7.4.2 Float Control and Treatment Measures
After the assembled tunnel ring is removed from the tail of the shield，due to the buoyancy of the surrounding groundwater or grouting slurry at the tail of the shield，when the earth pressure on the upper part of the segment and the weight of the segment cannot resist the buoyancy，the tunnel will float and cause passive damage to the soil above the segment.There are five control methods for the floating phenomenon.
1）Synchronous grouting control
In water-bearing fine sand stratum，solving the problem of segment floating is essentially a competition between synchronous grouting and stabilizing segment floating in time.The ideal grouting method should be that the shield tunneling along the axis，the grouting slurry fully fills the construction gap and rapidly solidifies to form a consolidation body with high early strength，so that the tunnel and the surrounding soil form an overall structure to achieve stability.
According to the relationship between the regular value of segment floating and shield advancing posture，the grouting hole position and grouting quantity are reasonably selected.According to the construction experience of Nanjing Yangtze River Tunnel，the grouting volume ratio of the upper，middle and lower rows of six grouting holes in the upper and lower rows of shield tail is about 5∶3∶2.According to the diameter of shield excavation and the gap of shield tail，the grouting volume of each ring segment（2 m wide）is 26-30 m3.
2）Control shield posture
Poor attitude control during shield tunneling will inevitably lead to frequent deviation correction.The process of deviation correction is the process of uneven stress on the segment torus.Therefore，in the tunneling process，the attitude of the shield should be strictly controlled，and the shield should be pushed along the design axis of the tunnel as much as possible to avoid the deviation correction snake.When the deviation is found，it should be gradually corrected，and sudden correction should be prohibited，so as not to cause large dislocation between segments，and the stress on the segment torus is seriously uneven.
3）Controlling tunneling speed
If the slurry cannot consolidate and stabilize the segment timely and effectively in the synchronous grouting process，the shield tunneling speed should be properly controlled.Since the synchronous grouting amount is 13-15 m3/m and the grouting speed is about 0.5 m3/min，the advancing speed should not be greater than 30 mm/min to ensure the balance between the gap formed when the segment is out of the shield tail and the grouting amount.If the advancing speed is too fast，the slurry cannot fill the gap between the segment and the tunnel in time.The slurry is concentrated at the bottom of the segment by gravity，and the segment is buoyed by the slurry buoyancy.
4）Reasonable control of shield advancing elevation
According to the shield construction experience of Nanjing Yangtze River Tunnel，the empirical values of segment floating after assembly in various strata are counted，and the shield posture is controlled to advance according to the axis elevation minus the segment floating during tunneling.In the construction of the shallow soil section in the middle of the river，the elevation of the shield propulsion axis is reduced to 20 mm below the design axis，so as to offset the buoyancy of the segment lining in the later stage.Practice has proved that the central axis of the formed tunnel is basically consistent with the design axis by controlling tunneling.
5）Treatment after tube floating
It is difficult to treat the segment after floating.Generally，the grouting hole can be opened at the bottom of the tunnel to release the grouting slurry at the bottom of the segment.According to the similar engineering construction experience，the effect of this method is not ideal，and the tunnel is polluted，and the construction risk is large.If it is found that the floating segment exceeds the limit，the shield tunneling must be stopped immediately，and the floating segment is grouted twice through the grouting hole.The grouting material is the best with instantaneous setting double liquid slurry，and the grouting sequence should follow the direction of tunnel slope，from the tunnel vault to the two waists，and finally the arch bottom is pressed.The principle of stopping grouting is to open the grouting hole at the arch bottom without water seepage，so as to prevent the segment from floating after the shield is restored to tunneling.