TBM

A tunnel boring machine (TBM) is a machine used to excavate tunnels through a variety of ground conditions. A TBM consists of a rotating cutter head at the front, a shield that supports the surrounding ground, thrust or propulsion systems, and a means to transport excavated material away from the face, typically a conveyor. As excavation progresses, precast concrete lining segments are installed to form the tunnel wall.

History and development: The concept of shielded tunneling emerged in the 19th century, with early advances by James Greathead enabling safer tunneling beneath crowded urban areas. Modern TBMs evolved through the mid-20th century, with advances such as Earth Pressure Balance and slurry technologies that allow operation in soft ground and under water or high water pressures. These innovations expanded the range of conditions in which TBMs can operate.

Types: Hard rock TBMs are used in solid rock and employ robust cutter heads and disc cutters for long, continuous drives. Soft-ground TBMs include Earth Pressure Balance (EPB) and Slurry TBMs. EPB TBMs balance the face pressure using excavated soil to provide ground support, while Slurry TBMs inject a bentonite-water slurry to stabilize the face and transport spoil.

Components: A TBM typically includes the cutter head, a circular shield, propulsion and thrust systems, a segmentation erector and lining installation system, a spoil removal conveyor, and power and hydraulic systems to operate the cutter head and support equipment.

Operation and applications: TBMs enable rapid, continuous tunneling with controlled ground settlement, making them suitable for urban, underwater, and environmentally sensitive projects. Drawbacks include high capital costs, complex logistics, potential for wear or breakdown of cutters, and limitations related to diameter and overall project length.

Notable projects: TBMs have been central to major infrastructure such as the Channel Tunnel between France and the United Kingdom and the Gotthard Base Tunnel in Switzerland, among many metro and road tunnel programs worldwide.