6.3.3 Replacement Technology of Shield Cutter Unde...
The earth pressure of the excavation surface is balanced by the air pressure instead of the earth pressure，and the stability of the air pressure on the excavation surface is used to provide the conditions for the construction personnel to enter the shield soil cabin safely.Air pressure stability can be roughly divided into three kinds：can prevent water gushing from the excavation face to prevent the collapse of the excavation face.Because the pressure on the palm surface can strengthen the stability of the palm surface.Due to the effect of compressed air on the surrounding rock gap to drain water，silt and clay layers are enhanced，the soil gap is reduced，and the formation stability is strengthened.The process of adding and reducing pressure is an important part of the tool changing process in the pressurized cabin.If the operation is improper，it is easy to cause the collapse of the excavation surface and the safety of the operators.Under the condition of compressed air，the personnel with pressure changer breathe compressed air，the general steps are as follows.
First of all，the pressure in the cabin is adjusted by the sequential pressure of the main and auxiliary cabins to prepare the personnel to enter the cabin.Secondly，when the pressure of the auxiliary cabin is balanced with the pressure of the main cabin and the working cabin，the operator enters the repair operation of the cutterhead and other equipment.Finally，after the completion of the operation，the decompression operation can be carried out.Generally，the decompression is only carried out through the main cabin，and the auxiliary cabin is always in the normal pressure state.After the completion of the decompression，the operator returns to the ground to rest under normal pressure.
This section takes Nanjing Yangtze River Tunnel as an example to illustrate the repair process of cutter head.The repair work of cutter head can be divided into two parts.
1）Preparation of cutterhead repair
（1）Template production.Before starting repair work，a template matching the original shovel seat should be made，including all bolt holes on the original shovel seat.The template will serve as a reference for the next repair of the tool holder.The cutter head repair is carried out by welding the blade holder，and the blade holder must be consistent with the wear shape of the outer edge region.In addition，the template will also be used for the lip welding of the edge blade.Templates are made of steel.
（2）Tool seat production.In order to repair the blade，two steel plates with right angles to the lip of the new blade are used as the blade seat（Fig.6.48）.The machining of tool holder must be carried out according to the shape of tool wear.To do this，you must first use templates.The welding seam should be reserved in the complete production process of the cutter seat，and the next step will be used in the repair work in the excavation chamber.
（3）Materials preparation.All materials used on TBM in each repair step should be prepared in advance.The materials include tools，welding equipment，blade holder，blade lip，screws，gaskets and all other necessities.
（4）Manufacture of outer edge shovel.The outer edge blade（blade）is connected with the adjacent blade by welding，as shown in Fig.6.49.The purpose is to make full use of the remaining original screw holes to make the lip of the repaired blade more stable，and to make the positioning of the repaired blade base more accurate.
Fig.6.48 The sketch of new shovel seat
Fig.6.49 Welding blade
Before welding blade，the blade must be fixed to the template with screws.Welding operation should be carried out outside the excavation chamber，so that the weld can achieve the best welding quality.In the welding process，deformation control must be carried out continuously to avoid problems due to limited operating conditions when installing blade lips.
2）Disc repair process
Fig.6.50 Adjustment schematic of cutter seat
（1）Blade positioning.The repair of the blade holder will be carried out step by step，and only one blade in the blade arm will be repaired in one operation.Therefore，before entering the excavation cabin，the cutter head must be fixed at the position above the required repaired cutter arm.After the cutterhead is located，transmission facilities such as platforms and hangers should be installed to ensure operation safety.
（2）Demolition of worn blade.Before repair，the old shovel must be removed.The demolition part shall be marked in accordance with the installation location and must be transported out of the mud tank.
（3）Installation of shovel seat.When locating the new blade holder，the template should be fixed to the remaining screw hole of the original blade holder，and then the blade holder should be fixed to the template（Fig.6.50）.At this time，the tool holder should be finely adjusted locally until the tool holder is completely consistent with the worn steel structure.The precise alignment between the new tool holder and the template is crucial.
After adjustment，weld the tool holder to the cutter head.Then take off the template，free up the welding position，the new tool holder welding.
（4）Installation of new blade.After the welding of the shovel holder is completed，the new shovel must be installed in place and connected with bolts.Finally，the wear resistant block next to the tool arm must be rewelded.