Rock blasting question

Last month I was traveling east on I-10, heading towards San Antonio before going south to Corpus Christi. When I was in the Hill Country there were a lot cuts through the hills. My first thought was, “Boy, the engineers sure trust the limestone around here to stay put.” The cuts’ walls were not far off of 90-degrees. As I rolled along, I notices in the walls of the cuts you could still see the scars of the bores the charges had been placed in; about every three feet they were quite distinctive along the rock face with the rock flat in between.

This got me to wondering. You’d think when the charge went off it would crack the rock in an even radius around the bore leaving a series of scallops in the face and no trace of the bore left. Is it possible to set a charge so that the main force is sideways to get a clean cut where you want it, leaving some of the drilled rock intact?

Shaped charges are a real thing, but you usually only see them in military weaponry.

Linear shaped charges (scroll down at above link) are also a real thing, often used in controlled demolition of buildings to cut through structural steel.

I don’t think either one of those finds heavy use in earth-moving applications.

When it comes to blowing up the earth itself, AIUI they’re just putting unshaped explosives in a borehole and setting them off, and the general effect depends on the geometry of the situation. If you set an explosive off on the surface of the rock, not much happens to the rock. If you bury your explosive a few inches in the rock, then the few inches of rock above the explosive gets blasted out, and the rock under the explosive is pretty much intact.

If your explosives are relatively close to an exposed face of the rock (e.g. several feet of rock to one side of the boreholes, and a hundred yards of rock on the other side), then you’ve got something like this; the few-feet-thick slab tends to get shattered and displaced, and the giant massif on the other side of the borehole stays put.

If you’ve got rock more or less evenly distributed around your explosive borehole, then you’ve got something more like this. You won’t blast much of that rock out to the side; you’ll just shatter rock evenly around the borehole (and more to the upper part of the borehole than the bottom).

I believe they use liquid explosives in bore holes like Tx hill country.

I may have heard that on a documentary.

I don’t believe those bore holes contained any explosives.

A couple of years ago I drove past a highway expansion, from 2 to 4 lanes, on a daily basis. It appeared to me that they simply drilled the bore holes and then ripped the face of rock off with an excavator.

I spent a summer in a gravel quarry. Strictly small scale compared to a highway project.

First, a guy operating an air powered hammer drill would spend one or more days making a grid of vertical holes 20 or maybe 30 feet deep. Depending on how crap the drill was acting, he might have to stop and cover the holes in case it rained overnight.

Down the holes would go a stick of stuff that looked like old-timey dynamite but wasn’t. It was like a cylindrical packed salt-lick with a cardboard wrapping and a nice neat hole in one end that accepted a detonator. The detonator wire or tube would be used to lower the assembly to the bottom.

Then we poured pink or orange coloured fish food sized pellets out of 50lb paper bags called ANFO to fill the hole nearly the rest of the way up. We’d heard legends that actual professional companies had trucks that pumped in a slurry of this from a big tank instead of schlepping it around by hand. On top we’d jam in loose gravel and drill dust to keep the energy from going straight up. We wanted the explosion to expand the most it could in every cardinal direction. This supposedly also reduced the distance the noise of the blast would carry and disturb fewer nearby homeowners. This step put the detonator connection in the most jeopardy so we’d do it carefully. It was hard to judge if there was any reduction of sound from my observation point.

All the detonators would be tied together using faster and slower acting detonators strategically to create a wave of explosions that turned the solid rock into particles most efficiently.

If it was electrical, we’d run a loooong wire over to the bunker and trigger after some warning sirens or if it was det-cord based light a burning fuse and then run to the truck idling nearby and get away. The fuse was always longer than it would take to briskly walk to safety because it was also a shitty truck.

Even though it was limestone and fairly soft as rocks go, the semicircles of the last row of drill holes would still frequently be visible in the newly exposed face and in lots of the bigger chunks.

Machine Elf is essentially correct about how rock blasting works, and icbm describes traditional quarry operations. The only thing I would add is that with an explosive in long bores, the majority of the momentum will be expended radially and will naturally fracture and push away at material on the free side. The “cylindrical packed salt-lick with cardboard wrapping” is probably some kind of cyclotol or pentalite-based plastique that is 70% or better NG equivalent used to activate the ANFO-fuel mix. (icbm, did they pour fuel oil or kerosene down the holes after they packed it with ANFO?) ANFO used to be common for blasting because it could be purchased cheaply as fertilizer and didn’t have any of the controls of high explosives, but after the truck bomb attack on Alfred P. Murray Federal Building in Oklahoma City in 1995 fertilizer materials and nitromethane were also more tightly controlled.

Today for blasting they would likely use something like Emulex or Seismopac/Tovex, which is easier to tag and trace and pre-packaged for different applications. Directional shaped charges are not typically used in rock blasting but sometimes shearing charges (which blast in a flat plane) are used to undercut an area to unstable to work on directly, or to deliberately collapse a tunnel or room. However, with the advent of mountaintop removal for coal the carefully controlled blasting operations in longwall mining are not employed and they essentially just blast away until uncovering the seam and then use earth moving equipment to remove coal, then dump a thin layer of fill (supposed to be topsoil but it is usually whatever crap soil and rock is easiest to access) back over the breech).


Confirmed, I’ve seen this on documentaries.

Have seen a demo of this on YouTube, can’t seem to find it now. Demonstrator had a series of 8-inch-thick concrete columns stood up at a test site, and did the following:

-affixed explosive charge to surface of column. Detonation did little damage, a bit of surface scarring.

