Re: blasting smallish tunnels in hard rock

On 24/03/2024 08:29, Richard Smith wrote:
[..]

Thanks for detcord idea.
You couldn't drill deeply at 1/2inch (clearance at 14mm)...
At the 32mm of traditional Cornish drilling, could get 2m to 3m (?) - (I
managed 1.5m in a few minutes).
 Ah - I see - way to cheaply practice - do it in miniature - make little
cubby-holes for storing tools in in the side of passageways, etc.

PLEASE NOTE: I AM NOT AN EXPERT ON ROCK BLASTING
I was thinking more like, for a person width and height tunnel, say 3x9 shots at 8" spacing, with 50cm deep holes with 40cm of 100g/m high loading detcord and 10cm stemming, connected by some low loading detcord.
Or possibly better, 15cm hldc, 10cm stemming, 15cm hldc, 10cm stemming, with a low loading detcord going past both hldc charges. Or even 8-8-8-8-8-10, if you can bear to do that.
Either no delays and only one detonator, or say divide it into four sections and use maybe three delay detonators.
That kind of cautious blasting, if it would work on the rock (it should be about right, but no guarantees), should not make too big a disturbance with aboveground at 20m. It might even be unnoticeable, even though it's a kilo of PETN.
For a hobby program.
As the tunnel isn't going to be very long - how long? one, a few, ten meters? - it shouldn't take too many shots.
Not that cheaply though; but a 50m roll of high loading detcord will get you maybe 5 to 8 shots. The low loading detcord will probably come in a much longer roll, and cost a little less.
Say £100 per shot, or a little more at 60cm of tunnel per shot.
PLEASE NOTE: I AM NOT AN EXPERT ON ROCK BLASTING

My instinct says one problem - with 14mm drill at longer lengths you can
get, could not get water to flow down flutes to where cutting happening.
Turn drillings into a harmless slurry, but also keep the drilling-tip
cool doing a lot of work in this hard granite.
You will be knowing - the drill-bits used in the likes of the Cornish
Holman rock-drill and the American Gardner rock-drill (?) have a hole
down the middle for the continuous supply of water which the rock-drill
supplies.

I didn't know that, specifically, though I do use through-fluid cooled drill bits from time to time. I don't know much about mining.
But I was only thinking of half a meter deep. That will still give you some rock to cart away between shots.
And if you can drill some larger diameter unfilled holes as well, so much the better. If the rock is only a few meters thick, you might drill through on the first try with a larger longer drill.
Peter Fairbrother