环流系统是泥水盾构的重要环节，通过环流系统的筛分机能及时分离泥水和渣土颗粒。筛分机的筛分效果对泥浆性质影响很大，关系到泥浆的造浆成本。某大型泥水盾构穿越粉细砂地层时，每掘进一环，泥浆密度会从1.12 g/cm3降为1.06 g/cm3，现场需要添加大量膨润土来提高泥浆的密度。针对粉细砂地层分析，根据环流系统中土颗粒的质量守恒进行计算，研究了环流系统的筛分效果对循环后的泥浆密度指标的影响，并与现场实际测量进行比较，得知地层中粒径小于75 μm的土颗粒对提高泥浆密度有利，应该充分利用。提出应当控制好筛分机的筛分效率，使地层中的细颗粒保留下来，为造浆服务，提高泥浆的密度。

Slurry circulation system is an important part in slurry shield tunneling. The mucks conveyed by the slurry can be separated from the slurry by the screening machine. The screening efficiency of the screening machine has great influence on the property of the slurry, and therefore has close relation with the slurry preparation cost. When a large slurry shield tunnels through fine silty sand strata, in each boring cycle, the density of the slurry falls from 1.12 g/ cm3 to 1.06 g/cm3. Therefore, at the job site, a large amount of bentonite is added to increase the density of the slurry. In the paper, the fine silty sand strata is analyzed and relevant calculation is made; the influence of the screening efficiency on the slurry density is studied and comparison is made between the study result and the measurement data. The study shows that the fine particles with less than 75 μm size in the strata are helpful to the increasing of the slurry density and therefore should be fully used. Therefore, it proposed that the screening efficiency of the screen machine should be well controlled so that the fine particles in the strata can be kept and used to improve the slurry density. 