| In recent years,with the continuous occurrence of severe cold and freezing weather in South China,the problem of pipeline in coastal cities has become increasingly prominent.Especially in winter,when the soil undergoes continuous low temperature environment,it often forms transient frozen soil layer.The formation and thawing of the instantaneous frozen soil layer will directly lead to the change of the temperature field around the pipe,and then lead to the brittle failure of the pipe or the frost heave and thaw settlement deformation of the pipe body.At the same time,the temperature carried by the pipeline will also cause the change of soil temperature field around the pipeline,which will lead to the thawing of frozen soil around the pipeline and the damage of soil surface.In this paper,the physical and mechanical characteristics of the soil around the pipe and the pipe body under the freezing and thawing conditions are studied by means of large-scale model test.Secondly,based on the Nishihara model,considering the seepage pressure caused by the unfrozen water in the frozen soil around the pipe,the stress and displacement solutions of the frozen soil around the pipe and at different positions are derived.Finally,combined with the finite element analysis method,the model test conditions are simulated.The parameters of model test results,theoretical calculation results and finite element calculation results are fitted to analyze the physical and mechanical properties of the pipeline and the soil around the pipeline under low temperature environment.The main contents include:(1)Based on the saturated soft clay in Ningbo area,this paper discusses the physical and mechanical properties of pipe soil from normal temperature to freezing state.In this study,the temperature of freezing pipe and test pipe is set as-10℃ and 20℃ respectively,and then5 k Pa load is applied on the soil surface twice.The thermodynamic characteristics of the soil around the pipe and the pipe during the phase transformation caused by the decrease of the soil temperature around the pipe were observed.The results show that:(1)At the initial stage of soil freezing,the temperature in the soil decreases sharply.When the temperature reaches the phase change temperature,the change range of temperature is much slower than before.(2)The change of temperature field in soil directly affects the distribution of water field in soil.When the phase change temperature is reached,the variation of water field is the most intense,and the variation of freezing front is the most obvious.(3)The change of soil water potential caused by phase change leads to a large amount of water migrating to the freezing front,which makes the soil produce linear frost heave phenomenon.When external load is applied on the soil surface,the development rate of frost heave can be effectively restrained.(4)A large cold shrinkage stress appears in the pipe body in a low temperature environment.When the frost heave develops to the position of the pipe body,the pipe flexes and arches.(2)According to the thawing characteristics of frozen soil in the operation of buried thermal pipeline with temperature,the model test of thawing settlement of soil around the pipeline under dead load was carried out.Under the condition of 10 k Pa upper dead load,the temperature of the test pipe was set at 60℃.In order to prevent the frozen soil from melting instantaneously under the high temperature,the temperature of the frozen pipeline is gradually increased by-12℃,-8℃,-4℃ and 0℃ until the soil is completely melted.During this period,the physical and mechanical properties of the soil around the pipe and the pipe caused by the thawing settlement of the soil were observed.The results show that:(1)In the early stage of pipeline heating,the soil temperature near the thermal pipeline rises faster.When the refrigeration is stopped,the heat balance in the soil is gradually broken.The temperature of each measuring point in the soil rises rapidly until it recovers to the ambient temperature.(2)The water field in soil begins to redistribute because of the thawing of soil.The pore pressure of the soil in the frozen area gradually recovers,and the pore water in the unfrozen area starts to move upward under the external load due to the melting of the frozen layer.(3)Due to the thawing of frozen soil layer,the bearing capacity of soil decreases gradually.The particle spacing caused by frost heave begins to produce consolidation reorganization under the combined action of external load and self weight,and its thawing settlement increases with the increase of water content.(4)The stress of the pipe body is mainly affected by the thermal stress generated by its own temperature,but the bearing capacity of the thawed soil is significantly lower than that of the frozen soil when the soil is thawed,so that the stress generated by the upper load is transmitted to the pipe body,resulting in the downward deflection of the pipe.(3)In order to study the influence of the rheological effect of frozen soil on the stress and deformation of the frozen soil around the pipe,this paper considers the influence of the seepage pressure produced by the unfrozen water in the frozen soil on the soil around the pipe.Based on Nishihara model,the viscoplastic constitutive equation of frozen soil around the pipeline is obtained by assuming that the first derivative of strain deviator of viscoplastic body is directly proportional to the difference between transient and steady stress deviator.The pressure produced by infiltration is added to the equilibrium equation in the form of volume force,and the analytical solution of viscoelastic viscoplastic of frozen soil around the pipe is derived by using Laplace transformation.The results show that:(1)The radius of plastic zone increases with time,and the increasing speed gradually slows down until it is stable.(2)The shear stress in the viscoplastic zone of the frozen soil around the pipe is greatly affected by the rheological characteristics of the frozen soil,and the closer it is to the pipe wall,the greater the change is.The influence of radial stress and tangential stress in viscoelastic region is weak.(3)Under different internal pressures,the radius of viscoplasticity increases with time and tends to be stable.The radius of viscoplastic zone is inversely proportional to the internal pressure.(4)Different internal pressure has little effect on the initial value of pipe wall displacement,but has great influence on the final stability value of pipe wall displacement.Under different internal pressure,the radial stress of the pipe wall does not change with time,which is equal to the internal pressure of the pipe itself.With the increase of time,the shear stress decreases gradually and tends to be stable,and the larger the internal pressure is,the smaller the decrease is.(4)In order to make the model test reliable,ABAQUS was used to simulate the freezethaw conditions of the model test.The test results are compared with the simulation results of the finite element software.The results show that:(1)The temperature in the soil gradually decreases with time,and there is an obvious latent heat process from 0℃ to-1.6℃.With the increase of cold end temperature,the temperature of thawed soil also increases gradually,and gradually reaches the ambient temperature.The temperature field of the two conditions is well fitted with the test value.(2)With the decrease of soil temperature,the excess pore pressure in the soil decreases rapidly from 0 k Pa to negative.Finally,with the stability of the temperature field in the soil,the excess pore pressure gradually returns to 0 k Pa.In the process of soil melting,the excess pore pressure in the soil gradually increases with time and then dissipates.(3)In the initial stage of freezing,the soil and pore water have a small downward displacement due to the effect of cold shrinkage.When the water in the soil reaches the phase change temperature,the soil surface begins to rise upward and produce frost heave.In the process of soil melting,under the action of external load,the soil surface has a downward melting consolidation phenomenon,and finally tends to be stable.(4)No matter in the frozen or thawing process,the stress of the pipe body has a relatively obvious response to the temperature.During freezing,the stress of the pipe body shows an increase in cold shrinkage,and the external load has a small effect on the stress of the pipe body.When thawing,the bearing capacity of the frozen soil weakens,which causes the effect of external loads on the pipe body to gradually become apparent. |
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