3.2.2　Driving Parameters and Gesture Control of Sh...

3.2.2　Driving Parameters and Gesture Control of Shallow Overburden Construction

3.2.2.1　Water Pressure Control of Incision

Because of the shallow soil cover on the top of shield，it is difficult to control the incision water pressure.The fluctuation of the incision water pressure is too large，which will increase the disturbance of the frontal soil and lead to the loss of the frontal soil.Therefore，the incision water pressure fluctuation should be reduced as much as possible.Technically，the operator changes from automatic control to manual control，and the fluctuation value of notch water pressure is controlled at-20 to+20 kPa to ensure positive stability.In the process of construction，due to the high strength and good self-reliance of the strengthened strata，the actual slurry pressure is less than the design value.According to the monitoring information，the initial setting value of slurry pressure is 1 kg/m2.

1）Theoretical arithmetic

（1）The upper limit value of incision water pressure.

PU—Upper limit value of incision water pressure（kPa）；

P1—Groundwater pressure（kPa）；

P2—Static earth pressure（kPa）；

P3—When the earth pressure is changed，30 kPa is generally taken；

γw—Bulk density of water（kN/m3）；

h—Tunnel depth below groundwater level（up to tunnel center，m）；

K0—Static earth pressure coefficient；

γ—Bulk density of soil（kN/m3）；

H—Tunnel depth（up to tunnel center，m）.

（2）The lower limit of incision water pressure.

PD—Incision water pressure lower limit（kPa）；

P2'—Active earth pressure（kPa）；

Ka—Active earth pressure coefficient；

Cu—Cohesion of soil（kPa）.

2）Determination of incision water pressure during excavation

The mud water pressure of shield tunneling is between the upper and lower limits of the theoretical calculation value，and is adjusted according to the situation of the surface structure and geological conditions.In the process of reverse washing，because the mud discharge pipe in the mud tank or shield is in a blocked state，the mud discharge rate is increased during reverse washing，but the incision water pressure is not reduced.The difference of notch water pressure deviation between shield propulsion，reverse washing and bypass switching is controlled at-20 to+20 kPa.

3.2.2.2　Strict Control of Main Tunneling Parameters

Total thrust，propulsion speed，mud discharge，reduce slurry pressure fluctuation，adopt low speed uniform excavation，avoid large disturbance to soil，strengthen mud management and unearthed monitoring，prevent over-digging and under-digging.The driving speed is controlled within 20 mm/min.Shield tunneling mainly controls the following parameters.

1）Excavation speed

Under the normal heading condition，the excavation speed is set to 20-40 mm/min；when the shield passes through gravel and gravel formation，and the excavation speed is controlled in 15-30 mm/min.

When setting the speed of shield tunneling，pay attention to the following points：

（1）When the shield is started，the shield driver checks whether the Jack is solid or not，and the speed should not be too fast before starting and ending the propulsion.At the beginning of each ring excavation，the excavation speed is increased step by step，and the starting speed is too large to disturb the formation.

（2）In the normal excavation process of each ring，the excavation speed value is kept constant as much as possible，and the fluctuation is reduced，so as to ensure the stability of incision water pressure and the smooth flow of mud discharge pipe.When adjusting the driving speed，it is adjusted step by step to avoid the impact disturbance caused by the sudden change of velocity to the formation and the excessive water pressure swing of the notch.

（3）The speed of propulsion must meet the requirements of grouting quantity of each ring，and ensure that the synchronous grouting system is always in a good working state.

（4）When selecting the excavation speed，we must pay attention to the matching with the geological conditions and the surface building conditions，so as to avoid the improper selection of the velocity to the shield cutter head，the abnormal damage caused by the cutting tool and the excessive disturbance of the soil around the tunnel.

2）Control of excavation quantity The actual excavation quantity R of shield tunneling is calculated by the following formula.

VR—Actual excavation quantity（m3）；

Q1—Discharge flow rate（m3/min）；

Q0—Mud flow rate（m3/min）；

t—Cutting time（min）.

When it is found that the amount of excavation is too large，check the mud water density，viscosity and incision water pressure immediately.In addition，it can also be used to investigate the collapse of soil and adjust the relevant parameters in time to ensure the stability of excavation surface.

3.2.2.3　Shield Plane Elevation Gesture Control

The shield machine keeps moving smoothly，reduces the deviation correction，and reduces the disturbance to the positive soil.Considering that the overlying soil is shallow，the shield and segment may float because of the shallow overburden.In order to offset the influence of floating，the deviation between the shield center and the tunnel design elevation is controlled within-30 mm and the plane deviation is controlled within±30 mm.

Strengthen monitoring and measurement，strictly control settlement.In order to ensure the safe passage of shield through shallow overlying soil section and strengthen the information construction，through the test data provided by the monitoring system，the construction parameters in the process of shield crossing are adjusted and controlled in time，and the grouting and ground tracking measures are taken to reduce the influence of shield construction on the ground.

Nanjing Yangtze river tunnel large diameter slurry shield in the initial period of shallow overburden，adopt these measures，and take effective measures and optimize the construction parameters，soil strengthening quickly and reliably establish slurry balance shield machine，ensure the 5.5 m ultra-shallow overburden under the condition of shield construction smoothly tank and safe starting，setting a world large diameter shield the most shallow overburden soil safe starting record（Fig.3.3）.