Tunnel hole-through

Tunnel hole-through, also called breakthrough, is the time, during the construction of a tunnel built from both ends, when the ends meet, and the accuracy of the survey work becomes evident. Many tunnels report breakthroughs with an error of only a few inches, for example:


 * the 1858 Blue Ridge Tunnel – 6 in
 * the 1944 Blue Ridge Tunnel replacement – 4 ft - 1944
 * the Lötschberg Base Tunnel – 10 cm
 * the Holland Tunnel - 1 cm