7.3.2 Determination of Mud Ratio and Index During ...
Taking Nanjing Yangtze River Tunnel as an example，combined with the actual construction situation，the determination method of mud ratio and corresponding indexes is introduced.
Fig.7.20 Filtration curve of No.4 mud film forming test
Fig.7.21 Mud film formed in No.4 mud film forming test
1）Density
Large mud density is conducive to stabilizing the excavation face，but too large mud density increases the burden of mud circulation equipment and affects the slag discharge efficiency.In order to maintain the stable grouting density of excavation face at 1.23-1.26 g/cm3 and the grouting density at 1.30-1.40 g/cm3.
Fig.7.22 Filtration curve of No.5 mud film forming test
Fig.7.23 Mud film formed in No.5 mud film forming test
2）Viscosity
In order to maintain the stability of the excavation face and form an effective mud film on the palm surface，it is necessary to improve the quality of circulating mud when preparing new mud in the mud treatment field.The mud viscosity of the slurry regulating tank is controlled at 23-25 s，while ensuring that the mud leakage is less than 10 m3/h and the water separation rate is not more than 5％.The mud parameters are determined by the construction experience of similar projects at home and abroad and the tests carried out by relevant scientific research units.In the construction of shallow soil section in the middle of the river，80-100 m3 new mud is added to each propulsion ring（because the mud leakage probability in the middle of the river is greater）to adjust the viscosity and density of the circulating mud.The concrete implementation also needs to be adjusted according to the actual situation to ensure the mud quality.
Fig.7.24 Filtration curve of No.6 mud film forming test
Fig.7.25 Mud film formed in No.6 mud film forming test
Table 7.5 Mud parameters in shallow overburden
3）Sand content
In the gravel layer with large porosity and sand layer with poor gradation，the sand particles in the mud have a blocking effect on the formation pores，so the formation of the mud film is closely related to the particle size and content of the sand in the mud.Through the mud treatment field screening precipitation，retain useful clay particles，remove more than 74μm most of the sand particles and more than 45μm part of the silt particles，the formation of appropriate solid particle gradation，ensure the formation of mud film in the excavation face.Therefore，the control of sand content in mud is also an important index in mud treatment，and the sand content in most sections of Nanjing Yangtze River tunnel project is controlled at 15％-25％.
4）Pump pressure control of slurry pump
In order to avoid the failure of equipment overload operation caused by high pump pressure of inlet and outlet slurry pump in slurry circulation system，it is necessary to control the pump pressure of inlet and outlet slurry pump，and the maximum pump pressure is less than 0.9 MPa.The mud parameters of shallow overburden are shown in Table 7.5.
5）Preparation of slurry in punching section
In order to ensure the stability of the excavation face in the middle-shallow overburden section of the Yangtze River，it is necessary to allocate 3000 m3 of the basic slurry according to the determined slurry parameters before entering the middleshallow overburden construction.The slurry viscosity reaches 25 s，and the density is controlled at 1.23-1.26 g/cm3.According to the tunneling experience of Nanjing Yangtze River Tunnel Project in⑧layer of silty fine sand and⑩layer of gravel sand，the mud loss in construction mainly includes the leakage of working face during tunneling and assembly，the carrying of slag and soil during the separation of mud and water，and the mud loss during the cleaning of sedimentation tank.The slurry loss of each ring is 110-130 m3 when tunneling in the shallow silt fine sand layer in the middle of the river.According to the velocity and distance of the mud pipe，80 m3 of the new mud is added to the tunnel face 20 min before the completion of each ring excavation to maintain the stability of the tunnel face.In order to keep the mud volume of slurry circulation system stable，30-50 m3 new mud was added to the slurry tank.