The Cumberland Gap Tunnel
The excavation consists of "top heading" - excavating and supporting the upper two-thirds of the tunnel - followed by "benching" - excavating and supporting ...
www.efl.fhwa.dot.gov/projects/cumgap/tunnel.htm -

[PDF]
chapter_6_of_rock_en
Figure 6.1: The 12 m span 8 m high top heading for the tailrace tunnel was constructed by ... Figure 6.3: A wedge failure in the roof of the top heading of ...
www.rocscience.com/hoek/pdf/Chapter_6_of_Rock_Engineering.p...

Howstuffworks "Tunnel Construction: Soft Ground and Hard Rock"
The second technique, shown in the diagram below, is the top-heading-and-bench method. In this technique, workers dig a smaller tunnel known as a heading. Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench. One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.
science.howstuffworks.com/tunnel3.htm

Top Heading => galeria de avanço superior
Dicionário Técnico - Lewis L. Sell