the side of the pipe is relatively small.(3)Based on the field model test,the finite element software Abaqus was used for numerical simulation analysis,and the pipe-soil finite element model of the directly buried thermal pipeline was established.The finite element simulation results were com Pared with the model test results to v erify the reliability of the model And through the finite element model to obtain the parameters that are difficu lt to observe in the field test,combined with the two for a comprehensive analysis.The results show that:(1)When the heat medium in the dir ect-buried thermal pipe runs at different temperatures,the distance between the soil around the pipe and the pip e decreases,the temperature decreases;the temperature is not easy to dissipate in the insulation layer,but it is more dissipated when conduc ting between the soil Obviously;the influence of the pipe temperature on the temperature field of the soil around the pipe is relatively greater when the temperature is lower.(2)When the pipeline begins to heat up,the surface displacement of the soil will bulge slightly due to thermal expansion.As the temperature of the soil around the pipeline and the pipe continues to rise,the surface displacement of the soil will show a tendency to settle.The effect of temperature on the displacement of the soil surf ace is relatively small,and the maximum settlement of the soil surface is only 2.33 mm during the temperature rise and fall cycle of the pipe body.(3)The pore pressure of the soil around the pipe will expand first and then dissipate during the temperature rise process.After the dissipation is stable,the distribution of the pore pressure value of the soil body has a high correlation with the buried depth;in the process of cooling,the soil closer to the pipeline The faster the pore pressure dissipates,the slower the pore pressure dissipation of the soil farther away from the pipeline.(4)Temperature has a great influence on the stress of the pipe body.The stress of the pipe body will increase sharply with the increase of the temperature of the pipe body,where the stress at the top and bottom of the pipe appears as a compressive stress,and the side of the pipe exhibits a tensile stress.In the process of cooling,the stress of the pipe body will decrease rapidly.At the time of stabilization,the stress at the top of the pipe approaches the initial stress value,the stress at the bottom of the pipe is greater than the initial value,and the stress on t he pipe side returns to the compressive stress and is slightly smaller than the initial value. |
