Study On The Deformation Effects Of Shield Tunnel With Shallow Overburden Crossing The Existing Subway Tunnel At Close Range

With the rapid development of urban subway tunnels,under the condition of limited underground space resources,the complex conditions of different forms of new tunnels crossing existing tunnels in close proximity will become an unavoidable huge problem in urban subway construction and put forward higher requirements for the safe construction of subway tunnels,and the discussion and research on tunnel construction in close proximity has been increasing in recent years.In this paper,numerical simulation of the shield construction of the upper span section is carried out using FLAC 3D in the context of the close span of Qingdao Metro Line 6 over the existing Metro Line 1,and the simulation results are compared with the measured data to analyze the influence law of the shield upper span construction on the ground surface and the existing tunnel.Finally,a numerical model was developed using the control single variable method to study the influence of the angle of the upper span of the new tunnel,the vertical clear spacing between the old and new tunnel,and the influence of the surrounding rock level on the existing tunnel and the degree of influence.The following conclusions were drawn:(1)Through the collation and analysis of the on-site monitoring data,it was found that the deformation of the ground surface and the existing tunnel mainly occurred after the shield crossed the monitoring section,and the overall settlement rate of the ground surface was faster and the settlement duration was shorter due to the shallow burial depth of the shield tunnel in the upper span section.The deformation of the structure at the intersection of the existing tunnel and the new tunnel is the most affected,and due to the different vertical spacing between the left and right lines and the new tunnel,the arch top rise and arch waist convergence values of the right line structure are larger than those of the left line.The deformation values of both the surface and the existing tunnel meet the monitoring and control requirements of the project,which verifies the safety of the construction of the shield span and the feasibility of the actual span construction plan of the project.(2)After the completion of the shield span,the lateral settlement of the ground surface and the Peck settlement curve behaved basically the same,the settlement value of the measured point above the axis was the largest,and the width of the settlement trough was about 30 m,about 5 times the tunnel diameter.The response range of surface settlement along the shield tunneling direction is about 15 m before crossing to 20 m after crossing,and the surface settlement rate and final settlement value at the intersection of the new tunnel and the existing tunnel are smaller than the surface settlement rate and final settlement value before and after the upper span,so the influence of the shield construction on the surface settlement is smaller than the influence of the shield construction on the surface settlement in a single line.(3)Before the shield reaches the monitoring section,the vault of the existing tunnel is first deformed by a very small amount of settlement due to the compression of the cutter disk on the soil ahead,and then starts to show a tendency of upward uplift.The response range of vault uplift is about 20 m before to 20 m after the shield crossing,and the uplift of the existing tunnel mainly occurs between the arrival of the cutter disc above the existing tunnel and the removal of the shield tail from the existing tunnel.Before the shield span,the existing tunnel vault is offset to the left side of the axis,and after the shield span,it is offset to the right side of the axis,and the value of vault offset and arch waist convergence is only half of the value of vault uplift.In the actual construction process,the focus of site monitoring should be concentrated on the bulge deformation of the existing tunnel structure.(4)The excavation and unloading of the shield tunnel is the main factor causing the stress change in the existing tunnel,and the stress change at the vault is the most obvious,mainly in compressive and shear during the upper span.After the shield excavation was completed,the percentage increase of compressive stress at the vault was 552.46% and the percentage increase of shear stress was 275.05%.The magnitude of stress change in the left line was slightly larger than that in the right line,but the maximum stress values did not exceed the bearing capacity of the existing tunnel support structure.(5)The larger the upper span angle,the smaller the unloading range of the upper part of the existing tunnel,and the larger the vertical clear spacing,the less the existing tunnel is affected by the upper unloading effect.The higher the rock level,the higher the deformation,deformation rate,initial structural stresses and stress growth of the existing tunnel,and the higher the rock level,the greater the impact of the shield on the existing tunnel deformation along the tunneling direction.When the upper span construction is carried out under the poor surrounding rock conditions,adequate protective measures should be taken in advance and the site should be monitored and measured during the construction to ensure the safe and smooth construction of the tunnel and avoid risks.