This is an older mining action describing the moment a tunnel or drill hole breaks through into another existing excavation. It marks the completion of a connection between two headings, which is often a critical milestone for establishing a continuous ventilation circuit. Surveyors must be extremely precise to ensure the headings meet exactly where planned when the final cut is made.
The night shift managed to thirl through to the main return airway just before knocking off.
Related terms
Thirling
A boundary mark used in traditional lead mining to show the end of an allocated working area.
Holing
The process of cutting a slot or groove at the base of a coal seam or ore face to facilitate blasting or breaking.
Crosscut
A horizontal underground tunnel driven across the main strike of the rock formation.