7.4 Shield Attitude Control and Risk Management Te...
The excessive snake movement of shield will inevitably cause frequent rectification.The process of rectification is the process of uneven force on the segment ring surface.Therefore，it is necessary to control the posture of shield in the tunneling process and make it do a small amount of snake movement along the tunnel axis as far as possible.According to the specification，the allowable deviation of the plane position and elevation of the central axis of the assembled segment in shield tunneling is±100 mm.The deviation should be gradually corrected to avoid sudden correction，so as not to cause serious uneven stress on the segment torus.
When the shield passes through the ultra-shallow overburden section，due to the thin overburden，on the one hand，in order to prevent collapse and roof fall，it is necessary to accurately control the amplitude of support pressure，resulting in a small variation range between the minimum limit and the maximum water pressure，which makes the support pressure of the excavation face difficult to control.On the other hand，the lining segment is affected by the buoyancy of surrounding groundwater and shield tail grouting slurry.When the upper soil pressure and the weight of the segment cannot resist the buoyancy of the segment，it will cause the segment floating，the longitudinal bolt of the segment brittle rupture，and cause engineering accidents.Therefore，how to safely pass through the shallow soil section is the key problem of underwater shield tunnel construction.In order to ensure the safety of construction，it is necessary to analyze the construction risks and formulate corresponding countermeasures.