Re: blasting smallish tunnels in hard rock

Peter Fairbrother <peter@tsto.co.uk> writes:

On 06/01/2024 21:17, Richard Smith wrote:
[...]

I was in a pub in Camborne with a lot of ex (and some current) tin
miners.  Who were explaining thing like "You just stick a stick of
dynamite to it with a lump of clay and it does a job" and "No, we
couldn't use gelignite in South Crofty - too hot - the rock was at
about 80degrees - it would have gone off by itself before we finished
loading it" (that would be an "ooops") - and the sort of things kindly
Cornish ex-miners will explain over a few pints of beer.

>
Talking of small diameter brisant explosives for small-scale mining,
how about PETN detcord?
>
While usually loaded at 10 g/m for transmitting trunkline shocks and
directly initiating the more sensitive charges, it is also available
in up to 100 g/m loading, which is about 1/2" diameter, and possibly
more. It can be used up to 70C plus - one manufacturer says 107C is
okay.
>
Not going to give you a huge face loading, but you could use fairly
close-spaced 1/2" holes?
>
Just an idea, I have no practical experience of this, but sometimes
you see detcord advertised for "pre-splitting", whatever that means.
>
Don't know about cost either, but I think a 50m roll of 100 or 150 g/m
should be a few £100's - and will last a long time. And it can be
detonated directly by standard 10g/m detcord, simple and providing a
saving on detonators, though you may need delays.
>
>
>
You didn't say whether the tunnel is through soft rock or granite?
>
>
Peter Fairbrother

Is all granite.
No killas (Cornish name for metamorphised sedimentary rock).
Up on top of Carn Brea, hence entirely granite.

What the granite looks like can be seen in my webpage on
boulder-splitting
http://www.weldsmith.co.uk/tech/minerals/240314_rocksplit/240314_rocksplit_drill_feathers.html
The split sections...

I have seen vids for using these about 1/4inch cartridges to split rock
in USA.
Apparently won't do much if not confined, so have dispensation to be
purchased without presenting explosives licence.

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.

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.


Rich S