Study On The Influence Of Hefei Metro Tunneling Adjacent To Viaduct And Its Construction Controls

Tunneling of urban metro will inevitably pass through the underground space of city center.In this context,the high-rise buildings,viaducts,underground pipelines,and other structures that are adjacent to the metro line can pose a great challenge to the construction risk control of the metro tunneling.The objective of this study is to investigate an approaching construction issue where the urban metro tunneling is near an existing pile foundations.To achieve this objective,research work that can be classified into three aspects has been implemented in this paper.In the first aspect,theoretical methods for calculating the ground deformation and additional stress induced by shield tunneling were summarized and then were used to elaborate the effect of shield tunneling construction on the response of the adjacent pile foundations.In the second aspect,numerical method was utilized to simulate the dynamic tunneling process of the shield twin tunnel based on a practical engineering entitled “Approaching Construction of Hefei Metro Line 2 Shield Twin Tunneling within the section between the Qingyanglu Station and the Xiyuanlu Station to the Adjacent Wulidun Flyover”.Combining the simulation results with the field monitoring data,the characteristics of the tunneling induced deformation and stress for the surrounding ground and the adjacent pile foundations were analyzed and interpreted.Additionally,the roles that the three considered variables play in twin tunnel construction's effect on the deformation and stress characteristics of the adjacent viaduct pile foundations were numerically simulated and discussed.In the last aspect,some useful suggestions and construction control measures for the potential shield twin tunneling projects that will be adjacent to the Wulidun Flyover were proposed.The main research work of this study and the derived primary conclusions are summarized as follows.(1)A three-dimensional finite-element model for simulating the construction of a twin shield tunnel that pass underneath through the Wulidun Flyover was established by using the Midas software.With the help of this model,the tunneling's influence on the ground surface settlement,the ground stress,and the deformation and stress characteristics of the adjacent pile foundations was numerically estimated.The estimating results was subsequently challenged by the data derived from field monitoring.It was shown that the ground surface settlement was changed gradually with the excavation step.With decreasing the distance between the tunnel face and the monitoring points on or below ground surface,the ratio of increase in the ground surface settlement was increased.However,with an increase in that distance,this ratio was decreased gradually to a stable near-zero value at which that distance was no less than 3.5D(D represents tunnel diameter).During the same process,the ratios of the increase in the vertical and the horizontal displacements of the pile foundation were increased initially and then decreased and eventually reached almost unchanged values.Besides,the distributions of the pile stress along the cover depth were almost identical for all the piles at different positions.(2)Three different variables,which are tunnel-pile horizontal distance,distance between the two tunnel faces,and pile length,were considered in the numerical simulation to analyze their effects on the characteristics of the deformation and maximum/minimum stresses of the adjacent single pile.The results indicated that the distance between the two tunnel faces was the least sensitive variable in affecting single pile' deformation and maximum/minimum stresses,and the tunnel-pile horizontal distance was the second influential one.With increasing the tunnel-pile horizontal distance,the pile vertical displacement,the pile-top horizontal positive displacement,and the pile-bottom horizontal negative displacement were decreased initially and then increased.And their magnitudes reached to the minimums when the pile foundation was located at the center of the two tunnels.Also,the single pile stress achieved its extremum at the center of the twin tunnel.Single pile length was most influential in affecting the pile vertical displacement and the pile maximum/minimum stresses,whereas it had a little effect on the horizontal displacements of the pile shafts at an identical cover depth.The variations of the pile maximum/minimum stresses were different for varying directions.In particular,the maximum ratio of the variation of the maximum shear stress in yz direction was the maximum which was 89.8%.(3)Considering the research results derived from both numerical simulation and field monitoring,some specific controlling measures which were useful in reducing the adverse effect of tunneling on the adjacent pile foundations were proposed for the construction of Hefei Metro adjacent to existing viaduct.