3.3.1 Split Extension Phenomenon of Slurry
In order to ensure the quality and safety of the shield，the most important problem is to ensure the stability of the excavation surface in the construction of a large crossriver and sea soft soil tunnel.When shield tunneling is carried out in the middle part of the river and sea，because the thickness of the overlying soil is generally small and the water pressure is large，the stability of the cutting surface is difficult to control.If there is carelessness，it is possible to split the formation with slurry to the bottom of the river（sea），resulting in major engineering accidents such as collapse and reverse irrigation of river（sea）water.
The problem of slurry eruption in the process of slurry balance shield tunneling is hydraulic fracturing in terms of physical phenomena.The study of hydraulic fracturing is still a relatively new topic in the field of soil mechanics and foundation，but it has been studied for a long time in the field of rock mechanics.Some research results are used in oil and natural gas exploitation，such as applying water pressure to drill holes to crack rock production in order to improve oil production efficiency.When the earth dam core wall is strengthened by split grouting，because the slurry pressure is too large，there will be leakage phenomenon，which also belongs to this category.In the field of grouting reinforcement，split grouting technology is used to strengthen soil in order to achieve the effect of improvement.The research in this field mainly focuses on the reinforcement range and reinforcement effect of split grouting.
There are few literatures on the split phenomenon of clay soil，and some existing studies have not been able to analyze the factors affecting the fracturing compression force，the direction of split and the extension of split.Many scholars in our country are also engaged in the study of fracturing phenomenon，mainly focusing on rock hydraulic fracturing and so on.The split study of slurry shield began with the Tokyo Bay Submarine Highway Tunnel in Japan.Li Yuan and Fu et al.studied the possible phenomenon of slurry split eruption in slurry shield construction by shield molding machine.In sandy soil layer，there is little possibility of slurry eruption except for the special condition that overburden is too thin，but in soft clay layer，the slurry pressure caused by split eruption is similar to the slurry pressure needed to maintain the stability of cutting surface，and an empirical formula of split eruption is given.However，the pressurized liquid used in the experiment at that time was water and did not take into account the effect of slurry viscosity.In addition，because it is a small model test，it is difficult to evaluate the phenomenon of splitting eruptions in the project without considering the effect of splitting extension.Morilin and Tamura Masahitoshi carried out laboratory experimental study on the split phenomenon of shield tail grouting，and studied the influence of various viscous slurry on the split pressure，and obtained the relationship between the split pressure and the clay and soil strength of slurry.the results showed that the split pressure was related to the viscosity of slurry，and the relationship between the split pressure and the viscosity of slurry and the strength of soil layer was obtained.
σ3—Minimum principal stress；
qu—Unconfined compressive strength；
α—Coefficient.
At the same time，Morilin and Tamura Masahitoshi pointed out that the coefficient α is related to the viscosity of slurry.When the viscosity is 25-28 s（the viscosity range of slurry used），α≈0.8.
The research on splitting extension is very low at home and abroad.
The actual split phenomenon is different from the indoor small triaxial split test，and there will be an extension process after the split occurs.Yuan Dajun has studied and summarized the clay formation mud water split and extension phenomenon，especially through a large number of field fracturing tests（clay formation，depth 15-21 m，multi-point mud water field fracturing test）.The empirical relationship between the split extension flow rate and the split extension pressure of slurry viscosity under the condition of 24-27 s is summarized：
qc—The split continues to extend the flow of slurry；
Pp—The effective slurry pressure of split extension is defined as the difference between the split pressure and the minimum principal stress where the split occurs.
a0—Field test hole split mud water inflow area（13.2 cm2）；
A—Shield machine and other split mud water inflow area.
Formula（3.6）cannot calculate and explain the occurrence and extension of slurry shield to reach slurry eruption，so it is necessary to study its characteristics according to the characteristics of slurry shield itself，so as to serve for the project.