5.1.2 Research Status
5.1.2.1 Research Status of Slurry Shield Mud Preparation and Mud Film Formation
Many scholars in China and abroad have done a lot of research work on the influence of slurry properties on film formation.In Japan，where slurry shield technology is relatively developed，Takeshi Watanabe et al.found through the self-developed test device that with the increase of slurry density，the slurry filtration gradually decreases.In high permeability formation，it is difficult to form dense mud film only by increasing the density and viscosity of mud.The sand content of mud plays a vital role in reducing mud filtration and forming dense mud film.Combined with specific projects in early Japan，a suitable mud formula was proposed.Similar to this study，the Swiss scholar Fritz et al.used the limit supporting pressure that the mud film could withstand to evaluate the quality of the mud film.By adding silt，sand，vermiculite，polymer and other additives to the mud to adjust the properties of the mud，a reasonable mud ratio was provided for the Zimmerberg tunnel（using a slurry pressure shield with a diameter of 12.35 m）to cross the coarse gravel sand stratum with the permeability coefficient of（1-4）x 10-3 m/s，which ensured the safe tunneling of the shield in the gravel sand stratum with a diameter of 780 m.German scholar Heinz studied the influence of additives on the rheological properties and penetration of bentonite mud.Fillers such as silt，sand and other particles can effectively reduce the penetration distance of mud in the formation.
Chinese scholars have also carried out research on mud preparation and film formation in specific projects in combination with the construction of Nanjing Yangtze River Tunnel，the Yellow River Tunnel of the Middle Route of South-to-North Water Diversion Project and some underwater tunnels in Shanghai.In 2001，Cheng Zhanlin et al.of the Yangtze River Academy of Sciences，took the tunnel of the Middle Route of South-to-North Water Diversion Project crossing the Yellow River as the background，carried out experimental research on the stability model test system of the shield excavation face，put forward the calculation formula of the critical slurry pressure of the slurry balance shield in the medium and coarse sand stratum，and obtained the correlation between the critical slurry pressure and the strength of the foundation stratum，the pore water pressure and the initial stress state at the excavation site.In 2007，Wei Liangwen and Zhang Qinghe of Tongji University took the Shanghai Fuxing East Road cross-river tunnel project as the background，through the indoor slurry characteristics test and microscopic analysis，as well as the engineering field test and monitoring data analysis，found that the PMS slurry system can quickly form high quality mud film，which is very beneficial to the stability of large slurry shield excavation face.
It can be seen that there has been a certain understanding of the formation law of mud film on the excavation surface of slurry pressurized shield，but these studies are mostly based on solving the problems of mud preparation and film formation in concrete slurry shield engineering.The influence of mud density，viscosity and particle composition on the formation of mud film is discussed，which promotes the development of slurry shield engineering.At present，there is still a lack of discussion on the mechanism of mud film formation in high permeability sand-gravel composite strata.
5.1.2.2 The Research of Water Shield Excavation Surface
There are many research results on the excavation face of slurry shield，but due to the importance of this issue，the work is still in-depth.At present，the research methods on the stability of slurry shield excavation face mainly include the simplified limit support pressure determination model which assumes the sliding shape of the soil ahead of the excavation，the complex centrifugal model test and the numerical analysis model which is convenient for a large number of research work and low cost.In 1994，Anagnostou et al.put forward a wedge“penetration model”considering slurry penetration，and proposed that in sandy soil，the support force of excavation face required for shield tunneling can be calculated by“film model”，and the slurry penetration will greatly affect the stability of excavation face under the static state of shield stopping excavation.In addition，due to the complexity of the actual geological conditions，when the shield is advancing，if special geological conditions are encountered，such as lens or local sand layer，if the mud parameters are not adjusted in time and no high-quality mud film is formed，the mud penetration is inevitable.At this time，the“penetration model”should be used to analyze the stability of this excavation face.At the same time，the study also found that in the sand with good particle size distribution，increasing the slurry pressure can effectively improve the safety factor of excavation face stability，while in the larger soil layer such as coarse sand and gravel layer，only increasing the slurry pressure has little effect on improving the safety factor of excavation face stability.In this case，increasing bentonite content to improve the dynamic shear strength of slurry can effectively ensure the stability of excavation face.However，increasing the content of bentonite will greatly increase the construction cost，so it is unreasonable to increase the viscosity of slurry to stabilize the cost of shield excavation face in gravel stratum.In 1995，according to the twodimensional test and the collapse situation during construction，the two-dimensional Murayama formula for the stability calculation of excavation face was proposed by Murayama scholars in Japan.In 2007，Li Yun and Zhang Zixin further applied the mud“permeability model”，combined with the Murayama formula，considering the geometric relationship between the seepage area of the mud and the sliding surface，the effective mud pressure was reduced，and the safety factor of the excavation face stability of the mud permeability model was obtained.
