What is a slot raise?

Answer 1

A slot raise is used in mining for shaft sinking and/or raising (drilling tunnels upwards at 45 to 90 degrees off horizontal).

When rock is blasted it expands to about 300% of it's original volume so when drilling a tunnel you need a void to give the blasted rock somewhere to expand into.

Rather than drill a large diameter tunnel upwards in one go (running the risk of the expanded rock compacting in place and "freezing" the tunnel) it is common to blast out a small diameter tunnel first - a "slot raise" - and then come back and drill and blast the larger diameter tunnel using the slot raise as a void to allow the blasted rock to expand.