7.4.1 Shield Attitude Control in Shield Tunnel Con...
The attitude control of shield in shield tunnel construction includes the rolling control of the body and the control of the forward direction.In the tunneling process，the shield operator adjusts the attitude of the shield according to the data displayed on the computer screen by the laser automatic guidance system.The operation principle of shield attitude control is as follows：
（1）The roll angle of the body should be appropriate.The shield roll angle is too large，and the shield cannot maintain the correct attitude，which affects the assembly quality of the segment.At this time，the roll angle can be reduced by reversing the cutterhead.
（2）The thrust of the right jack partition needs to be increased if the shield moves horizontally to the right.Conversely，the thrust of the left jack partition needs to be increased.If the shield head is downward，the thrust of the lower jack partition needs to be increased，and vice versa.
1）General rules of shield attitude control
In general，the direction correction of shield should be controlled between±20 mm.In the transition curve and circular curve section，the direction correction of shield machine should be controlled between±30 mm.Try to keep the shield axis parallel to the tunnel design axis，otherwise it may cause too small shield tail clearance and segment dislocation cracks due to bad posture.
When the soil on the excavation face is uniform，the attitude control of the shield is easy，and the direction deviation angle is usually controlled in the±5 mm/m.
When the left and right soft and hard strata in the excavation face are uneven and are in the curve section，the shield attitude control is difficult.At this time，the tunneling speed can be reduced and the jack thrust of each partition can be reasonably adjusted.If necessary，the overbreak cutter（shield equipped with overbreak cutter）can be considered for overbreak in hard rock area.
When the shield encounters the soft and hard soil layer，in order to prevent the shield from“raising”，keep the downward attitude.On the contrary，we should maintain the upward posture.Attention should be paid to the jack travel difference between the upper and lower ends and the left and right sides can not be too large，generally controlled between±20 mm.
In the curve section tunneling，generally according to the curve radius of the shield to the curve inside a small amount of offset，offset generally take 10-30 mm.
In the shield attitude control，the stroke control of the propulsion cylinder is the key point.For 1.5 m wide segment，in principle，the stroke is controlled in 1700-1800 mm，the stroke difference is controlled in 0-40 mm，the stroke is too large，the shield tail brush is easy to expose，the segment is more out of the shield tail，and the deformation is large.If the stroke difference is too large，it is easy to increase the angle between the shield and the segment，which is easy to cause the damage and dislocation of the segment.
2）Control technology of shield tunneling posture in different geological environments
（1）Control of shield tunneling posture in muddy soil.
When shield tunneling in soft soil layer，due to the extremely poor self-stability of stratum，in order to control the horizontal and vertical deviation of shield within the allowable range and avoid excessive disturbance to stratum caused by excessive shield snake，it is appropriate to control the shield tunneling speed at 30-40 mm/min and the cutter head speed at about 1.5 r/min.During tunneling in this section of strata，the thrust of the four groups of jacks should be relatively balanced，so as to avoid the large stroke difference of jacks during tunneling.Otherwise，it may cause that the thrust axis and the segment center axis are not in the same straight line.In the tunneling process，a certain amount of additives should be added according to the actual situation in order to keep the excavation smoothly and keep the continuous tunneling of the shield as far as possible.At the same time，the synchronous grouting amount should be strictly controlled to ensure the effective filling of the pipe back gap.
（2）Control of shield tunneling posture in sand.
Due to the poor self-stability and high water content of water-bearing sand layer，shield tunneling in full-section water-rich sand layer is prone to“bump”phenomenon，and the shield is also prone to floating in water-bearing sand layer.
In order to avoid the“bump”phenomenon of shield tunneling in water-bearing sand layer，the shield machine should maintain the upward trend in the process of advancing.If the“bump”trend is found，the upper and lower pressure should be adjusted immediately to maintain the upward trend of shield.
In order to avoid the floating phenomenon of shield tunneling in water-bearing sand layer，the rotational speed of cutterhead should be reduced and the disturbance to surrounding sand layer should be reduced in shield tunneling.If the buried depth of the tunnel is less than 2/3 times of the shield diameter，measures such as geological improvement and ground surcharge should be implemented in water-bearing sand layer.
（3）Control of shield tunneling posture in strata with large fluctuation of strata.Rock layer fluctuation will lead to uneven soft and hard rock in tunnel excavation face.The shield tunneling in this stratum is difficult to control the attitude of the shield，and it is easy to cause excessive hunting in the vertical direction of the shield，resulting in dislocation and cracking of the segment.Taking the soft and hard strata above as an example，under such geological conditions，the force of shield cutterhead is uneven，and the tunneling speed is unbalanced.This requires that during the tunneling process，the shield attitude data provided by the measurement system must be observed at all times，and the thrust and total thrust of and total thrust of the jacks in each partition must be continuously adjusted in combination with the travel difference between the propulsion jack and the hinged jack to maintain the stability of the shield attitude.If you don't pay attention to adjusting the travel difference of the jack，it will cause large changes in the selection of the segment，and even cause too small shield tail clearance to make the segment not smoothly out of the shield tail.Therefore，in the process of advancing，we should not only pay attention to the shield attitude provided by the measurement system to guide tunneling，but also take into account the travel difference of jacks in each partition.
（4）Control of shield tunneling posture in the hard rock stratum of the whole section.
The whole cross-section hard rock stratum belongs to uniform rock stratum.The axis attitude of shield tunneling in this kind of stratum can be well controlled.When tunneling，the thrust of jacks in each partition is uniform，the total thrust and tunneling speed are uniform，and the good attitude of shield can be maintained.
3）Correction measures for shield tunneling
Shield always deviates from the design axis during tunneling，and rectification must be carried out in a planned and step by step.Correction measures are as follows：
（1）Attention should be paid to the change of roll angle during tunneling，and the rotation direction of cutterhead should be adjusted according to the roll angle of shield in time.
（2）The tunneling parameters should be adjusted according to the geological conditions of each section.
（3）In the process of deviation correction，the tunneling speed should be slowed down，and attention should be paid to avoiding the damage to the segment caused by the excessive force on the unilateral jack during deviation correction.
（4）The reasonable segment type should be selected as far as possible to avoid the excessive influence of human factors on shield attitude，and the segment assembly quality should be strictly controlled to avoid the adjustment of shield attitude.
（5）When correcting the deviation，it is necessary to pay close attention to the attitude of the shield，the selection of the segment and the gap between the shield tail and the segment.The gap between the shield tail and the should be uniform.
（6）When the shield deviates from the design axis，it shall not be adjusted violently to avoid excessive deviation in the opposite direction.
4）Shield attitude control requirements of Nanjing Yangtze River Tunnel
According to the construction experience ofNanjing Yangtze River Tunnel，the shield posture and formed segments are stable in the sand layer，and prone to floating in the shallow overburden.The shield posture in the shallow soil section of the middle reaches of the Yangtze River is vertically controlled at-30 to+10 mm，and the shield posture is controlled at+10 to+30 mm inside the curve due to the advance of the curve section on the plane.