Book 24, Question 78
What holds up the roof while a tunnel is being dug?
Digging leaves an open space that could sag or collapse, so builders add support as they go. In hard rock, workers put long steel bolts into the ceiling and spray concrete on the walls. In soft ground, they use a strong shield and rings of concrete or steel. Engineers study the ground before digging to choose the right support.
More fun facts
- Supports for a tunnel or mine roof are either passive, which stop rock from falling, or active, which actually make the rock stronger.
- Timber posts and steel arches are passive supports that miners have used for a very long time.
- Weak rock layers over an opening can sag like an overloaded bookshelf until the lowest layer breaks and falls, then the next one.
- Roof bolts, an active support, are the most common way to hold up the roof in most underground mines.
- A hole is drilled, the bolt goes in, and turning its head spreads a shell that locks into the rock and pulls a plate tight, clamping the layers into a strong beam.
- The bolts are 1/2 to 1 inch thick and must be anchored in solid rock, sometimes 4 feet up and sometimes 6 feet or more.
- In tricky ground, a bag of resin glue goes in with the bolt. The bag breaks as the bolt spins, and the glue fills the hole.
- Tunnels meant to last for years get extra help, such as concrete liners, steel arches, more bolts and sprayed concrete called shotcrete.
Questions kids ask
Why not just hold up every roof with posts?
Posts get in the way of big machines, much like posts set across a basketball court would get in the players' way.