2.4.4　Control of Driving Parameters in the Arrival...

2.4.4　Control of Driving Parameters in the Arrival Section

1）Slurry pressure

When calculating mud pressure，the groundwater level is 5 m.After entering the influence range of precipitation in the arrival stage，the groundwater level is adjusted by the measured groundwater level.The mud water pressure shall be strictly controlled according to the tunneling parameters，with the deviation range of±10 kPa.The mud water pressure value shall be reduced by 10-15 kPa according to the calculated value.

2）Driving speed and cutter head speed

The driving speed and cutter head speed are set according to geological conditions and current construction experience，as shown in Table 2.4.

3）Backwall grouting

In order to prevent slurry leakage from shield tail，tunnel floatation and formation instability，it is necessary to strengthen the control of grouting behind the wall to ensure the quality of synchronous slurry.The volume mass of the slurry is 1.96 g/cm3，and the slump of the slurry is controlled at 18-22 cm in this area.The grouting quantity is generally controlled at 25-30 m3，and the grouting pressure is controlled to prevent the penetration of shallow overlying soil layer.

4）Setting and preparation of mud index

The volume mass of feed mud water is 1.23-1.26 g/cm3，the pulping mud water is 1.30-1.45 g/cm3，and the mud viscosity is controlled at 18-20 s.

5）Ballast quantity control

During shield tunneling，the amount of ballast produced by each ring should be strictly controlled to prevent surface subsidence caused by over excavation.A small amount of under digging is allowed and over digging is not allowed to protect the stability of the palm surface.

6）Shield gesture control

According to the construction experience，the shield attitude and shaped segment are stable in the sand layer，easy to float in the shallow overlying soil layer，and easy to cause settlement deformation in the 10-20 ring near the tunnel gate.The shield attitude is strictly controlled when the shield is driven，and the horizontal and vertical deviation between the shield excavation and the design axis is controlled at-20 to+20 mm，and the shield tunneling attitude is in good agreement with the design axis.

Through the implementation of the above comprehensive construction measures，Nanjing Yangtze River Tunnel has safely and smoothly implemented the arrival and receiving of the shield on the left and right lines（Fig.2.27）.By canceling the sealing curtain of the tunnel door and receiving it by irrigation（soil）in the receiving well，the soil and water pressure of the longitudinal leakage channel is balanced.The tunnel door is temporarily blocked and sealed with arc steel plate，and then the 10 rings pipe piece of the hole mouth is filled with secondary grouting，and then the permanent sealing construction of the hole door is carried out under the condition of precipitation.The effect is very satisfactory，and there is no leakage trace of the hole door after construction.The use of cement mortar pedestal not only solves the problem of height difference received by guideway，but also provides reaction force for the assembly of several ring segments at the mouth of the hole，which is of great significance to improve the assembly quality of senior executives and prevent the occurrence of misalignment.