In the field of tunnel engineering, tunnel boring machines and shield machines are both commonly used large-scale mechanical equipment, and they play an important role in tunnel construction. However, many people are confused about the concepts and differences between the two machines, and even think that they are the same equipment. In fact, although tunnel boring machines and shield machines have certain similarities, there are obvious differences in characteristics, uses and working principles.

   1. Tunnel Boring Machine (TBM)

   ① Features

   Tunnel boring machine is a large-scale mechanized equipment used to dig underground tunnels, which is characterized by high efficiency, safety and high degree of automation. It usually consists of a cutterhead, a support system, a propulsion system, a slag discharge system, a hydraulic system, an electrical system and other parts. Various types of cutters, such as hobs, tooth cutters, etc., are installed on the cutterhead to crush rocks. The support system provides stable support for the machine to ensure stability during the excavation process. The propulsion system pushes the machine forward through a hydraulic cylinder, and the slag discharge system is responsible for discharging the crushed rock slag out of the tunnel.

   ② Application

   Tunnel boring machines are mainly suitable for tunnel construction in hard rock formations, such as railway tunnels, highway tunnels, water conservancy tunnels, mining tunnels, etc. In hard rock formations, tunnel boring machines can quickly and efficiently dig tunnels, improve construction efficiency, and reduce construction costs. It can dig tunnels with diameters ranging from a few meters to more than ten meters, and is suitable for tunnel projects of different sizes.

   ③ Working principle

   The working principle of a tunnel boring machine is to use the cutter on the cutter head to crush and cut the rock, push the machine forward through the propulsion system, and discharge the crushed rock slag from the tunnel through the slag discharge system. During the excavation process, the machine will automatically adjust the pressure and speed of the cutter according to the geological conditions to adapt to different rock hardness and strength.

   2. Shield machine

   ① Features

   The shield machine is a large-scale mechanical equipment used to dig tunnels in soft soil formations, with unique design and functions. It is mainly composed of a shield body, a cutter head, a propulsion system, a segment assembly system, an excavation system, a hydraulic system, an electrical system, and other parts. The shield is the main structure of the shield machine, which plays a supporting and protective role to prevent ground collapse and groundwater from infiltrating the tunnel. The cutterhead is used to cut soil and rock, the propulsion system pushes the shield machine forward through the hydraulic cylinder, and the segment assembly system is responsible for assembling the prefabricated segments into tunnel linings to form a stable tunnel structure.

   ② Application

   The shield machine is mainly suitable for tunnel construction in soft soil layers, such as urban subway tunnels, cross-river tunnels, sewage tunnels, etc. In soft soil layers, due to complex geological conditions, collapse and groundwater leakage are prone to occur. The shield machine can effectively solve these problems and ensure the safety and quality of tunnel construction. It can excavate tunnels with diameters ranging from a few meters to more than ten meters, and is suitable for different types of soft soil layers.

   ③ Working principle

   The working principle of the shield machine is to use the shield as a support. While excavating the tunnel, the prefabricated segments are assembled into tunnel linings through the segment assembly system to form a stable tunnel structure. During the excavation process, the cutterhead cuts soil and rocks, and the cut slag is discharged from the tunnel through the excavation system. At the same time, the propulsion system pushes the shield machine forward, allowing the shield machine to continuously dig deeper into the tunnel.

   3. The difference between the two

   ① Different applicable strata

   This is one of the main differences between tunnel boring machines and shield machines. Tunnel boring machines are mainly suitable for hard rock strata, such as granite, sandstone, limestone, etc. These rocks are relatively hard and require strong crushing capabilities to dig tunnels. Shield machines are mainly suitable for soft soil strata, such as clay, sand, silt, etc. These strata have low strength and are prone to collapse and deformation. The shield body of the shield machine is needed to provide support and protection.

   ② Different structures and functions

   The structure of the tunnel boring machine is relatively simple, mainly focusing on rock crushing and excavation. The cutters on its cutterhead are mainly hobs and tooth cutters, which are used to crush hard rocks. The structure of the shield machine is more complex. In addition to the excavation function, it also has the functions of segment assembly, excavation, and waterproofing. The shield body is equipped with a segment assembly system and an excavation system, which can complete the assembly of the tunnel lining and the discharge of slag during excavation.

   ③ Different working principles

   As mentioned above, the tunnel boring machine mainly relies on the cutter on the cutter head to break the rock and push the machine forward through the propulsion system. During its working process, it does not need to assemble the tunnel lining, but performs secondary lining construction after the excavation is completed. While the shield machine assembles the prefabricated segments into the tunnel lining through the segment assembly system during excavation to form a complete tunnel structure. During its working process, it does not need to perform secondary lining construction, which improves the construction efficiency.

   ④ Different construction costs and efficiencies

   Due to the different structures and functions of tunnel boring machines and shield machines, there are certain differences in their construction costs and efficiencies. Generally speaking, tunnel boring machines have high construction efficiency in hard rock formations and can quickly dig tunnels, but due to their complex structure and high equipment cost, they are suitable for long-distance, large-diameter tunnel projects. Shield machines also have high construction efficiency in soft soil formations and can ensure the safety and quality of tunnel construction, but because they require prefabricated segments, the segment cost is high, and they are suitable for tunnel projects with high construction environment requirements such as urban subways.

   Therefore, although tunnel boring machines and shield machines are both large-scale mechanical equipment used for tunnel construction, they have obvious differences in applicable formations, structures and functions, working principles, construction costs and efficiency. In actual projects, it is necessary to select appropriate equipment based on geological conditions, tunnel scale, construction requirements and other factors to ensure the safety, efficiency and economy of tunnel construction.