Study On Safety And Construction Stability Of Shield Tunnel In Coastal Sandy Soil

Economic and social activities in coastal areas are very active,and many large-scale projects are also located in coastal areas.With the rapid development of economic construction and the increase of population in coastal areas,the traffic pressure of cities is increasing.Most coastal cities have owned or are accelerating the pace of subway construction.By means of site survey,numerical analysis and model test,the construction stability of Metro Shield Tunnels crossing buildings in sand stratum,the construction stability of Metro Shield Tunnels under tidal action and the construction safety risk of shield tunnels in water-rich sand stratum are studied in this paper.Specific research contents,methods and conclusions are as follows:(1)Based on field experience and theory,the chamber pressure,grouting pressure and grouting volume(indirectly expressed by grouting thickness)in the construction parameters of metro shield tunnel are analyzed,and the optimum values of the above parameters in the construction process are obtained: chamber pressure 0.1MPa,grouting pressure 0.19 MPa,and grouting thickness 0.21 m.PLAXIS3D software is used to analyze the influence of the excavation process on the shield tunnel underpass complex in this section.By comparing the results of numerical analysis and on-site monitoring data,the results show that the influence of shield tunneling crossing buildings in Metro sections on surface buildings is within the safe range.(2)By using the parameters in(1)to carry out numerical analysis and model test comparative study,this paper explores the influence of tide on groundwater during the construction of Metro Shield Tunnel in coastal sandy soil layer,and then on the shield tunnel.The results show that with the rise of water level,the soil on the surface of the model will be uplifted by buoyancy,and the maximum uplift value of the soil surface at each excavation stage of the tunnel is above the excavation surface when the water level rises to the tunnel vault.The increase of groundwater level will lead to the rebound uplift of surface soil.The rebound value of shield tunnel structure under the soil is larger than that of tunnel structure without shield.When the groundwater level rises to the elevation of the arch bottom,the center and the vault of the shield tunnel,there is a "bowl" shaped soil above the face of the shield tunnel.Its uplift value is larger than that of the surrounding soil.With the gradual increase of the water level,the "bowl" shaped soil will gradually spread back and its uplift value will also increase.Moreover,the shield tunnel structure will have corresponding rising displacement,which will increase with the rise of water level,and the structural deformation of the tunnel will also increase.(3)By combining content(1)and(2),the main factors affecting the construction risk of shield tunnels crossing water-rich and weak strata are separated,and combined with fuzzy mathematics and Bayesian network,a fuzzy Bayesian model is constructed to evaluate the safety and stability of shield tunneling in sandy soil strata.The research shows that segment rupture,waterproof layer rupture and shield tail seal failure of shield tunnel have great influence on the construction risk of shield tunnel crossing water-rich soft stratum.