Research On Underpass And Reinforcement Measures Affected By The Construction Of The Overlapped Shield Tunnels

In recent years,under the influence of station setting form,environmental assessment and space utilization limitation,many tunnels overlap each other in urban subway sections,which will undoubtedly increase the disturbance of tunnel construction to the surrounding strata and cause adverse effects on construction safety.Taking the overlay shield tunnel under construction from Guanzhuang Station to Yongshun Station as the engineering background,this paper mainly studies the ground settlement law of completely overlay section tunnel excavation,the influence of ascending tunnel excavation on the construction of the established descending tunnel and the reinforcement measures,and the influence of some overlapping section tunnels under the railway and the reinforcement measures.The research is as follows:(1)By establishing a three-dimensional numerical simulation model,the land surface settlement law caused by the shield tunneling process in the completely overlaid section is analyzed.Based on the random medium theory and superposition principle,the prediction formula of transverse surface settlement of the overlaid tunnel under two excavation sequences is proposed.The influence of three factors on the land surface settlement including the clear distance of tunnel,the buried depth of tunnel and the spacing of tunnel excavation is considered by numerical simulation orthogonal test.The model analyzes the change of the downward tunnel displacement deformation and structural stress caused by the upward tunnel construction under the excavation sequence of "first down then up",which is often adopted in practical engineering.The analysis shows that the vertical displacement and principal stress of the downward tunnel arch are greatly affected by the upward tunnel construction.In order to control the deformation and structural stress of the tunnel,it is necessary to adopt the reinforcement measures of grouting between the tunnel and setting the trolley support in the downgoing tunnel.(2)The numerical simulation method was used for modeling and comparative analysis of different strengthening conditions,and the grouting parameters in the grouting reinforcement measures were further analyzed to determine the optimal grouting reinforcement scheme;The results show that the combination of trolley support and grouting reinforcement has the most obvious reinforcement effect,which can effectively reduce the uplift deformation and structural stress of the tunnel.The vertical displacement of the downgoing tunnel decreases with the increase of grouting thickness,Angle and elastic modulus,but the decreasing range slows down gradually.Therefore,the optimal grouting scheme proposed for this stacking project is 2m grouting thickness,120° grouting Angle and 200 MPa grouting elastic modulus.(3)Modeling analysis is carried out on the construction process of shield tunnel under the railway in some overlapping sections.In view of the excessive settlement and deformation of railway subgrade,different grouting reinforcement conditions are set for analysis and comparison,and grouting reinforcement measures are selected in the radial part of the tunnel and the stratum below the subgrade.Furthermore,the rationality of grouting reinforcement length is further studied,and the optimization scheme of grouting reinforcement is proposed.The modeling analysis shows that the control effect of grouting reinforcement measures on subgrade settlement in underpass engineering is remarkable,which ensures the safety of underpass railway construction.For this underpass project,it is suggested that the length of radial grouting reinforcement of upper and lower tunnels should be 3D ～ 6D,and the length of stratum grouting reinforcement under the subgrade should be 3D ～ 5D(D is the tunnel diameter),which can provide a certain reference for grouting construction design.