The basic structure of a tunnel boring machine (TBM) includes: An excavation section (cutterhead, its main shaft, and drive unit); an excavation reaction support section (support shoes); and an advance section (advance jacks).
During operation, the following actions are repeated: expand the support shoes to fix the machine body to the tunnel wall; rotate the cutterhead and activate the jacks to advance; after one stroke, retract the support shoes, move them to the front, and return to the starting position.
Cutterhead Structure: Spherical cutterheads and planar cutterheads: Cutting cutters are arranged at intervals at the front of the cutterhead. Generally, there are center cutting cutters, front cutting cutters (excavation face cutting cutters), and side cutting cutters. The spacing of the cutting cutters depends on the load capacity of the cutting cutters, the rock strength, and the target daily tunneling progress. On the outer perimeter, to prevent the cutter bodies from flying off the cutter heads, they are arranged at a certain angle, and the cutting cross-section of the cutter head is arc-shaped. This is the meaning of "spherical". In this case, the side cutting cutters are used in the same way as the front cutting cutters.
To avoid installing special edge cutters, flat cutters can be used. Recent trends emphasize cutter interchangeability, leading to the increased use of spherical cutters.
Peripherally Supported and Central Spindle Type Cutterheads: Peripherally supported cutterheads consist of a cylindrical body and a main frame. They utilize large-diameter bearings and feature a large-opening peripheral support structure at the rear. Excavated rock is lifted from the gaps on the outer circumference of the cutterhead, using the main frame as a hopper, and transported to the excavation device. This cutterhead design is similar to that used in tunnel boring machines (TBMs) for soft rock. It is highly effective in collapsible geological conditions. The cutter protrusion can be relatively small, and cutters can be replaced internally. Central spindle type cutterheads are circular plate structures with a central spindle supported by small-diameter bearings. The cutterhead is mounted on a circular plate. Muck is collected by scrapers located on the outer periphery of the cutterhead and then fed to the waste disposal device from above via hoppers on the outer periphery.
The cutterhead is mounted on a saddle at the front of the plate. Because it is not restricted by a main shaft, its configuration is relatively flexible. This structure is commonly used in open-type TBMs. However, the muck discharge opening is limited, so depending on geological conditions, it may not be effective for waste disposal. The cutterhead is driven by a drive motor located at the rear of the TBM, using the cutterhead's main shaft as the drive shaft.
The support shoe configuration serves as a reaction force during propulsion; this reaction force is the propulsion force and the cutterhead torque. To fully withstand this reaction force and avoid damaging the tunnel walls, it should be made as large as possible to reduce grounding pressure. Typically, the grounding pressure is 3–5 MPa. Support shoes for this purpose are called main support shoes; there are also various support shoes for purposes such as vibration control and direction control.
Shield-type TBM support shoes: In shield-type TBMs, there are main support shoes (tail) and face support shoes (front) used for propulsion reaction. The main support shoes are generally arranged in pairs horizontally on the left and right, but in large-diameter cases, sometimes 4 to 5 are arranged around the perimeter.
Open-type TBM support shoes: There are two types: single support shoe and double support shoe. The single support shoe type has a pair of support shoes on the left and right sides of the main beam. This can accommodate changes in the orientation of the main beam during propulsion.
The double support shoe type has a pair of support shoes at the front and a pair at the rear. The front support shoes can have 4 (X-shaped), 2, or 3 (T-shaped).
Directional correction is performed before the support shoes are installed in either type, but in the single support shoe type, the direction can be changed during excavation.
The double support shoe type cannot change direction during excavation, but it is less affected by geological changes and has good straight-line performance.