Cake Filtration And Face Stability Of Slurry Shield Tunnel

In order to improve the transportation efficiency of the civilians,the cities need more constructions on the net of tunnels.Shield tunneling is widely used to build the tunnels,and more and more large-diameter tunnels under deep river were completed by the shields.The tunnel with large-diameter is best choice for a passageway through the urban river.Due to the high hydraulic pressure,thin cover and high-permeability soil in the river bed,the stability of the tunnel face is prone to fail when the slurry shield is advancing.Based on the engineering requirement,two typical issue on the slurry shield were mainly investigated in this thesis.One is the behaviors of slurry penetrating and filter cake forming;the other is the face stability.The major works and results of this dissertation are as follows:(1)The author conducted a series of slurry infiltration tests under different slurry pressures by a self-developed infiltration column.The behavior of filter cake forming and the influence of slurry penetration on the soil were investigated.The results of tests show that for the slurry with the bentonite content of 4.76%,under the various slurry pressures(ranges from 71 to 119 kPa),infiltrate into the Yangtze River sands(hydraulic conductivity is 2×10-4m/s),the time for forming micro-permeable filter cake(the hydraulic conductivity of 35 mm-thick equivalent filter cake ranges from 7.6×10-5 to 1.3×10-6 m/s)is 5 s.And the time for forming scarcely-permeable filter cake(the hydraulic conductivity of 35 mm-thick equivalent filter cake ranges from 4.4×10-9 to 7.9×10-9 m/s)is 300 s.This indicates that only micro-permeable filter cake can be formed on the tunnel face when the shield is advancing.When the slurry pressure is 71 to 119 kPa the average penetration distance ranges from 8.5 to 21.4 cm.Thus the hydraulic conductivity of the soil keeps constant during the slurry infiltration.(2)By introducing the cake filtration theory,the mechanism of filter cake forming is revealed.A series of fluid loss experiments were carried out under the slurry pressure from 10 to 90 kPa,and the basic parameters(? ?0.2,e0=1.85,? = 2 ?k0=-5.513)of the constitutive model of the experimental filter cake were obtained.This filter cake is formed by the slurry with the bentonite content of 4.76%.Test results show that the greater the slurry pressure,the permeability of the filter cake worse.Based on the analytical solution of the cake filtration theory,the dynamic distribution pattern of filter cake thickness and hydraulic conductivity on tunnel face is revealed.The method of calculating the average thickness of the filter cake based on the pressure drops in the filter cake is proposed.It is found that the average thickness of the filter cake is related to the angle between the adjacent cutter-arms and angular velocity of the cutter wheel.(3)In order to calculate the pore pressure in front of tunnel face,A two-dimensional numerical model with the considering of the micro-permeable filter cake is established by SEEP/W.The numerical result shows that the pore pressure drops quickly in the filter cake(its hydraulic conductivity is 1×10-6 m/s),and decreases slowly in the stratum(its hydraulic conductivity is 2×10-4m/s).At 30 min front of the tunnel face the excess pressure equals to 0 when the excess slurry pressure is 45 kPa.The seepage forces on the filter cake and failure zone are computed according to the pore pressure and failure mechanism.The result shows that the seepage forces on the filter cake and failure zone maintains the stability of tunnel face.The efficiency of stabilizing the tunnel face deteriorates when the slurry pressure increases.Because for a higher slurry pressure,more pore pressure drops beyond the failure zone.The seepage force on the prism and filter cake plays the important roles to maintain the stability of the tunnel face,at vertical and horizontal direction individually.(4)In order to calculate the hydraulic head in front of tunnel face,A three-dimensional numerical model with the considering of the micro-permeable filter cake is established by COMSOL.The result shows that the excavation time and excess slurry pressure have little influence on the normalized hydraulic head.The distribution of the normalized hydraulic head shows that,the seepage force on the filter cake increases when the hydraulic conductivity of the soil increases,the seepage force on the wedge decreases when the hydraulic conductivity of the soil increases,the hydraulic conductivity of the soil has little effect on the seepage force on prism.In the both filter cake and wedge,the thickness of cover has little influence on the normalized hydraulic head.When the cover is larger than twice the diameter,the effect of cover on the normalized hydraulic head cannot be neglected in the prism.(5)A new calculation approach for the limited excess slurry pressure is proposed based on the limit equilibrium analysis.By comparing with the former methods,the solution of this approach is safer.The result of parameter analysis shows that the normalized limited excess slurry pressure increases significantly with the increase of the cover,for the soil whose effective friction angle is smaller than 25° and the ratio of cover to diameter is smaller than 2.5.The normalized limited excess slurry pressure is proportional to the ratio of hydraulic conductivity of filter cake to the hydraulic conductivity of soil.The normalized limited excess slurry pressure is inversely proportional to the effective friction angle and cohesion.At last,the normalized diagrams are provided,which allow a quick assessment of the stability of the tunnel face.