3.3.4 Prevention and Control Measures of Slurry Sp...
During the mud water shield tunneling，measures to prevent mud water splitting include：Controlling slurry pressure，increasing the strength of overburden，increasing the thickness of tunnel overlying soil and increasing the viscosity and specific gravity of slurry are the most important measures to control the pressure of slurry.Especially in the process of crossing the river and crossing the sea，improper setting of slurry pressure is easy to occur slurry split，erupt to the bottom of the river to lead river water recharge and other engineering accidents，such as the second tunnel of Heinenoord.In order to prevent slurry split in shield tunneling，it is necessary to study the mechanical properties of slurry split and put forward the control index to prevent split，and then to control slurry pressure in engineering，and to put forward the setting principle of slurry pressure under different excavation conditions，which is of great engineering significance.
Mud pressure is an important tunneling parameter in slurry shield construction，and its setting value is also difficult to choose.At present，most of the shield tunneling sections are set，such as 100 m before shield tunneling，and then the mud support pressure suitable for the current geological and topographic strip is selected by trial and error method.
By summing up the construction experience，this section divides the setting of slurry pressure into two situations：①General shield driving section；②Special shield driving section.The following is a classified discussion of these two situations.
1）Setting of slurry pressure in general shield excavation section
The general shield driving section refers to the situation in which the tunnel passes through the soil layer relatively single，the terrain is flat or the slope is flat，the fluctuation is small，and the tunnel line is smooth.Generally speaking，the shield driving section has the characteristics of simple operation and low risk of engineering accident，and its slurry pressure setting also has a lot of engineering experience，and the setting is relatively simple.Generally speaking，the slurry pressure in the center of shield machine is between its active earth pressure and static earth pressure：
γ'—Floating bulk density；
h—The vertical distance from the surface of the soil to the center of the shield；
φ'—Effective value of friction angle of soil；
γw—bulk density of water；
H—The vertical distance from the horizontal surface to the center of the shield.
c—For non-clay soil，c=0.
For a specific tunnel crossing the river，the slurry pressure in the center of the shield is P0.According to the existing engineering experience，there are：
In general，the setting value of slurry pressure has a certain range of selection，which will be beneficial to the construction unit to adjust according to the specific situation，such as strict control of settlement，appropriate increase of slurry pressure，and so on.In fact，the setting of slurry pressure is also affected by its own performance parameters，such as slurry bulk density and so on，which need to be set in combination with slurry characteristics and formation characteristics.
2）Setting of slurry pressure in the excavation section of special shield
The special excavation section refers to the continuous change of geological environment in the process of shield tunneling，such as passing through buildings，or the shield body is in an abnormal working state，such as shield with pressure to change knives and so on.In the process of shield tunneling，it often crosses the building.The existence of building objects makes the setting of shield tunneling parameters different from that of general excavation section：①Due to the overload of buildings，greater slurry support pressure is needed to maintain the stability of the excavation surface.②Due to the limited area of overload，overload can not effectively increase the formation split resistance.On the one hand，in order to maintain the stability of the excavation surface，the influence of overload should be taken into account when setting the slurry pressure；on the other hand，the setting of the slurry pressure must prevent the formation from being split because of the excessive pressure.
To sum up，the slurry pressure determines the maximum support pressure according to the formation split resistance，and the minimum support pressure according to the active earth pressure considering overload，see formula（3.15），formula（3.16）.
γf—Bulk density of slurry；
D—The diameter of the shield.
Then the setting value of mud water pressure of shield is satisfied：
In addition，from the point of view of engineering application，controlling slurry pressure is not the only way to prevent slurry split.It is also an effective method to prevent the crack of slurry by increasing the strength of overburden，increasing the thickness of tunnel overlying soil and increasing the viscosity and specific gravity of slurry.The reinforcement of the overburden of the tunnel can be constructed in advance，but the cost of the project is high，and priority can be given to the projects with high split risk；increasing the thickness of tunnel overlying soil is affected by tunnel route selection，which may lead to higher cost；to improve the viscosity and specific gravity of slurry is a temporary measure to prevent the split of slurry in the construction process，and its engineering effect is also relatively weak.The control of slurry pressure has the characteristics of real-time，effective and economical，and can be carried out smoothly and effectively in most cases，which has become a better method to prevent and control accident risk in engineering construction.