3.2.3 Slurry Quality and Grouting Control After Wa...
In the process of shallow overburden construction of shield tunnel，the quality of mud film formation in front of the cutter head is determined by the quality of mud water，and the grouting technology behind the wall is an important measure to ensure the overall stability of the tunnel and the stability of the surrounding soil.
3.2.3.1 Mud Water Quality Control
In order to strengthen the support of front soil mass and prevent grouting on the ground，heavy slurry shall be adopted for advancing.The density of slurry is 1.26-1.3 g/cm3，and the viscosity is 25-35 s.In order to ensure the quality of slurry，in the process of advancing，increase the frequency of mud test，adjust the mud quality in time，and ensure the smooth progress of mud.The flow of slurry quality control is shown in Fig.3.4.
Fig.3.4 Mud water quality control process
3.2.3.2 Strengthen Synchronous Grouting Control Behind Wall
Synchronous grouting is an important measure to prevent formation subsidence and control the synchronous grouting pressure and grouting quantity of shallow overburden section.The grouting capacity is controlled at 150％-250％.In order to prevent the grouting pressure from breaking the overburden，a pressure limit valve is set up in the control system of the grouting machine.
Taking the synchronous grouting construction behind the wall of Nanjing Yangtze River Tunnel as an example，this part introduces the slurry preparation and construction technology.
1）Grouting flow
The formation loss caused by shield construction and the reconsolidation of reconstructed soil disturbed or shear destroyed around shield tunnel and the infiltration of groundwater are the important reasons leading to the settlement of surface and buildings.In order to reduce and prevent settlement，in the process of shield tunneling，a sufficient amount of slurry material is injected synchronously behind the lining segment of the shield tail as soon as possible to fill the ring construction gap of the shield tail.
As a very important process in shield construction，grouting behind segment lining has the following three main purposes：
（1）Fill the gap of shield tail building in time，support the rock mass around the segment，and effectively control the surface subsidence.
（2）The condensed slurry will be used as the first waterproof barrier of shield tunnel to enhance the waterproofing ability of tunnel.
（3）Providing early stability for segment and integrating segment with surrounding rock mass is beneficial to the control of shield tunneling direction and can ensure the final stability of shield tunnel.
In general，three double piston grouting pumps with shield are injected at the end of the shield at the same time，which can fill the annular gap between the pipe and the formation in time，control the deformation of the formation，stabilize the segment structure，control the direction of shield tunneling，and strengthen the self-water prevention ability of the tunnel structure.
2）Slurry parameters and ratio
（1）Grouting ratio.Because most of the shield tunnel passes through fine sand and gravel layer，and the tunnel passes through sand layer and river water directly，combined with the construction case of large section mud water shield in China，cement mortar is used for synchronous grouting，and the material ratio is shown in Table 3.1.
（2）The main performance indexes of slurry.
Gelation time：Generally speaking，it is 3-10 h.According to the formation conditions and excavation speed，the gelation time is adjusted by adding accelerator and changing the ratio in the field test.For strong permeable strata and areas requiring high early strength by grouting，the mix ratio is further adjusted and early strength agent is added through field tests to further shorten the gelation time，obtain early strength and ensure good grouting effect.
Table 3.1 Preliminary proportion of synchronous grouting materials
The strength of solidified body is not less than 0.2 MPa（equivalent to unconfined compressive strength of soft rock）and not less than 2.5 MPa（slightly higher than the natural compressive strength of strong weathered rock）.
The rate of slurry stone formation is：＞95％，that is，the consolidation shrinkage is＜5％.
Slurry consistency：12 cm±2 cm.
Slurry stability：the pour rate（the ratio of floating water volume to total volume after static precipitation）is less than 5％.
3）Synchronous grouting management
（1）Grouting mode.Automatic control or manual control is selected according to the need of grouting.The automatic control mode is to set the grouting pressure in advance，adjust the grouting speed automatically by the control program，and stop the grouting when the grouting pressure reaches the set value.The manual control mode is to adjust the grouting flow rate at any time according to the driving situation，so as to prevent the grouting speed from being too fast and affect the grouting effect.Generally do not grouting from the reserved grouting hole，in order to greatly reduce the possibility of leakage from the segment.
