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Overview of tunnel construction using the shield method: The shield method is suitable for tunnel projects in soft soil areas at great depths; it can be used to pass beneath rivers, lakes, the seabed, as well as areas with numerous surface buildings and underground pipelines. Regarding the types of shield tunneling construction methods, they can generally be classified by cross-sectional shape, excavation method, head structure, etc. Based on the structure of the head (whether there is a partition to separate the excavation face from the shield tail lining workspace), they can be divided into closed-type and open-type. In terms of usage, the closed-chest type is more widely used. Open-type shield machines include extrusion-type shields and fully open-type shields, but they are rarely used in urban underground construction projects in recent years, so no further explanation is given here. Based on the principles and methods of face stability, the characteristics of various shield machines can be summarized as follows. (1) Closed-chest shield: A mechanical shield type that maintains the pressure within the pressure chamber, which is filled with sediment or slurry, by using a sealing partition placed between the cut ring and the support ring, thereby creating a pressure chamber between the partition and the working surface, in order to ensure the stability of the working surface. Based on the mechanism of face stability, they can be divided into earth pressure balance shield machines and slurry pressurized shield machines. ①The earth-pressure balance shield transforms the excavated mud and sand into slurry, and controls the pressure of this slurry to maintain stability at the working face. This shield is of a type equipped with machinery for cutting the surrounding rock, mixing equipment to turn the excavated soil into slurry, machinery for discharging the cut soil, as well as control mechanisms to maintain the pressure of the cut soil. It can also be divided into earth pressure shield machines and slurry pressure shield machines, depending on whether they have additive materials that promote slurry formation. The earth pressure balance shield is shown in the figure. a. Earth-pressure shield: It uses a rotating cutter head to cut through the surrounding rock, filling the space between the working face and the diaphragm with mixed soil and sand. This construction method obtains stability at the working face by using the thrust of the shield to compress the cut soil and apply that pressure to the entire working surface, while the soil is removed via a spiral conveyor. b. Soil-pressure shield: By rotating the cutter head while injecting additives, it forcibly mixes and cuts the soil and additives to bring them into a plastic, fluidized state. Similar to the earth-pressure shield, this construction method also maintains the stability of the working face while discharging soil through a screw conveyor. ②Slurry pressurized shield: This type of shield maintains the stability of the excavation face by applying pressure to the slurry, and transports the cut soil and debris out in a fluid form through slurry circulation. This type of shield is equipped with excavation machinery for cutting through the surrounding rock, slurry circulation equipment, slurry feeding and discharging devices that apply pressure to the slurry, and separation equipment for removing the slurry. In addition, some slurry-pressure type shields also have slurry conditioning and treatment equipment to ensure the proper properties of the slurry. (2) Open-type shield: The main difference between an open-type shield and a closed-type shield is the absence of a partition; it is a shield design in which the excavation face is entirely or mostly exposed, based on the premise that the working surface can remain stable on its own. For working surfaces that cannot be made self-stable, auxiliary methods must be employed to enable them to meet the conditions for self-stability. ①A mechanical shield is a type of shield equipped with a cutting disc at its front, allowing for the continuous mechanical excavation of soil and sediment. Some soil retention effect can be achieved through the support of the cutterhead. ②Semi-mechanical shield: Among the open-type shields, the semi-mechanical shield is one that consists of the original prototype of shields – the manually excavated shield – to which an excavator, a loader, or a machine capable of both excavation and loading is added. To prevent the excavation face from collapsing, movable cutting rings and semi-circular jacks can be installed. However, due to the installation of the excavation device, it is actually difficult to set up soil-retaining devices such as jacks at the front; as a result, the excavation surface remains largely open during the excavation process, and the stability conditions of this surface require further study. The next two images: the first is a schematic diagram of the shield machine structure ； The second is the shape of the tunneling machine.