-Drilled horizontal hole to center of column, placed charge at deepest part of hole (i.e. at center of column). Detonation blasted a fair crater on one side of the column.

-Drilled horizontal hole to center of column, placed charge at deepest part of hole, filled remainder of hole with sand. Detonation blew column in two: the sand had acted as a dead mass to help contain the explosion and make it do its dirty work on the concrete instead of just escaping out of the open bore hole.

I’ve had quite a lot to do with blasting in open pit gold mines and I’m guessing what you saw were presplit holes

To leave a clean face behind a blast a line of holes is drilled along the required face, they will be fairly close spaced and will be loaded with special types of explosive that create a very hard impact but not much disruption, these holes will also be left open to release pressure, this line of holes will detonate a fraction of a second before the main blast and if everything goes right the ground will crack along this line and the force of the main blast won’t go past it


Okay, steepone, that makes the most sense. Knowing next to nothing about rock blasting, I’d pictured something like the video link in Machine Elf’s post: A line of charges spalling off a slab of rock which is then carted away. The part I was fuzzy on was why the entire bore hole was not obliterated by the force exerted into the rock face. Your explanation of a sort of instant toilet roll perforations keeping the main blast from being exerted past the point the engineers want is clear.

Question: Is this done one every blast as they’re widening and deepening the cut, or just the last one to definitely define where the cut wall will be?

It would only be done to define the final face, it’s quite time consuming to set up so it would only be done when needed

A case of confirmation bias? Where are all the boreholes completely obliterated by the detonation?

ANFO = Ammonium Nitrate (and) Fuel Oil.
If it’s ANFO, you don’t add (more?) fuel oil to make it go boom.

Ammonium nitrate never made it to the “tightly controlled” status that you state. However: the ATFE was allowed to make a recommendation that suspicious purchases of ammonium nitrate be voluntarily reported by suppliers.

Market based influences have reduced the popularity and availability of ammonium nitrate as a fertilizer in recent years (replaced by urea-based fertilizers for nitrogen supplements), but ammonium nitrate’s scarcity is not caused by governmental regulation.

It is still a commonly used oxidizer in commercial explosives.

Emphasis mine.
I don’t understand. What you describe here seems contradictory and geometrically impossible, in and of itself.
Would you please rephrase?
ETA: The drilled rock is gone, it will never be “intact” again.

Mostly, Yes! There are many ways to set charges. Directional, general direction. Down pressure etc. If you want to blow up a large rock, lay the charge directly on top or in a slight hole. Lay a medium size stone over the charge. It will destroy the lower giant rock! Just don’t be anywhere close when it goes off!
In my past work days, we had to destroy several hundred lbs. of HE. Some was class A, some B and some prima cord, detonators, Some was directional charges. After the initial destruction of the main explosives, we saved one directional charge, a detonator, and a piece of prima cord for an experiment. A large (12"x18"x 4 foot piece of timber) was selected for the experiment. The shaped charge was placed on the upper part of the standing wooden tie. Taped in place, (the funnel type shaped charged) on the smaller side so it would have the 18 inches to travel with a piece of prima cord (four foot or so) with the electrical detonator attached, we remotely set it off from about a hundred yards away. The charge was set on the longest width and completely went through burning a 3 inch hole splitting the timber in half! This was a nice experiment! I enjoyed it very much. Hope you have nice and SAFE experiments if you ever do try anything like this!

Never did it, but have seen it done.
A medium sized Amish mill in our area splits logs that are too big for the carriage with black powder charges.

No, we did not add oil. This is pre-1995 and I didn’t find it very hard to obtain ANFO. I showed up at the CIL storage facility with a purchase order and away I went. The only time they wouldn’t give me explosives was when they asked where my fire extinguisher was and I told him the boss didn’t think I’d need one. He was wrong, no extinguisher, no ANFO.

Sometimes, and I don’t know why, we would also add plastic encased chubby tubes of something squishy on top of the salt-lick cardboard tubes.

Fun summer job.

Interesting. I wasn’t aware that premixed ammonium nitrate and fuel oil (ANFO) was ever sold over the counter for transport, to anyone.
This certainly has not been applicable for decades.

Pedantically, yes. But in practice, the people doing the blasting call just the prilled NH[sub]4[/sub]NO[sub]3[/sub] “ANFO” even before the diesel is added, at least in my (underground gold mine geologist) experience.

I believe the OP is looking at something like this - and is surprised that the sharp outlines of the blasting boreholes are still visible as semicircular channels in the rock face - the surprise arising from a lay person’s expectation that the inside surface of the boreholes themselves should have been obliterated or pulverised beyond recognition by the blast.

Premix is still sold by Nobel in Aus.

As described above, ANFO is so safe that it’s difficult to set off with just a detonator, or by pumping it down holes. You use a booster or put dynamite/gelignite down the hole to make an explosion to set off the ANFO

The 2kiloton 1947 Texas City explosion (500 dead)* was ammonium nitrate waxed with heavy fuel oil for use as fertilizer. It was (1) a bad idea, and (2) safe enough so that they still do it – but now using handling protocols for explosive.

*Hiroshima was around 15 kt ???
PS: yes, blasting doesn’t smash the rock. Unless you want it to do that. It’s like pulling an orange apart: you could squash the orange halves if you wanted to (joice), but if you want sections, you just pull the two halves apart. Using ANFO :slight_smile: