A critical ventilation structure in an underground mine that routes intake air over a return airway, or vice versa, keeping the two streams completely separate. It is typically constructed by blasting down the roof to create height, then building airtight walls and a deck. This prevents fresh air from being contaminated by exhaust air carrying dust and explosive gases.
The ventilation plan requires us to build a concrete overcast at the third crosscut before we can advance the panel.
Related terms
Undercast
A ventilation tunnel built to route one airstream safely beneath another.
Stopping
A solid wall or partition built across a mine passage to control ventilation airflow.
Airway
Any underground gallery, drift, or shaft designated specifically for routing ventilation air through a mine.
Return
An underground passage that carries exhausted or contaminated air away from the working face.