3.1.2 The Present Situation of Technology
1）Tunnel anti-floating technology
When the tunnel passes through the saturated upper layer，it will be subjected to the buoyancy of water.When the buoyancy exceeds the weight of the overlying soil on the tunnel and the self-weight in the tunnel and tunnel，the tunnel will float up.When the segment leaves the tail of the shield，the tunnel is surrounded by the grouting slurry behind the wall，and the buoyancy of the slurry is much greater than that of the saturated soil.At the same time，the earthwork excavated by the shield will lead to foundation unloading，and the assembled tunnel will deviate upward from the central axis by foundation rebound.Under the combined action of buoyancy and foundation spring back，the overburden of the tunnel produces uplift.If the maximum uplift is not effectively controlled，the overburden will be cracked，resulting in permeable cracks.The river flowing into the shield tail along the permeable crack will seriously affect the safety of the tunnel and tunnel construction.
Considering the formation loss caused by shield propulsion and the counteracting effect of foundation spring back on the tunnel，the anti-floating of the tunnel can be controlled by the following type：
q—Support pipe slice weight，q=
γc，Rois the outer radius of the tunnel，Ri is the inner radius of the tunnel，γc is the bulk density of condensed soil；
Fy—Slurry buoyancy，Fy=γyπ
，γy grouting bulk density after wall；
p—The overlying soil，p=2 Roh'.
2）Underwater reinforcement measures for shallow overburden
When the thickness of shallow overburden is too small or the anti-floating setting cannot be taken，the reinforcement measures of underwater overburden should be taken.The commonly used reinforcement measures are as follows.
（1）Throwing soil under the water to increase the thickness of the overburden of the tunnel.This method is generally used in the reinforcement of deep-water river tunnel，including the famous Detian Yibeihe tunnel reinforcement.This method can not only meet the minimum thickness of overlying soil required by shield propulsion and tunnel safety，but also save cost，which is simple and easy to use.
（2）Grouting reinforcement at the bottom of the water.Due to the requirements of shipping and drainage，it is not suitable to strengthen the shallow overburden in shallow water channel by throwing soil by river channel.Therefore，underwater grouting reinforcement and anti-floating plate are the first choice of shallow water channel reinforcement.Through grouting，the overlying soil at the bottom of the river forms an impermeable hard shell，and the value of c，Ø increases accordingly.When the overlying soil is impermeable，the increase of the value of c，Ø can effectively reduce the calculated value of the minimum thickness of the overlying soil required for shield propulsion.The water pressure will effectively act on the overlying soil layer to prevent the tunnel from floating；at the same time，replacing the floating weight with the actual weight of the upper overlying soil can also increase the weight of the tunnel overlying soil，prevent the tunnel from floating，and reduce the calculated value of the minimum overlying soil thickness.
（3）Adding anti-floating plate to provide reaction force for shield propulsion and preventing tunnel floating.In the particularly shallow overlying area，the antifloating structure，such as anti-uplift pile and anti-floating plate，should be added while grouting and reinforcement，which should be used to balance the lateral pressure produced by the soil during shield propulsion so that the shield can pass through the underwater dangerous area quickly with more appropriate propulsion force and propulsion speed.
Taking the first phase test section of Nanjing Metro North and South Line as an example，the reinforcement measures are introduced.
The test section of Phase I of Nanjing Metro North Line starts from Yingma Lane in the south and reaches Sanshan Street in the north，which consists of four parts.From south to north，it is divided into open section，buried section，shield working well and shield section.The shield section is 679.35 m long，in which crossing the inner Qinhuai River section is the key point of this section.The width of the shield crossing the Qinhuai River is 16.8 m，and the elevation at the bottom of the river is only 0.97 m from the top of the tunnel，which is less than 1/6 of the diameter of the tunnel.It is a typical construction case of shallow overlying tunnel at the bottom of the river.
Due to the requirements of dredging and waterway，the reinforcement method of adding uplift pile and anti-floating plate and grouting at the bottom of the water is adopted in this project.Steel reinforced concrete anti-floating slabs and anti-pull piles can not only prevent the tunnel from floating，but also provide a reaction force to prevent the shield from drifting when the shield is pushed forward，as shown in Fig.3.2a.The grouting depth is 7 m below the bottom plate，AB liquid is used，A liquid is cement slurry with water-cement ratio of 0.7，B liquid is water glass，the ratio of A liquid to B liquid is 1：（0.8-1.0），and the initial setting time is controlled at about 90 s after double liquid mixing.The purpose of grouting is to prevent the river from pouring into the working face through the shallow overlying soil gap when the shield is pushed forward.In addition，a hard dielectric layer is formed between the anti-floating plate and the shield section，and the pressure measurement produced by the shield propulsion or the floating force on the tunnel is transferred to the anti-floating plate，and the grouting area is strengthened as shown in Fig.3.2b.