Page "Tunnel boring machine" Paragraph 17

In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines ( EPB ), Slurry Shield ( SS ) and open-face type.

Both types of closed machines operate like Single Shield TBMs, using thrust cylinders to advance forward by pushing off against concrete segments.

Earth Pressure Balance Machines are used in soft ground with less than 7 bar of pressure.

The cutter head does not use disc cutters only, but instead a combination of tungsten carbide cutting bits, carbide disc cutters, and / or hard rock disc cutters.

The EPB gets its name because it is capable of holding up soft ground by maintaining a balance between earth and pressure.

The TBM operator and automated systems keep the rate of soil removal equal to the rate of machine advance.

Thus, a stable environment is maintained.

In addition, additives such as bentonite, polymers and foam are injected into the ground to further stabilize it.