5.1.1　Technical Problem

In recent years，there have been many large-scale cross-sea cross-river tunnel projects at home and abroad.The common feature of these tunnels is that they are large in diameter and often pass through coarse-grained strata such as flowing sand，gravel and sandy silt rich in groundwater.In the harsh working conditions，especially in a large number of water-bearing gravel layers，non-cohesive，extremely unstable soil layers and shallow overburden tunnel projects，as well as the construction of ultra-large diameter shields and areas with special high requirements for ground deformation，slurry shield construction is mostly used.

Slurry shield is a kind of sealed mechanical shield.In the construction process，the slurry needs to meet the following three requirements.①A reliable mud film is formed on the excavation to stabilize the excavation surface.②Slag soil was successfully transported to avoid sediment.③Slurry separation was completed by screening equipment.The stability of excavation face is maintained by the following three factors：①Mud pressure balances soil pressure and water pressure.②The impervious mud film is formed on the excavation surface，so that the mud pressure can effectively play a role.③The slurry penetrates into a certain stratum from the excavation surface，and the stratum on the excavation surface increases the cohesion.With the continuous excavation of the excavation face，the slurry pressure is always required to be one.The form of surface acts on the excavation surface.In this process，the formation of impervious cement film on the excavation surface is the core factor.

In low permeability clay and silty clay strata，the formation pore is small，and the mud is easy to form mud film on the excavation surface.However，in the high permeability formation such as fine sand and gravel sand，mud easily penetrates the formation and directly seeps out，and cannot form mud film on the excavation surface，which will lead to a large number of mud filtration，mud pressure is difficult to maintain，thus inducing excavation surface instability and other accidents，the consequences will be very serious.At the same time，large size particles in the formation are unfavorable for carrying slag，but the separation of mud and water is relatively easy.Therefore，how to form a good mud film on the surface of high permeability stratum is an urgent problem to be solved in the cross-river tunnel project.