According to the working principle, the shield machine is generally divided into a hand-drilled shield, a squeezed shield, a semi-mechanical shield (local air pressure, global air pressure), a mechanical shield (open chest-cut shield, a pneumatic shield) , muddy water pressure shield, earth pressure balance shield, hybrid shield, shaped shield).
The mud-water shield machine stabilizes the excavation surface by pressurized muddy water or mud (usually bentonite suspension). There is a sealing partition behind the cutter head, and a muddy water chamber is formed between the cut surface and the mud is filled with mud. The excavated soil material and the mud mixture are transported by the mud pump to the outside separation plant, and the mud is reused after separation. The earth pressure balance type shield machine uses the soil material (adding foam to the soil if necessary) as the medium for stabilizing the excavation surface, and the earth chamber is formed between the rear partition plate of the cutter head and the excavation surface, and the cutter head is rotated and excavated. The soil material is increased, and then the soil material is transported out by the screw feeder. The soil pressure in the soil chamber can be adjusted by the rotary cutting excavation speed of the cutter head and the unloading amount (rotation speed) of the spiral output feeder.
According to different classifications of shield machines, shield excavation methods can be divided into: open type, mechanical cutting type, net format and extrusion type. In order to reduce the disturbance of the shield construction to the ground layer, the shield can be driven into the soil layer by means of a jack, and then the soil excavation and transportation are carried out in the incision.
open
Both hand-drilled and semi-mechanical shields are semi-open excavation. This method is suitable for geologic conditions. The excavation surface can be stable during excavation or it can maintain stability with auxiliary measures. Excavation is generally carried out from the top and down from the top. If the soil layer is poor, the jacking surface can be temporarily supported by the jack and the gusset. With open excavation, it is easy to deal with isolated obstacles, rectification and over-excavation. In order to minimize disturbance to the formation, it is necessary to properly control the amount of over-excavation and exposure time.
Mechanical cutting
Refers to the full-section rotary cutter disc excavation method similar to the diameter of the shield. According to the quality of the geological conditions, the large cutter can be divided into two types: the non-sealing plate and the sealing plate. There is no sealing plate between the tool holders for the better soil conditions. Large cutter excavation method is not as good as open excavation for over-excavation in corner construction or rectification. In addition, clearing obstacles is not as open as opening. The shield structure using the large cutter head has a complicated mechanical structure and consumes a large amount of power. At present, the advanced mud-water pressure shield and earth pressure balance shield at home and abroad adopt this excavation method.
Web format
Excavation is carried out in a mesh format, and the excavation face is divided into a plurality of lattices by the grid beam and the grid plate. The support of the excavation face is caused by the cohesion of the soil and the resistance within the thickness of the mesh. When the shield advances, the soil is squeezed out of the grid. Adjust the opening area of ​​the mesh according to the nature of the soil. When excavating in the net format, after all the jacks are retracted, a large shield retreat phenomenon will occur, resulting in surface settlement. Therefore, effective measures must be taken during construction to prevent the shield from retreating.
Squeeze
Full-extrusion and partial-extrusion excavation, because there is no excavation or only partial excavation, there is a large disturbance to the ground layer. When constructing the axis, the ground building should be avoided as much as possible. When constructing during partial extrusion, carefully control the amount of soil to reduce and control surface deformation. In the case of full-extrusion construction, the shield compacts the soil within a certain range around it.
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