Question

A 20 m thick orebody is dipping at an angle of \(75^\circ\). The orebody and host rock are strong. It is proposed to deploy a 150-200 mm diameter drill for production. The suitable stoping method is

• A. block caving
• B. long hole stoping
• C. shrinkage stoping
• D. post and pillar stoping

Hint:

Match the method to a steep, thick, strong orebody drilled with large diameter blastholes, think about which method uses that kind of production drilling.

Answer 1

The Correct Option is B

This question is really about matching a stope layout to the deposit's dip, thickness and strength, plus the drilling equipment mentioned. Let's check each stoping method against a 20 m thick, $75^\circ$ dipping, strong orebody in strong host rock, worked with a 150 to 200 mm diameter drill.

1. Block caving: this method relies on the ore mass breaking up under its own weight once it is undercut, which only works well when the ore, and often the host rock, is weak to moderately strong and well jointed. A strong orebody in strong host rock resists caving rather than collapsing on its own, so this method does not suit the conditions given.
2. Long hole stoping: also called sublevel open stoping, this is built for exactly this situation, a thick, steeply dipping orebody with strong ore and strong host rock that can stand open without immediate filling. Long parallel blastholes, matching the stated 150 to 200 mm diameter, are drilled from sublevels and fired in large rings or rows, with the broken ore drawn out from below by gravity because the $75^\circ$ dip is steep enough for that.
3. Shrinkage stoping: here the blasted ore is deliberately left in the stope so miners can stand on it to drill the next lift, using smaller diameter hand-held or stoper drills rather than large mechanised longhole rigs. Managing that much broken ore in a 20 m wide stope becomes impractical, and it does not match the large diameter drilling described.
4. Post and pillar stoping: this method depends on leaving regular pillars of ore to hold up the roof, which works for flat or gently dipping deposits where pillars sit under a roughly vertical load. At a dip of $75^\circ$, pillars of this kind cannot be formed or kept stable, so this method is out of place here.

The deposit's thickness, steep dip, strength of both ore and host rock, and the large diameter production drilling all point to the same method.

Let's summarize:

• Strong ore and strong host rock rule out caving methods, since nothing will collapse on its own.
• A steep dip lets broken ore flow by gravity, which large diameter longhole blastholes are designed to exploit.
• A 20 m width and mechanised large diameter drilling both point away from shrinkage stoping's hand-held, ore-supported style of working.

So long hole stoping is the suitable method for this orebody.