Blast Pushing Large Slab Laterally

[phillyfrank]

I am looking for any case histories of a quarry-type blast pushing un-shot rock around it by gliding on a bedding plane. I'm not talking small stuff. The incident I'm working on is a block over 100 ft on a side over ten feet thick moving several feet.

 

[exploengineer]

Hi Phillyfrank,

I've only seen that happen on that scale once in a coal mine with large diamter holes (12", 300mm).

Block movement can always happen, but it takes a lot of energy to make it happen on a large scale.

What type of rock and geology?

If it has recently happened somewhere, I'd love to see some pictures and the blast plan.

If you are worried about it happening, you can prevent excessive overbreak and block movement by angle drilling the blast holes on a 15 degree angle or better. Over 20 degrees, they get hard to load, but you still can. I know some coal mines that drill 25 degrees and still load them.

Also, by increasing the time between rows, finishing the blast pattern on an angle, and loading the perimeter holes with smaller diameter product, you can cut down on movement.

If there is one weak zone, you can always put in a deck to cut down on energy migration into the weak seam. Always shoot the top deck first or shoot them on the same delay.

Hope my rambling helps.



Frank Lucca

http://www.terradinamica.com

 

[rkdoc]

I posed your question to a friend with much blasting experience; this is his reply:

"I've actually seen it happen in the Iron Range a few hundred feet back from the blast. Usually happens at the back of a blast and is due to poor relief on the blast. Of course if it is stability related then that's another matter."

From my own observations in the Iron Range, the bedding dips somewhat but not a lot, with the rock varying from very strong and tough to moderately strong and brittle. There are pre-existing fracture sets that could cause stability problems that might not be apparent well behind a blast.

Don't know if your situation is in similar geology or stress regime.

 

[phillyfrank]

Situation was three sides free, very flat dip on bedding, VERY crowded shot with little relief. You should see the videos! Put rock into orbit. Thanks, all.

 

[rkdoc]

Some more on the mechanics of the Iron Range occurrences:

It happens where the bedding dip is low and the friction angle on the bedding planes also is low. If there isn't enough relief, as for example when the last shot wasn't fully mucked out, the blast can wedge rock layers behind the shot up a bit and back a lot. The back of the moved wedge block can form a ridge 6 ft high.