7.2.1 Stability Analysis of Excavation Face
1）Influence factors of excavation face stability of slurry pressurized shield
In the process of slurry pressurized balanced shield tunneling，the stability of excavation face depends on the following three factors：
（1）Mud pressure balances water pressure and soil pressure.
（2）The mud forms an impermeable mud film on the excavation surface，so that the mud pressure can be fully exerted.
（3）The slurry penetrates into the stratum from the excavation face，increasing the cohesion of the stratum in a certain range in front of the excavation.
In order to ensure the stability of the excavation face during the advancement of slurry shield，the appropriate slurry pressure should be set according to the stratum and soil water pressure of the excavation face.When the set mud pressure is too small，the risk of collapse of the excavation surface will increase.If the pressure is too large，the possibility of mud gushing will occur.Slurry pressure can be set according to the active earth pressure or static earth pressure generated by the soil.The horizontal displacement of excavation face and ground settlement in front of excavation should be considered comprehensively when setting slurry pressure.
In order to transmit and maintain mud pressure to the excavation face，the formation of mud film is crucial.Due to the existence of slurry pressure，the slurry continuously penetrates into the soil of the excavation face.Reducing the porosity of the soil of the excavation face makes the excavation face form an approximately impermeable mud film.Due to the formation of the mud film on the excavation surface，the effective mud pressure acting on the excavation surface increases，thus effectively maintaining the stability of the excavation surface.If the mud film is not formed or the quality of the mud film is not good，the mud will penetrate into the stratum under pressure.Not only the mud pressure cannot effectively act on the excavation face，but also the pore water pressure of the soil will rise and the effective stress will decrease，which is unfavorable to the stability of the excavation face.Therefore，effective measures must be taken to accelerate the formation of mud film in sandy soil and sandy gravel stratum with large loose permeability coefficient，so as to prevent collapse under the action of stratum pressure and cause instability of excavation face.
It can be seen that the formation of the mud film plays a vital role in the slurry balance，so it is very important to manage the mud indexes such as the specific gravity，viscosity，yield value and filtration characteristics of the mud.In addition，the stability of the tunnel excavation face is also related to the buried depth of the tunnel，the diameter of the tunnel and the stratum conditions of the tunnel.
2）Stability analysis of slurry shield advancing in the middle of Yangtze River
In the process of slurry pressurized shield tunneling，the cutterhead destroys the original balance of the excavation face，and the excavation face will produce active earth pressure，static earth pressure or passive earth pressure acting on the cutterhead.When the slurry shield is advancing，if the slurry pressure acting on the excavation surface is too small，the excavation surface soil will experience the displacement towards the cutterhead.In order to prevent the slip of the soil，the shear stress in the soil will continue to increase，and then the lateral stress of the soil will gradually reduce to the minimum.At this time，the state of the soil is in the active limit equilibrium state，and the relative soil pressure is called the active soil pressure.If the slurry pressure acting on the excavation surface is too large，the excavation surface soil will experience the displacement of the movement behind the cutterhead.In order to prevent the upward slip of the soil，the shear stress in the soil will increase，and then the lateral stress of the soil will gradually decrease to the minimum.At this time，the state of the soil is in a passive limit equilibrium state，and the relative soil pressure is called passive soil pressure.
Therefore，when the shield is advancing in the saturated soil layer at the river bottom，the slurry pressure of the slurry tank is equal to the active earth pressure，the slurry pressure of the slurry tank is equal to the static earth pressure，and the slurry pressure of the slurry tank is equal to the passive earth pressure.The three critical stress equilibrium states are shown in Figs.7.4～7.6（when c=0，the stratum is clay-free）.
Pg—Shield slurry tank slurry pressure.
Ft—Measured value of shield thrust.
Ff—Calculation value of shield friction.
L—Shield length.
D—Exterior diameter of shield.
