A tunnel boring machine (TBM) also known as a "mole", is a machine used to excavate tunnel

Tunnel

A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...

s with a circular cross section through a variety of soil and rock strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

. They can bore

Boring (earth)

Boring is drilling a hole, tunnel, or well in the earth.-Earth boring:Boring is used for a wide variety of applications in geology, agriculture, hydrology, civil engineering, and oil and natural gas industries...

 through anything from hard rock to sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

. Tunnel diameters can range from a metre (done with micro-TBMs) to almost 16 metres to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling

Directional boring

Directional boring, commonly called horizontal directional drilling or HDD, is a steerable trenchless method of installing underground pipes, conduits and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig, with minimal impact on the surrounding area....

 rather than TBMs.

Tunnel boring machines are used as an alternative to drilling and blasting

Drilling and blasting

Before the advent of tunnel boring machines, drilling and blasting was the only economical way of excavating long tunnels through hard rock, where digging is not possible. Even today, the method is still used in the construction of tunnels, such as in the construction of the Lötschberg Base Tunnel...

 (D&B) methods in rock and conventional "hand mining" in soil.