7.3.1 Experimental Study on Slurry Ratio and Film ...
The strata crossing the channel section of Nanjing Yangtze River Tunnel are mainly silty sand strata.When the shield punching section passes through the silty sand layer，due to the single particle size and poor gradation of the silty sand layer，the slightly disturbed stratum is easy to be liquefied.The dense mud film is formed on the excavation surface to maintain the stability of the excavation surface and ensure the safety of the excavation surface.However，the particle size of this kind of fine sand stratum is small，and the mud is not easy to penetrate into the formation to form a permeable zone，which can only form mud skin in the surface siltation.Since the mud is prone to damage under disturbance，in order to ensure the stability of the excavation face in such strata，it is necessary to ensure the formation of mud with high strength and strong anti-disturbance ability on the surface of excavation face.This paper focuses on how to form dense mud skin on the surface of fine sand.
1）Test mud
Slurry used in this experiment：the old slurry+pulping agent in the field conditioning tank，the slurry ratio and the property parameters are shown in Table 7.4，and the particle curve of the old slurry is shown in Fig.7.13.Due to the flocculation phenomenon of No.7 slurry，the first six kinds of slurry were selected for film forming test.
2）Test strata
In this experiment，the fine sand with particle size of 0.075-0.25 mm was selected to simulate the fine sand layer in practical engineering.The dry density and permeability coefficient of the stratum used in the experiment were consistent with those in practical engineering.The dry densityρd=1.73 g/cm3，and the permeability coefficient K=10-3 cm/s.
3）Film forming test results
Table 7.4 Proportion and property parameters of mud used in laboratory test research for mud proportioning in fine sand stratum
Fig.7.13 Particle size distribution curves of old slurry
The test results of slurry film-forming test in each group are as follows：
The test results of No.1 mud are shown in Figs.7.14 and 7.15.
The test results of No.2 mud are shown in Figs.7.16 and 7.17.
The test results of No.3 mud are shown in Figs.7.18 and 7.19.
The test results of No.4 mud are shown in Figs.7.20 and 7.21.
The test results of No.5 mud are shown in Figs.7.22 and 7.23.
The test results of No.6 mud are shown in Figs.7.24 and 7.25.
Fig.7.14 Filtration curve of No.1 mud film forming test
Fig.7.15 Mud film formed in No.1 mud film forming test
4）Analysis of test results
From the above results，it can be seen that the filtration curve changes step by step with the pressure gradient.The greater the viscosity of the mud，the smaller the 24 h precipitation rate，the smaller the amount of filter water formed by the mud，the thinner the mud，and the denser the mud.
According to the test results，it is recommended to use No.3 or No.5 formula should be used when the project crosses the silty sand stratum of shallow overburden section of Jiangzhongchongcao，that is，the slurry is prepared by adding NSHS-1 or NSHS-3 slurry preparation agent to the old slurry（mixed slurry of bentonite and clay）in the slurry mixing tank.The density of the old slurry is controlled above 1.12 g/cm3，the addition amount of NSHS-1 or NSHS-3 slurry preparation agent is about 2％，and the viscosity is controlled within 23-25 s.
Fig.7.16 The variation curve of filter water in No.2 mud film forming test
Fig.7.17 Mud film formed in No.2 mud film forming test
Fig.7.18 Filtration curve of No.3 mud film forming test
Fig.7.19 Mud film formed in No.3 mud film forming test