Tunnel boring machines reduce disturbance in the excavation area, which makes them ideal for use in heavily urbanized areas. They also produce a smooth tunnel wall that reduces the cost of the final lining. The total excavation time of long tunnels is significantly reduced with TBMs compared to conventional excavation methods.
A variety of TBMs have been introduced during the past decades - these include the Slurry and the Earth Pressure Balance (EPB) for soft ground, the Grippers for hard rock and the Single Shield TBM for tunneling through rock and other stable, non-groundwater-bearing grounds. A smaller form of TBM called a microtunnel boring machine is used extensively for underground infrastructure tunneling, since it allows fast excavation without the need to disturb the surface.