| With the rapid development of China ’s urbanization process,the cross-domain construction among different cities has occurred in recent years,which often crosses rivers,lakes,and seas.Considering the impact of bridges on the ecological environment of waters and the increasing shortage of bridge resources,the demand for underwater tunnels is greatly enhanced.Due to the advantages of construction safety,strong geological adaptability and good working environment in the tunnel,the slurry shield method has become the most widely used method for underwater tunnel construction.During the construction of underwater tunnels,the pressurized slurry with good rheological properties penetrates into the tunnel face to form a filter cake.Based on the impermeable filter cake,the infiltration of subsequent slurry can be effectively prevented,and at the same time,the slurry pressure acting on it is converted into effective stress to balance the water and soil pressure in front of the tunnel face,thus maintaining the excavation stability.Therefore,the quality of the filter cake determines whether the slurry pressure can be effectively transmitted to the tunnel surface,which is an extremely important safety index in the slurry shield project.Nowadays,considering an increasing number of underwater tunnels are constructed with the characteristics of long distance,large diameter,and high water pressure,the construction of slurry shield tunnels inevitably crosses the highly permeable stratum.In this case,a large amount of slurry penetrates into the stratum,and the filter cake is difficult to form in time,leading to the fact that the tunnel face cannot be effectively supported and even the collapse of the excavation face,which seriously affects the construction safety and construction quality of the underwater shield tunnel.The mechanism of filter cake formation in saturated sand layer is complex,and its essence is the slurry penetration in sandy layer.Therefore,it is of great significance to investigate the characteristics of slurry infiltration and behaviors of filter cake formation,which is the key to the stability control of the tunnel face during the construction of slurry shield tunnels.In the respect of the above problems,this paper studied the slurry penetration characteristics and filter cake formation law during the slurry shield tunnels being constructed in saturated sand layer.The constitutive relationships and flow patterns of slurries with different concentrations commonly used in slurry engineering was investigated.In addition,combined with the characteristics of high-permeability strata,the numerical model was established and developed to analyze the hydrodynamic properties of slurries.Furthermore,the laws of filter cake formation on the tunnel face under the coupling of multi-parameters such as slurry rheological parameters,sand layer parameters and slurry chamber pressure were also analyzed.The main research work and results of this paper are as follows:(1)Based on the indoor rheological measuring system,the rheological tests of slurries with different concentrations were carried out to investigate the influence of CMC content and bentonite concentration on the rheological parameters(yield stress,plastic viscosity,etc.)and flow patterns of slurries.The viscosity enhancement mechanism of CMC in bentonite slurries was microscopically clarified by SEM.In addition,according to the change characteristics of slurry rheological properties,the mathematical model of the relationship between powder contents and rheological parameters was established,and the theoretical system of slurry rheology was formed,which lays a foundation for the proposal of slurry penetration theory in sand layer and the establishment of numerical model.(2)The Herschel-Bulkley-Papanastasiou(HBP)rheology model was introduced to solve the problem that the existing slurry constitutive relationship was numerically divergent and cannot describe the shear thinning characteristics of slurries.On this basis,the HBP rheology model was integrated with the Navier-Stokes equations under the framework of Finite Volume Method(FVM)to establish a numerical model of slurry dynamic process.The proposed model was developed and implanted in OpenFOAM,an open-source platform,to realize the solution of the numerical model.Furthermore,numerical study on hydrodynamic properties such as pressure field,velocity field and plug zone of bentonite slurries with different concentrations in the fully developed laminar flow state was carried out,and the penetration relationships of slurry in saturated sand layer was investigated.(3)Based on the mass conservation theorem of slurry liquid phase and solid particles,considering the non-Darcy infiltration of slurries in sand layer,an improved theoretical model of slurry infiltration and diffusion was established to clarify the migration law and penetration behavior of slurries in sandy soils.Based on it,the influences of key parameters of slurries and sandy soils on slurry infiltration and the development of filter cake formation were further studied.(4)Aiming at the formation characteristics of underwater tunnels with high permeability,considering the three-dimensional infiltration behaviors of slurries in practical engineering,a three-dimensional slurry infiltration column test system with variable cross-section was developed independently.Based on the variable crosssection test system,the three-dimensional spatial infiltration process of bentonite slurries in slurry tank was simulated,and the evolution laws of key parameters such as excess pore water pressure and filtration loss during slurry infiltration process was studied.And the migration law of slurry in stratum and the formation law of filter cakes were clarified.Furthermore,by exploring the interaction between the slurry particles and the pore structure of the sand layer,the necessary conditions for the rapid formation of impermeable filter cakes in sand layer with high permeability were investigated,and the formation mechanism of filter cakes was revealed. |
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