Splitting Rock with Superficial Heat

[Brav24]

TL;DR Summary

Fresnel Lens on Granite

Hi. I'm an amateur mason - I currently use drills and plugs to split granite boulders. I've used sledge hammers, chisels and levers. I've dropped rocks, used hydraulic presses and tended fires to crumble and split rock. It's hard work. It's long hour work.

I'm wondering if there's not a better way: what would happen if I used a fresnel lens to liquify a single, superficial point on the face of a granite boulder and then shocked this point with water? Could it potentially split the boulder?

 

[Tom.G]

Summary: Fresnel Lens on Granite

what would happen if I used a fresnel lens to liquify a single, superficial point on the face of a granite boulder and then shocked this point with water? Could it potentially split the boulder?

Probably not, but you would have one heck of a steam explosion. See this site for some examples:
https://greenpowerscience.com/BLOGGER111/FresnelLensGranite.html

(above found with:
https://www.google.com/search?&q=melting+temperature+of+granite)

The cracking you are after is caused by differential expansion of a material with temperature. I suspect that a localized effect would not supply enough force to the large mass involved.

The melting point of granite is well within the temperature of a carbon arc as generated with an arc welder. I suppose you could try some small scale trials that way.

Another search turned up some interesting possibilities:
https://www.google.com/search?&q=boulder+splitter

I knew a fellow that built a boulder splitter.

• Drill a hole in boulder
• Fill with water
• Using a large/heavy home-built tripod, suspend a shotgun shell aimed at the hole
• Throw a heavy blast-mat over the whole thing (contains the shrapnel)
• Use a remote release to drive a firing pin to the shotgun shell

I haven't heard from him in a couple of years though.

Be Safe and Live Long,
Tom

p.s. Please let us know what works for you. We like learning new things.

 

[Rive]

I knew a fellow that built a boulder splitter.

Could not decide if (from safety standpoint) a mammoth Freshnel Lens is worse or that one.
Using a ramset would be less concerning.

It's a bit of an advertising, but there is a regular, established, safe (as safe as a small dose of explosive can be) product for 'microblasting' solid materials.
These homebrew solutions are... ugh.

...what would happen if I used a fresnel lens ...

Those things (at that size) are really dangerous. Kind of like dancing around a left-alone cutting torch which you can't see.
Not really recommended.
Really not recommended.

 

[Baluncore]

Welcome to PF.

Any heating of one point, a patch, or an area on the surface of granite, will result in exfoliation. That is why granite outcrops and hills are rounded. Without a line of holes, the intended cut orientation will not be defined on the surface, nor into the rock.

Wedges and shims require a line of short holes to be drilled. Once wedge pressure begins to be applied, the rock must be given time for a crack to form and migrate, while air enters the crack. If you try to work too fast, the surface will spawl near the hole. The cut follows the line of holes because all the wedges are oriented to widen the intended plane of the cut. The wedge generated stresses sum in the plane of the intended cut.

I see two other similar ways of splitting granite. Two things are required, a line of holes and a splitting force equivalent to the wedges.

Consider a gang of water jets containing a cutting compound such as soda. The holes could be directed and drilled to an increasing depth, while water under pressure would fill the forming and opening crack. The viscosity of water is greater than air, so that will reduce the flow into the crack, which must be compensated by pressure. It would be necessary to understand the water pressure distribution within the hole during the drilling process.

Consider a similar gang of laser drills. As the holes become deeper, the annulus of rock around the blind hole will be heated by the exhaust, and so expand. That expansion will act like a wedge to open the crack. Unfortunately, the laser will fuse the wall of the hole, so air must enter the developing crack from the surface of the rock between the drill holes. Will it be possible to contain the expansion while the hole is being drilled, to prevent a spawl of the surface near the hole.
Laser drilling of rock; https://www.osti.gov/servlets/purl/894903

 

[Brav24]

Hello all, many thanks for your insights and knowledge.

I suspected the superficial heating wouldn't achieve the desired result, but I'm glad I asked before I experimented. Thank you very much Tom.G for walking me through it and giving an alternative.

The shotgun shell method was new to me - and after some research, it seems microblast is the same technology, though developed as a method/system. Unfortunately, while effective, it seems to shatter rock rather than split it, which would result in a lot of waste. As I am working with field stone on site, water and lasers are impractical - I did look at the water angle quite a bit, though. So much great technology out there, but stone seems content to outlast us yet.

Wedge and feathers it is until my next bright idea - the control and precision they allow can only be matched by an industrial hydraulic splitter, and those only come in 6 figure budgets and 3-phase. Maybe, if I ever finish my project here, I can at least drop a picture.

Cheers for the enlightenment all!
Brav

 

[berkeman]

I haven't heard from him in a couple of years though.

 

[Baluncore]

The shotgun shell method was new to me - and after some research, it seems microblast is the same technology, though developed as a method/system. Unfortunately, while effective, it seems to shatter rock rather than split it, which would result in a lot of waste.

That will be the case for any pressure wave that might bounce around within the rock. The fractures occur where weaknesses reflect energy and so double the amplitude of a pressure wave. Hammers and explosives are the same in that the pressure waves concentrate at changes in acoustic impedance.

The presence of high pressure clay in cracks can absorb applied vibration energy, so rocks are most efficiently split or fractured above the surface, where sound waves can echo within, and where they can simply fall apart. Daily thermal cycling also increases once a rock is separated from its nest.

To control the line of a fracture accurately requires a very slow and even increase in wedge pressure over the intended line of fracture. You may have noticed that after you insert the wedges and have gently increased the pressure, if you go away for lunch or leave it overnight, you will get a cleaner cut. Do not hurry the process. Geology is all about giving rock sufficient time to change.

Hydraulic splitting is more accurately done with a hand pump, or with grease filled screw expanders, not with 100 HP of brutal hydraulic crush, that is only good for crushing rock fast.

 

[hmmm27]

The shotgun shell method was new to me - and after some research, it seems microblast is the same technology, though developed as a method/system. Unfortunately, while effective, it seems to shatter rock rather than split it, which would result in a lot of waste.

Maybe black powder ? lower detonation speed : more whomp, less crack.