Research On Surrounding Environmental Effects Of The Curved Shield Tunnel Construction In Compound Stratum

With the rapid development of urban underground space led by subway construction in China,the spatial distribution of underground structures is increasingly intensive.New metro lines often avoid existing underground structures in the way of curve and proximity,which causes the safety risk of subway tunnel construction.Relying on the shield tunnel crossing overpass project of Nanjing Metro Line 7,the line between Wanshoucun station and Dingjiazhuang station passes through the upper soft and lower hard compound stratum in many sections.The upper part of the excavation face is mainly silty clay,and the lower part is in many cases of diorite,siltstone and mudstone.The curve tunnel with r=450m is close to the pile foundation of 29~#pier of Qixia Avenue viaduct across Jingwu road.The pile type is bored pile with a diameter of 1.5m and a length of 27m,with a minimum horizontal clear distance of 1.26m from the section.After turning southeast,the line is close to the pile foundation of tujiaying bridge,with a diameter of 1.0m and a length of 23m,with a minimum horizontal clear distance of 3.6m from the section.Compound stratum,curved tunnel and pile foundation of close bridge are the major risk sources of this section project.In this paper,numerical simulation and on-site monitoring are used to study the surrounding environment effect of curved shield tunnel construction in compound stratum.The main research results are as follows:(1)Based on the statistical analysis of the construction and driving parameters of the shield tunnel between wanshoucun station and Dingjiazhuang station and the field monitoring data of the surrounding environment,it is found that the total thrust of the jack,the pressure of the earth bin and the torque of the cutterhead are significantly affected by the changes of the stratum.The greater the stratum strength,the greater the total thrust,the greater the earth pressure and the torque of the cutterhead.When the shield machine is driving in silty clay,the total thrust should be 10000～12000k N,the earth pressure should be 0.12～0.14MPa,and the cutter head torque should be 1800～2000k N·m;when driving in the diorite,the total thrust shall be at least15000k N,the earth pressure shall be at least 0.20MPa,and the cutter head torque shall be at least 3000k N·m;when the shield machine is driving in the composite stratum,the appropriate shield parameters shall be selected according to the proportion of soil in the tunnel face;(2)Using gap gap parameter method,the calculation formula of shield tunnel stratum loss is established considering the factors such as shield machine size,hinge device parameters and curve radius.The stratum loss of shield tunnel under typical hinge angle and curve radius is calculated;(3)The three-dimensional numerical simulation model of the compound stratum curve shield tunnel is established,and the reliability of the numerical model is verified by the field measurement data.Through calculation and analysis of the influence law of the curve radius and stratum distribution of the tunnel on the construction environment,it is found that the overbreak caused by the construction of the curve tunnel has a significant impact on the surface settlement.The larger the overbreak is,the larger the settlement is,and the 10 mm overbreak will lead to a settlement increment of about 2.08 mm.When the tunnel is excavated in the compound stratum,the larger the proportion of soft soil in the excavation face,the larger the surface settlement,and the higher the proportion of soil 10%higher,the surface subsidence increased about 2.1mm;(4)The three-dimensional numerical simulation model of the pile foundation of the bridge close to the composite stratum curve shield tunnel is established to explore the influence of the curve radius of the tunnel,the horizontal clear distance between the tunnel and the pile and the stratum distribution on the displacement of the pile foundation of the existing bridge.It is found that the smaller the curve radius of the tunnel,the greater the stratum loss and the greater the lateral displacement of the pile body.When the radius of the curve is 300m,the lateral displacement of the pile top is about 14.56mm;the deformation of the pile itself decreases with the increase of the horizontal clear distance between the pile and the tunnel,the net distance increases by1m,the lateral displacement and the longitudinal displacement of the pile decrease by 1.67mm;the differential settlement between the piles is significantly affected by the stratum distribution,the larger the proportion of the soft soil in the excavation face of the tunnel,the greater the differential settlement between the piles.In the single silty clay layer,the differential settlement between piles caused by tunnel excavation is 6.77mm,and the differential settlement between piles caused by tunnel excavation in the composite stratum with 30%soft soil is 2.34mm;(5)The influence of shield tunneling on the pile deformation of Jingwu viaduct and dujiaying bridge is calculated and analyzed.The maximum lateral displacement,the maximum vertical displacement and the differential settlement between piles of Jingwu viaduct are 12.7mm,8.4mm and 4.9mm respectively.The maximum lateral displacement,the maximum vertical displacement and the differential settlement between piles of tujiaying Bridge are 6.8mm,8.3mm and 5.4mm respectively.The lateral displacement of Jingwu Viaduct Pile and tujiaying bridge pile exceeds the control value of the lateral displacement of the bridge pile,so it is necessary to take control measures to ensure the construction safety.According to the results of numerical simulation,some measures are put forward to control the deformation of pile,such as monitoring the deviation of tunnel and controlling the grouting quantity behind the wall,so as to ensure the construction safety.