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Types of Tunnel Boring Machines

Tunnel boring machines ,or TBMs, have become an essential tool for many of the world's most ambitious tunneling and engineering projects. Tunnels for all sorts of purposes, including hydroelectric projects, mines, sewers, aqueducts, nuclear waste storage and tunnels for transportation are all made possible by these machines. Several different types of TBMs are in use throughout the world today. Some of these machines are enormous, as much as 40 feet or more in diameter and as much as a mile long, including the conveyor systems for removing excavated material.

Open Hard Rock TBMs
- Open hard rock TBMs are designed for use in hard, stable rocks. These machines tunnel through the rock and use a complex system of hydraulics to pull themselves along through the tunnel as they dig. They do not have built-in systems for reinforcing the tunnels they dig, relying on the structural integrity of the rock to support itself.
Shielded Hard Rock TBMs
- Shielded TBMs are designed for operation in hard rocks that are not stable or have a high degree of fracturing, making cave-ins likely. These single shield TBMs have a system of automated machines which place concrete segments on the tunnel wall as a reinforcing sheath as they dig. These machines cannot propel themselves like open TBMs; they can only push off from the concrete segments they lay as they go along. A special type of shielded TBM called a double shielded TBM is designed to work in either type of condition. These machines are used where engineers expect to encounter both stable and fractured rock during a tunnel project.
Soft Ground TBMs
- Soft ground TBMs are designed for tunneling through softer types of earth like clays, soft rocks like sandstone, and loose or very wet ground. Earth pressure balance TBMs use sophisticated controls to maintain a balance between forward motion and excavated material to keep the tunnel stable. Soil stabilizers are often injected into the surrounding soil or soft rock to help keep it stable. Slurry shield TBMs operate in soils or rocks that are exceedingly wet, or even saturated. Some of these machines are completely airtight, and use a slurry and pump system to remove excavated material. Both types of soft ground TBMs are shielded; they lay a sheath of concrete segments as they go.
Raise and Shaft Boring
- Raise and shaft boring TBMs are used for tunneling vertically; these machines dig ventilation shafts, access shafts, and maintenance shafts as well as traditional mine shafts and drainage tunnels. These machines are designed in much the same way as other TBMs, but are made to tunnel vertically instead of horizontally or nearly so.

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