Tunnel hole-through

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Tunnel hole-through, also called breakthrough, is the time, during the construction of a tunnel built from both ends, when the ends meet,^[1] 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 inches (150 mm)
• the 1944 Blue Ridge Tunnel replacement – 4 feet (1.2 m) - 1944
• the Lötschberg Base Tunnel – 10 cm^[2]
• the Holland Tunnel - 1 cm^[3]