（2）Preparation of grouting equipment.Mortar is prepared by mortar mixing station and transported to shield by special mortar tank of segment carrier.Each line is equipped with two special moving mortar tanks with a single tank volume of 12.5 m3 and a fixed mortar tank for shield machine with a volume of 25 m3.
（3）Main parameters.
①The pressure of grouting.Synchronous grouting requires that the slurry pressure in the formation is greater than the sum of static water pressure and earth pressure at this point，so that it can be filled as far as possible and should not be split.If the grouting pressure is too large，the tunnel will be disturbed by slurry，resulting in later formation settlement and tunnel itself settlement，and easy to cause slurry running；but grouting pressure is too small，slurry filling speed is too slow，filling is not enough，will increase the surface deformation，and synchronous grouting pressure should be basically balanced with soil cabin mud water pressure，to avoid mud or mud forward channeling.The synchronous grouting pressure of this project is set to 0.3-0.6 MPa，and the appropriate adjustment is made according to the monitoring and measurement results.
②The amount of grouting.The synchronous grouting amount is 150-250％of the building gap，that is，33.3-55.5 m3/ring.
③The time and speed of grouting.At the same time，synchronous grouting is carried out，the speed of synchronous grouting is matched with the propulsion speed of shield machine，and the excavation and grouting end at the same time.
④Grouting sequence.Six grouting holes are used to inject at the same time，and a pressure detector is set up at the exit of each grouting hole in order to detect and control the grouting pressure and grouting quantity of each grouting hole，so as to realize the symmetrical and uniform injection behind the segment.
（4）The end standard of grouting and the inspection of grouting effect.The grouting pressure standard is adopted，that is，when the grouting pressure reaches the design pressure，it can be considered that the grouting meets the design requirements，and the injection can be stopped at this position.The grouting effect inspection mainly adopts the analysis method，that is，according to the P-Q-t curve，combined with the excavation speed and lining，the deformation measurement results of the surface and the surrounding buildings are comprehensively analyzed and judged.Ifnecessary，nondestructive testing is used to check the effect，and the field radar detection is shown in Fig.3.5.
Because the strata of shield tunnel are mainly silty sand layer and soil layer，the permeability of fine sand layer is large and the permeability is strong，which is suitable for the need of shield tunneling through this section of strata.The shield synchronous grouting system has the function of injecting double liquid slurry.
Fig.3.5 Detection of synchronous grouting effect by radar detection technology
4）Secondary grouting management
After synchronous grouting，the annular gap behind the segment is filled，and the formation deformation and settlement in most areas are controlled.In the local area，affected by groundwater，there may be some problems in the solidification process of synchronous slurry，such as local inhomogeneity，solidification shrinkage of slurry and dilution loss of slurry.In order to improve the waterproof and compactness of the backlining grouting layer and effectively fill the annular gap after the segment，according to the monitoring results，the secondary reinforcement grouting is carried out if necessary.In the construction，the feedback of surface subsidence monitoring information is adopted，and combined with the subsidence and leakage of pipe pieces in the tunnel，it is comprehensively judged whether it is necessary to carry out secondary reinforcement grouting.The secondary reinforcement grouting material is mainly cement slurry and sodium silicate，and the slurry ratio（1 m2）is cement slurry（water cement ratio 1∶1）：water glass（Baume degree 35，modulus 2.4）=1∶0.5.
KBY 50/70 grouting pump is used for secondary reinforcement grouting.The secondary reinforcement grouting pipeline is self-made，which can realize the function of quick connection and unloading and sealing and non-leakage，and equipped with slurry stop valve.The grouting pressure of secondary reinforcement grouting is 0.6-1 MPa，the gel time of slurry is adjusted to 4-15 min，and the amount of water glass is added if it's necessary to shorten the setting time.The grouting quantity is determined according to the monitored gap and the results of monitoring measurement.Grouting pressure is mainly used to control grouting.