6.3.1 Technical Difficulties in Cutter Replacement...
The so-called pressure into the cabin refers to the pressure of compressed gas to replace the mud pressure or soil pressure in the pressure cabin.According to the principle of compressed air construction，the excavation face is supported by air pressure，and the operator enters the pressure cabin under a certain air pressure environment for operation.Although the construction of large-diameter slurry shield in China has been successful，the research on the cutter replacement technology with pressure in ultra-high water pressure（above 0.68 MPa）and strong permeable strata is still in its infancy，and there are still many difficulties in the operation of pressurized cabin entry.In general，there are the following points.
1）Stability of excavation face
The most basic and core issue in shield opening is the stability of the excavation face.In the process of cutter head repair，the high-pressure gas leakage in the working space must be effectively controlled to avoid the damage of environmental pressure fluctuations to the operators and a large number of gas leakages that cause the formation of seepage channels in a certain area of soil layer lead to soil instability，and the gas leakage per unit time exceeds the air supply per unit time of the equipment，resulting in the failure to maintain the spatial pressure.When the high permeability strata such as sand cannot maintain the stability of the air pressure，it is necessary to increase the air closure performance of the strata to complete the auxiliary construction.At present，the most commonly used method to reduce the permeability of soil is to inject mud into the front of the excavation，but when the bottom of the river with high water pressure and high permeability is opened，how to form a mud film with good air tightness is still an important problem in shield opening technology.
2）Personnel safety under pressure
When it is necessary to repair or replace the cutterhead and tools，especially for longterm operations such as welding operation，this kind of pressurized cabin opening generally lasts for several hours，and the excavation surface needs to be supported by compressed gas to form a local relatively stable working space.When foreign countries use compressed gas to maintain the stability of the excavation surface in the cabin opening，due to improper operation，the workers in the tunnel have decompression symptoms，such as ear membrane rupture and hearing impairment.At the same time，when conducting fire operations under compressed air conditions，a large number of harmful gases will be generated，which will threaten the personal safety of staff.
3）Welding repair operation under pressure
In order to ensure the welding quality，it is necessary to protect the molten pool to avoid contact with nitrogen and oxygen in the air，which is difficult to guarantee in high pressure environment.At the same time，compared with the ignition operation under atmospheric pressure，the influence of compressed air on the welding behavior and the possible combustible gas in the cabin should be considered in the welding repair of shield cutterhead in the compressed air environment.In the high pressure environment will cause welding arc column arc pressure increase，arc shrinkage and a series of problems，affect the cutter welding repair operation.