5.3.1 Material Balance Calculation and Screening E...
Taking the fine sand and gravel sand of Nanjing Yangtze River Tunnel as an example，the balance calculation is considered according to the material quality of the shield tunneling ring，and the characteristics of the two typical sand strata are calculated and analyzed.The basic physical properties of the two formations are shown in Table 5.11.In the calculation，favorable and unfavorable conditions are considered respectively，as shown in Table 5.12.
Table 5.11 Basic physical properties of the stratum
Table 5.12 Calculating the formation conditions
1）Calculation of mud circulation system in fine sand formation
The main component of the stratum is fine sand，and the proportion of particles in each stratum is shown in Table 5.13.
The particle size of＜0.075 mm is only about 20％of the whole formation particle，and about 70％of the formation particle size is in the range of 0.075-0.25 mm.The clay particle content of this formation powder is small，and the separation of mud and water is not difficult.At the same time，since the particle size of about 70％of the particles is between 0.075 mm and 0.25 mm，and the pores between the formation particles are small，it is not difficult to form impervious or slightly permeable mud film on the excavation surface.The difficulty is how to smoothly discharge the sand excavated from the excavation surface through the circulation system without forming two flow velocities of sand and mud.
It can be seen from Fig.5.35 that the black cyclone separation system has a good screening effect on the mud in the silty sand stratum.After the secondary screening，only 16％of the mud particles above 75μm are left，which is far less than the proportion of the particles above 75μm in the silty sand stratum.The sand in the stratum can be basically screened out.However，the classification screening effect of the black cyclone is in series and parallel，and the separation effect is gradually reflected.
2）Calculation of mud circulation system in gravel sand formation
The stratum is a non-full-section gravel layer，the upper fine sand stratum，and the lower part is a gravel sand stratum.The proportion of particles in each layer in the formation is shown in Table 5.14.
Table 5.13 List of soil particle content of each group of fine sand stratum
Fig.5.35 Mud particle fraction curve at various locations in the fine sand formation circulation system
Table 5.14 Summary of soil particle content of each group of gravel sand stratum
The local layer has a large amount of large particles and a small amount of small particles.The existing screening equipment can smoothly complete the separation of mud water.How to smoothly discharge the sand excavated from the excavation surface and avoid blocking the pipe is one of the main problems to be solved.