Mechanism Behaviour Of Existing Buried Pipelines Due To Twin Shield Tunnelling

Tunnel excavation is inevitable associated with the ground movements and the changes of soil stressing,leading to additional stressing and settlements of nearby existing buried pipelines.The serviceability of the buried pipelines encountered by tunnelling has gotten momentum recently,especially for the pipelines being close to or having deteriorated to a critical state,which was an urgent problem to solve during the design work and construction.Although most previous studies focused on the responses of pipeline to single tunnelling and treated the tunnel-pipeline-soil interaction as a plane strain problem.There is still a lack of research on the responses of a pipeline to twin shield tunnelling.With the growing utilization factor of underground spaces in cities,the limited region for building and the increased construction of metro tunnel with a larger cross section,twin adjacent tunnels or even multiple tunnels are increasingly favored and constructed.The problem of the mechanism behaviour of an existing buried pipeline due to twin tunnelling has motivated the study in this paper.In this dissertation,a series of centrifuge model tests and numerical simulations were carried out to investigate the effects of twin tunnelling with different conditions on the adjacent pipelines.It is emphasized on the analyses and discussion about the greenfield ground surface settlement,pipeline settlement,pipeline bending strain,stress and strain of the soil around the pipeline,stiffness of the soil around the pipeline,the pipe-soil relative stiffness and the load transfer mechanism in the pipeline duo to twin tunnelling.In order to satisfy the need for an effective evaluation method of the potential risk on the pipeline due to twin tunnelling,based on the results obtained from centrifuge model tests and numerical simulations,a new approach was presented for evaluation the effects of twin tunnelling on the adjacent continuous pipelines and jointed pipelines.The main research contents and conclusions are as follows:(1)The conventional tunnel model,which was only used for simulating the volume loss effect,was improved.In the centrifuge model tests,both the volume loss effect and the weight loss effect were simulated by using the improved tunnel model.(2)A serise three-dimensional centrifuge model tests were conducted to investigate the effects of the construction sequences of twin tunnels at different depth on the adjacent pipeline in dry sand.In the centrifuge model test,both the volume loss effect and the weight loss effect were simulated.A displacement controlled method(DCM)based on ground loss,which was accomplished by a subroutine named DISP,was applied for the three-dimensional numerical back-analyses of the centrifuge tests.In the numerical simulations,an advanced hypoplasticity constitutive model incorporating strain-and path-dependent soil stiffness at small strains was adopted to describe the soil behaviour.Thereafter,the tests results were compared with the results obtained from a published dimensionless chart and an analytic solution.It is shown that the greenfield surface settlements,pipeline settlements and pipeline bending strain were greatly affected by the buried depth of the twin tunnels.The distribution of the sagging regions of the pipeline was significant affected by the second tunneling with shallow depth.The response of the pipeline should be closely monitored when the tunnels advance within the range of ±1.2DT.It is suggested that the superposition principle should be cautious of using to predict the bending strain in a pipeline induced by twin tunnelling.Because the effects of the accumulated shear strain on the relative pipe-soil rigidity should be reasonably considered.(3)A serise three-dimensional centrifuge model tests were conducted by using the same improved tunnel models to investigate the influences of twin tunnelling with different construction sequences and different relative position on an existing buried pipeline.Meanwhile,numerical back-analyses of the extended conditions were also carried out.Under the condition of piggyback twin tunneling,the maximume greenfield ground settlement,pipeline settlement,pipeline bending strain were induced when the lower tunnel was excavated first.Nevertheless,under the condition of stacked twin tunneling,the maximume greenfield ground settlement,pipeline settlement,pipeline bending strain were induced when the upper tunnel was excavated first.It is shown that twin tunnels with different relative location greatly affect the distribution of pipeline bending strain and the pipeline sheer stress.It is suggested that the superposition principle should be cautious of using to predict the bending strain in a pipeline induced by twin tunnelling.Because the effects of the accumulated shear strain on the relative pipe-soil rigidity and the shielding effects from the upper tunnel should be reasonably considered.(4)A constitutive model with explicit consideration the effects of different pipe-soil relative movements on the soil stiffness was presented,and an empirical curve described the ground surface settlement due to twin tunnelling proposed by Vorster[1]was adopted.On that basis,a series of numerical parametric studies were performed to investigate the responses of continuous pipelines to twin tunnelling.Based on the computed results,a new equation for calculation the relative pipe-soil stiffness was proposed.Meanwhile,two regression equations for describing the relationship between the relative pipe-soil stiffness and the relative pipe-soil curvature and the relationship between the relative pipe-soil stiffness and the relative pipe-soil maximum settlement were also estimated.Moreover,the applicability and reliability of the two regression equations mentioned above were validated.And both of them can be used to directly estimate the maximum pipeline bending strain and the maximum pipeline settlement due to single tunnelling and twin tunnelling,moreover,which can also be used to assess the tunnelling-induced risk to continuous pipelines.(5)The same constitutive model and the same empirical curve were adopted into the parametric studies to investigate the responses of the existing buried jointed pipelines to twin tunnelling.In the calculation procedure,pipeline joint mechanical behavoiurs were simplified as hinges.Based on the calculations results,a new equation for calculation the relative pipe-soil stiffness for the jointed pipeline was proposed.Meanwhile,a regression equation for describing the relationship between the relative pipe-soil stiffness and the normalized maximum joint rotation angel was also estimated.Moreover,the applicability and reliability of the regression equation mentioned above was validated.With pipeline dimension,pipeline material properties,cover depth of the pipeline,soil properties and ground settlement profile estimated before twin tunnelling,the regression equation can be used directly to calculate the maximum joint rotation angel due to single or twin tunnelling,moreover,which can also be used to assess the tunnelling-induced risk to jointed pipelines.