Theoretical Analysis And Experimental Research On Deformation Of Buried Pipelines Due To Subway Tunnel Excavation

In recent years,the scale of urban rail transit construction in China has been ever growing.The large-scale construction of subway tunnels has an impact on existing underground structures,with buried pipelines being one of them.The subway construction causes soil movement,which brings additional displacement loads on the pipeline,resulting in additional deformation and internal forces that thus increase the risk of pipeline damage.Additional deformation is an important index for pipeline safety assessment,and correct calculation of pipeline deformation and internal forces is a key to pipeline safety assessment.At present,calculation of pipeline deformation induced by tunnel excavation relies on numerical simulation and empirical methods,while theoretical methods are not sufficiently studied.The traditional elastic foundation beam method cannot solve the problems of axial deformation of pipelines,pipe-soil detachment,and deformation of jointed pipelines.Therefore,this paper used a combination of theoretical analysis,numerical simulation,and centrifugal test to explore the law of pipe-soil interaction,and proposed theoretical models and calculation methods for solving pipeline deformation.The main results are as follows:(1)According to the bending moment transmission performance of pipeline joints,pipelines were divided into jointless pipelines and jointed pipelines.In terms of jointless pipeline,a theoretical model considering the axial force and geometric nonlinearity of the pipeline was established,and the governing differential equations of the axial and vertical deformation were derived.The optimization method was used to solve the differential equations.The law of pipe-soil relative displacement was studied,and a double-layer spring elastic foundation beam model considering the detachment between pipeline and soil was proposed.Combined with the ideal elastoplastic constitutive relationship of the foundation spring,four kinds of broken line models for describing the relationship between pipe-soil relative displacement and interaction forces were given.The governing differential equations of pipeline deformation were deduced and solved by transfer matrix method.(2)In terms of jointed pipeline,pipe sections were regarded as beams on elastic foundation,and pipeline joints were simplified as“free hinge”and“spring hinge”according to whether they have the ability to transmit bending moments.The transfer matrix method was used to solve the deformation and internal forces of the jointed pipeline.The“equivalent load”and the joint stiffness reduction coefficient were introduced into the governing differential equation of the beam on elastic foundation.Based on the Fourier series method,the analytical solution of the relative rotation angle of jointed pipeline was given.The relationship between the joint stiffness reduction coefficient and the joint rotation stiffness was figured out,which improved the practicability of the Fourier series solution.It can be seen from the accuracy of calculation results,as an analytical solution,the Fourier series solution has a higher accuracy than the transfer matrix method.In Fourier series method,the series coefficients of load function need to be computed first,which increases the difficulty of using Fourier series method and preliminary workload.(3)Using the“drag-and-drop”method,three sets of centrifugal tests were conducted to simulate excavation of the tunnel perpendicularly undercrossing the pipeline.Test results show that,the higher the roughness of the outer surface of the pipeline,the greater the horizontal displacement load on the pipeline and the greater the axial force of the pipeline.When the pipeline stiffness and volume loss of soil are small,there was no obvious pipe-soil detachment.After increasing the volume loss of soil and pipeline stiffness,the interaction force between the bottom of the pipeline and the soil above the midline of the tunnel was close to zero,indicating that detachment between pipeline and soil occurred.There was an abrupt change of rotation angle at joints of jointed pipeline,and the pipe segments behaved more rigid.In order to obtain the pipe-soil interaction parameters,an indoor loading test was carried out,and the vertical foundation coefficient and the horizontal foundation coefficient were calculated respectively based on the test results.The results of the centrifugal test were compared with that by theoretical methods and the correctness of theoretical methods was verified.(4)The ANSYS finite element software was used to establish a finite element model of the tunnel undercrossing the pipeline.The pipeline and the soil were meshed by solid elements,and the pipe-soil interaction was simulated by contact elements.The relationship between the vertical foundation coefficient and the contact stiffness was deduced,and a trial calculation method of the normal contact stiffness was given.Substituting the ground displacement fitting curve obtained by numerical simulation into theoretical methods,the theoretical calculation results of pipeline deformation and internal forces were basically consistent with the results by numerical simulation,which further verified the correctness of the theoretical method.(5)The theoretical method of this paper was used to analyze the parameters of the influence factors of the pipe-soil interaction.Results show that,the displacement load of soil can cause significant axial tension and compression strain of the pipeline,which should be considered in the calculation.When the tunnel perpendicularly traverses the pipeline,the pipe-soil detachment was easy to occur.When the tunnel drives parallel to the pipeline,the settlement difference between pipeline and soil was small,and the pipe-soil detachment was not easy to appear.A dimensionless pipe-soil relative stiffness coefficient was proposed,and a simple method for estimating the bending moment and settlement was given based on the theoretical method considering the pipe-soil detachment.Parameter analysis of the jointed pipeline shows that there is a limit value for the relative rotation angle of the pipeline joints.When the tunnel traverses perpendicularly underneath the pipeline,the limit value is 1.1S_max/i_s.When the tunnel excavates parallelly underneath the pipeline,the limit value is 0.33S_max/i_s.(6)The theoretical method presented in this paper was used to calculate in-situ cases.The theoretical calculation results of pipeline settlement were compared with the measured data.Both were consistent with each other,which proves the effectiveness and practicability of the theoretical method.