| With the rapid development of urban subway,the new lines will inevitably cross the existing lines,which will cause many problems.It is of great significance for design and construction to clearly understand the influence of constructing new line on existing lines.This paper is based on the Xinfadi station ? Caoqiao station section of Beijing New Airport Subway line constructioned by PBA method,which overpassing the existing subway No.10 and underpassing the Zhenguosi North Street.Combined with site monitoring,this article carries on numerical simulation to the project.The tunnel rock parameters were firstly calculated through displacement inversion analysis method,and then,the impact of surface subsidence caused by underground excavation and uplift of existing lines caused by large section shallow embedded excavation and the protection scheme of active control measures for uplift are studied.Based on the above research,the deformation control mechanism of the existing tunnel is summarized,and the plan of the excavation and the protection measures of the existing tunnel is optimized.The main research conclusions are as follows:(1)The surface subsidence caused by the construction of upper pilot tunnel accounts for 73.1% of the total,and the subsidence caused by the construction of the arch accounts for 13.87%.Therefore,the key to control the surface subsidence lies in the excavation and support of the upper soil.The simulation results show that using advanced support,pre-grouting and using pipe sheds can effectively reduce the surface subsidence value during the construction of the upper pilot tunnel.When constructing the the arch,it is necessary to skip when breaking the central partition wall of the pilot tunnel,the main reinforcement and section steel support of the grid are retained.After the second lining reaches the design strength,the temporary support will be removed.(2)The construction of the upper pilot tunnel resulted in the accumulated uplift of the existing line of 2.7mm,the accumulated settlement of 2.33 mm and the final uplift of 0.37 mm.Compared with the scheme of constructing each pilot tunnel in turn and finally constructing the side piles,top beams and other tunnel structures at the two sides,constructing side piles,top beams and others in side pilot tunnels and then constructing middle pilot tunnels can significantly reduce the maximum uplift value in the process by 36.04%.(3)The influence of underground excavation on the uplift increment of existing lines mainly occurs during the construction of lower soil,and the uplift amount of existing lines caused by lower pilot tunnel construction accounts for 72.3% of the final uplift.Four load reduction methods are analyzed,including vertical stratification and longitudinal segmentation.Excavating soil above the tunnel firstly can reduce the maximum uplift amount of the tunnel by about 20%.(4)Excavating has little influence on the horizontal convergence displacement of shield tunnel,which is within the deformation control range.After the construction,the central section of the shield tunnel converges horizontally by 0.2mm,and draws vertically by 0.8mm.The tunnel is in the state of horizontal flattening.(5)Digital simulation shows that pre-grouting reinforcement and steel pipe-roof applied to soil around shield tunnel have significant effect on controlling tunnel uplift.Under this scheme,the maximum uplift amount of tunnel is 1.33 mm,which is about 75% less than before.The field monitoring result of the tunnel is 1.30 mm,slightly smaller than the simulation result,which may be due to the accumulation of excavated soil on both sides.The anchor cable support with the pre-stress of 900 k N has the best effect on reducing the deformation of the existing shield tunnel,which can reduce the deformation of the existing shield tunnel by 12%,but the stress of the existing shield tunnel increase 24 % at the same time. |
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