Akira Mori et al.studied the increase of excess pore water pressure caused by slurry infiltration and the influence of excess pore water pressure on the stability of sand excavation face by using slurry filtration test device and shield simulation construction device.In 2002，Broere et al.from the University of Delft in the Netherlands established the groundwater flow model based on the wedge-cabin model，and analyzed the influence of the excess pore water pressure in the stratum on the stability of the excavation face.The study showed that the formation of excess pore water pressure in the stratum would reduce the effective mud pressure，the effective stress on the soil and the friction of the soil.It was proposed that the excess pore water pressure in front of the excavation was proportional to the penetration distance and the shear strength of the bentonite mud，and inversely proportional to the characteristic particle size d10 of the stratum.In 2009，Li et al.used the upper bound solution analysis method and FLAC3D three-dimensional numerical simulation method to study the influence of the gradient of slurry pressure on the excavation face and the soil characteristics changing with depth on the stability of the excavation face for the stability of large-diameter slurry shield excavation face in soft soil layer.The study shows that the gradient of mud pressure will significantly improve the local stability of the excavation face，and the formation characteristics changing with depth will affect the overall failure mode.At the same time，the upper bound solution that can estimate the possible failure mode of the excavation face under given geological conditions is obtained.In 2010，American scholars Seung Hankim and Fulvio Tonon assumed that an ideal mud film was formed on the excavation surface.The three-dimensional finite element method was used to analyze the influence of tunnel diameter，the ratio of overburden thickness to tunnel diameter，soil lateral pressure coefficient and soil strength parameters on the stability of tunnel excavation surface.It was proposed that the minimum mud pressure should be determined by the relationship between mud pressure and expected displacement，and the influence of tunnel diameter on supporting pressure was small in non-viscous soil.
It can be seen that the research on the stability of excavation face has made great progress in the theoretical limit support pressure and numerical simulation，and it is believed that the penetration of slurry into the excavation face has an important influence on the stability of the excavation face.At present，there is a lack of research on the stability of excavation face in sandy gravel composite stratum，and the mechanism of mud film in the stability of excavation face is not revealed from the perspective of force transformation.
5.1.2.3 Research Status of Excavation Face Stability Control for Slurry Shield with Pressure Opening
In the domestic and foreign construction of shield with pressure cabin maintenance，there are many examples of successful cabin opening under pressure conditions.Typical examples of cabin opening with pressure in foreign countries are：the fourth tunnel of Yibei River in Germany in the bottom of the sand drift and other strata，in 0.4-0.45 MPa pressure cabin opening repair.The slurry shield of Weser tunnel in Germany is maintained in iced gravel stratum under pressure above 0.45 MPa.When the red line subway of St.Petersburg in Russia is constructed by slurry shield in silty sand，the repairable open cabin under pressure of 0.55 MPa is carried out.The Westerschelde Tunnel in the Netherlands conducts active cabin opening maintenance in clay stratum.The cabin opening is carried out after the mud pressure is replaced by the pneumatic method，and the air pressure is 0.45 MPa.The most representative example of pressure cabin opening in China is the pressure cabin opening ofNanjing Yangtze River Tunnel Project in gravel stratum at about 60 m below the river bottom.In this project，the slurry film with good air-tightness is formed on the excavation surface through the two-step slurry scheme of first penetrating and then forming the film，and then the slurry level within 3.0 m of the shield upper arch is reduced，and the compressed air is replaced by 0.6 MPa.However，in these cases，more attention is paid to the process of cabin opening and the health problems of cabin entry personnel in the process of decompression，and there are few analysis and experimental studies on the stability of excavation face.
At present，although the cabin opening with pressure has attracted enough attention in the engineering field，the research on its system is rare，especially the research on the air tightness of mud film.The research on water and gas movement in porous media such as clay and clay can be used for reference.Richards found that the air pressure was applied on the side of the saturated porous ceramic plate.When the air pressure reached a certain value，the ceramic plate began to drain，and the air entered but could not penetrate the ceramic plate.The concept of intake value and foaming point was proposed.Ye et al.found that the gas in saturated clay exists initial pressure value，only when the pressure reaches a certain value，will appear obvious permeability.According to the research of Jieying Revolution，the bulk density of soil will change during the dehydration process，and there is no inflection point in the soil water characteristic curve due to the abrupt change of slope caused by the change of intake value.These studies have important reference significance for studying the air-closure law and mechanism of the mud film.
It can be seen that although there have been many successful cases of pressurized cabin opening，there are few studies on the failure mode and action mechanism of mud film under the condition of high permeability gravel composite stratum.