Research On Laws Of Dynamic Filter Cake Formation And Face Stability During Slurry Shield Tunnelling In Sand Stratum

The slurry shield machine has been the preferred option for river-cross and sea-cross tunnels because of its particular support mode for excavation face.With the underwater tunnel developing toward longer,larger and deeper,the slury balance shield would encounter various challenges from complex stratum inevitably.Especially in sandy,gravel and pebbled stratum which have strong permeability,lower cohesive force and poor stability,the slurry pressure keeps difficultily and excavation face would easy to instability in slurry shield.In this paper,the law of slurry penetration and filter cake formation in sandy stratum was researched,and then the ultimate support pressure on excavation face and its stability problem were analysed considering about the influence of slurry shield cutting tools on slurry pressure support mode on excavation face.The major contents and achievements of the study are as follows:(1)Based on filtration effect of porous media,mathematical model which can describe the penetration and deposition of slurry particles was established through motion equation of slurry frontal surface and concentration equation of slurry.The judgment method of state transition from slurry penetration to filter cake formation was proposed used by soil penetrability,and the criterion of slurry penetrability was deduced by pipe bundle theory.Combining the volume conservation principle of slurry particles and constitutive equations of filter cake,the mathematical model of slurry filter cake formation was established,which unified the overall process contain slurry penetration and filter cake formation.(2)A test system of slurry penetration and filter cake formation was developed,and the method of electrical resistivity dynamic scan was proposed for real-time monitoring of slurry and soil interaction.The tests of slurry penetration under different conditions,such as soil particle size and gradation,were carried on by using this system.The seepage water flux,pore water pressure and soil electrical resistivity were monitored and analysed during test.The test results show that the thick slurry has an ability of forming an inner filter cake in coarse grained stratum with strong permeability which plays an important role for transforming slurry pressure to support pressure on excavation face.(3)Soil filtration test was carried out based on BBD response surface test design method.The influence of slurry concentration,slurry pressure,stratum porosity and soil particles characteristic(soil particles size d₁₅ and nonuniform coefficient C_u),et al.,on soil filtration coefficient were analysed.Calculation model of soil filtration coefficient during slurry penetration,and the fitting degree of this model was certified perfect through single sample K-S test which comparing the test value and predicted value.The model can be forecasted the ability of soil retention for slurry particles.(4)Based on the analysis of shield cutting tools layout principle and cutting characteristics,the slurry penetration and filter cake formation test with cutters cutting was carried out.And the effect of periodic cutting of cuters on the law of slurry penetration and filter cake formation was analysed and summarized.The research results show that it is benefit to forming a larger area filter cake timely under lower rotate speed and slower advance rate of shield,and higher slurry pressure which can improve the conversion rate of slurry pressure on excavation face.(5)A new analysis model of slurry shield face stability which can reflect the effect of shield machine tunneling parameters was established based on the wedge and prism shape limit equilibrium model.This model took into full consideration of the dynamic characteristic of slurry penetration and filter cake formation with cutters cutting,and the seepage force for wedge-shape and prism-shape caused by slurry penetration was calculated by numerical analysis software.Utilizing this model,the calculation method of slurry ultimate support pressure was proposed.The influence of slurry pressure,slurry viscosity and slurry particles characteristic on ultimate support pressure was discussed.Finally,based on the research results of this paper and previous engineering practice,the control strategy of excavation face active instability"lower rotate speed,slower advance rate,appropriate high pressure,when encountering coarse grain,applying viscous slurry"was proposed.