Fig.7.4 Force analysis chart when slurry pressure equals static earth pressure（Pg=P0）
Fig.7.5 Stress analysis diagram of active earth pressure equal to slurry pressure（Pg=Pa）
Fig.7.6 Force analysis chart when slurry pressure equals passive earth pressure（Pg=Pp）
μ—Friction coefficient between shield periphery and soil，μ=0.4 in subclay，μ=0.3 in muddy clay.
Pr—Mean radial earth pressure around shield.
Pa—Active earth pressure.
P0—Static earth pressure.
Pp—Passive earth pressure.
Ka—Active earth pressure coefficient.
K0—Static earth pressure coefficient.
Kp—Passive earth pressure coefficient.
Hw—Water depth.
γw—Water bulk density.
h—Cover thickness.
γ'—Gravity of soil floating.
c—Soil cohesion.
φ—Soil internal friction angle.
φ'—Effective internal friction angle.
When the slurry pressure Pg of the shield slurry tank is less than the active earth pressure Pa of the working face，the soil of the excavation face will collapse into the soil tank，which will cause the overburden collapse and the ground subsidence.The river will enter the soil tank along the collapse crack and cause the water inrush accident.When the slurry pressure Pg of the shield slurry tank is between the active earth pressure Pa and the static earth pressure P0（Pa＜Pg＜P0），there will be a certain settlement in the overburden layer ahead of the shield.The superposition of the settlement caused by the subsequent process will not be conducive to the control of the total settlement of the tunnel.At the same time，due to the disturbance of the front soil，it is difficult to control the shield，and it is difficult to coincide with the front earth pressure acting point on a straight line.The working condition suitable for the normal advance of shield is that the front extrusion pressure Pg is between the static earth pressure P0and the passive earth pressure Pp，so that the front overburden layer has a certain uplift to compensate for the settlement caused by the subsequent process and not to crack the overburden layer.When the positive extrusion pressure Pg is greater than the passive earth pressure Pp，the front covering soil will be cracked to produce cracks，and the river water will flow into the shield along the cracks to cause engineering accidents，and even cause the whole tunnel to be scrapped.
The value of slurry pressure in the actual construction process is between the upper and lower limits of the theoretical calculation value，that is，the positive extrusion pressure of shield is between the active earth pressure and the static earth pressure.In order to give full play to the advantages of good rock strata independence，the underpressure propulsion mode is adopted，that is，under the condition of maintaining the overall balance between the level of the excavation chamber and the flow rate of the inlet and outlet slurry，the support pressure in the construction is slightly lower than the theoretical calculation value.The construction control method of rapid excavation，rapid assembly and high-quality slurry supplement after the excavation is adopted，which can well stabilize the working face.The slag discharge and slag discharge of each ring of the slurry field and the change of the slurry tank level are monitored at all times，and the extrusion pressure of the shield front is adjusted according to the actual monitoring data.However，in practical engineering，the control of support pressure may not be accurate due to the mutation of geology ahead，the inconsistency between excavation speed and slurry flow or the mismatch of slurry index.When the pressure of the excavation chamber reaches the limit pressure of the slurry，it may lead to the instability of the excavation face.The common methods to judge the stability of the excavation face are：①Observing the stability of the support pressure，if the support pressure rises suddenly，it shows that the excavation face is most likely to have a large area of soil collapse，otherwise，it shows that the excavation face has a roof fall.②Based on the observation of the support pressure，the stability of the level of the excavation chamber and the change of the inlet and outlet slurry flow are observed.If the level fluctuates greatly，it can be considered that the excavation that the excavation surface has been unstable.
3）Minimum thickness of shallow overburden
According to the above analysis，the earth chamber pressure should be kept between the static earth pressure and the passive earth pressure during the earth pressure balance shield construction，namely：
From the formula（7.1），formula（7.4）and formula（7.5），the formula for calculating the minimum covering thickness h required for safe shield propulsion can be obtained.
