Explosions and Water

Maybe they were trying for a timed series of booms and somebody measured the detcord lengths wrong? Instead of a few little booms it was one big boom?

Or maybe the mine's boommaster was PO'd at standing out in the rain and decided two sticks would get the job done twice as fast as one stick?

I am also afraid of the 2 sticks are better than one "policy" but matting should still prevent rocks from flying around.

I think they are using delayed / series explosions.

Without seeing the location is is hard to tell exactly how the charge pattern was layed out, however at first blush sounds like too much charge was used and not enough or any matting to contain the resulting uplift.

To answer your other question, no dynomite is not affected by water, the installer is.

I imagined water would not be the reason - The explanation was raised by an official.

Hi Hendrik,

Could they have hit a gas pocket with the dynamite explosion?

Just a thought.

Mike

Considering this is a quarry, they should have been aware of potential gas pockets and added extra matting to compensate. Any gas pocket would also have been vented somewhat during the drilling operation. Contrary to what you see on television there really is not much of an ignition flash, however depending on the gas deposit and volume it is not beyond possibility, just not very likely. It is also hard to call not knowing the actual blasting compound they were using.

Hi DWT,

I had found one dolomite-related mining disaster before I posted. The mine in the story was an excavated underground mine, not a quarry one. It was with subsequent Googling that I found the actual story - complete with a satellite photo, whereupon I realized this was a quarry/open pit mine incident. It makes sense to me that the incidence of accidently hitting gas pockets in an open-pit mine would be far lower than in an excavated underground one.

Now, to me, it seems more likely that human error was the cause.

BTW, not knowing much about mining, I'm curious as to how they search for dangers such as gas pockets / voids, etc. Do you know?

Mike

I am not much of a mining expert either, just knowledgeable about explosives and how to move rock with them. I would expect most gas pockets are discovered by drilling. Gas pockets can also be found in similar ways to finding petroleum. I believe modern mines also install gas detectors just in case something is found while drilling is in operation.

Could an aviation fuel leak 3 km away (that caused a 80m deep sinkhole) about 15 years ago supply enough gas?

Hi Hendrik,

I would be surprised if aviation fuel would anything do with limestone except penetrating and may be modifying the friction.

Lowering friction may have caused an existing cave to break down.

(80 m deep is a pretty big chunk of material, aviation fuel is not a solvent for carbonates, so what else may have happened?)

If a lot of the disappeared fuel found a passage (nearly horizontal strata layered with a little bit of inclination) to the site of the explosion then there is a hypothetical possibility that a two-stage explosion happened.

(Passages of this type are existing and used by water to flow in unexpected ways. In Germany there is an underground pathway from the Danube via Neckar to the Rhine, in France from South-Pyrenees to North feeding the Garonne-river.)

The first explosion is breaking up the rock into fragments (may be cubic meters in size) the high energy of the blast vaporising the fuel that sits in faults and veins and where-else and subsequently igniting this vaporised fuel in air mixture.

If you inspect the area you would find solid rock and broken fragments that still contain the fuel and smell accordingly - if this maybe explanation is near reality.

Have a look, very improbable events happen!

RHABE

There are huge dolomite cavities containing super saturated Mg or Ca CO3, below or in the region. About 300 - 400 m below the surface.

I would imagine CO2 would be possible but not Metane or Hydogen.

Could be a leak via a fisher on the other hand is it was ignited the boom would be rather spectacular and the resulting hole rather large.

Hendrik, Dynamite is NOT affected by water but ANFO can be. Ammonium Nitrate is very Hygroscopic and will absorb water readily. If the crevice had filled with water, the incompressibility of the water may have enhanced the detonation rate of the ANFO by helping to confine the explosive. Are you sure of the explosive used? ANFO is far more common that dynamite for mining operations.

In a typical quarrying operation ANFO isn't poured into the drill holes until the blast is ready to go, and typically a plastic plug is inserted into the hole after the ignition charge is inserted. A few holes with water from rain would not cause much change in the blast pattern. I however am not aware of SA regulations regarding blast procedures, but they can hardly vary greatly from those in place in North America. The blast master would be aware of the incompatability of the ANFO and H2O and would take procedures to mitigate problems one would hope.

Afrter thinking about the initial question for a while I think that there may have been a fisher in the blast area they were unawair of, and such a fault in the structure could cause some of the mineral to brake away more rapidly than expected. Just a guess however, you kind of have to see the work face to tell for sure.

Hi Rhabe

You are most probably right about the pockets. (a GA for that) We also have hollow fingers pointing up from below at different angles and shapes.

We had a CCTV camera with lights down a hole once. The images were poor but there could have been potholes in the finger.

Please have a look at #13. What do you think?

Hi Old Coal Man

I agree with you.

How much "error" will cause the rock that was up to now restricted to 100m reach 700m.

The rocks also were flying in one direction only. Maximum 10 degrees spread and almost perpendicular to the face.

Would a compulsory high speed video recording off all blasts not solve many problems?

What is your opinion about post #13?

About post #18 True but some are real cowboys - We had a case here of a blaster that kept a container of (I think) ANFO on a shelve in his garage.

The video would help to see where the "big rock" came from, and I have dome this in the past, but it will not give you all the answers.

I can not imagine a fuel leak from 3km away, 15 years past having anything to do with this. The groundwater and seepage would have diluted the fuel long ago, and would have already gone much deeper after 15 years.

ANFO is very stable and I have seen guys toss cigarettes and burning matches into it only to see them doused by the liquid. It takes a high explosive, aka dynamite to set off the ANFO. Also, it takes a higher explosive balsting cap to detonate the dynamite that detonates the ANFO.

Most gel-based "dynamite" used today is very stable. I have seen it dropped from hieghts, run over, you name it. It takes the fast, high explosive of a cap to ignite the dynamite.

When you are driving around the site, you keep the dynamite up front next to you, and the caps in the back away from your body. If a stray radio signal sets one of them off, you will be missing a hand, a foot, whatever is close.

The water had nothing to do with it other than a little better dust suppresion. It is likely the material didn't require the loading and pattern used. The kind of blasters use the one shoe fits all mehtod instead of specific thought concerning the task at hand. The presence of a drainage ditch in the blast area will shoot rock like a cannon.

You can not say that about all blasters...it's like saying all doctors are abortionists. Iw ould like to know how long this mine has been operating and how many "acceptable" shots have been fired without any problems. I have witnessed over ten thousand shots, both underground and surface. I have only experienced 3 that didn't go as planned....that is a pretty good record. No one is perfect.

{* Disclaimer: I have zero experience in blasting and only halve a cup of coffe so far this morning, these are just thoughts *}

Air compresses, water doesn't.

So could there have been a crack/fissure through the rock that would have normally sent a relatively low power air burst up it when the explosion went off. Now fill that fissure with water, and a higher power water pressure wave is sent, possibly creating a rock mortar for any loose debries in it's way...?

After having worked around the hard rock mines on the Canadian shield for a number of years, it became common knowledge that every now and then a blast goes in an unexpected direction. An explosion is a pressure wave, and waves get reflected and refracted, even in rock. If the miners don't have a full understanding of the strata below their blast zone, they can get surprised when an inclusion (bubble) or layer of harder rock below reflects some of the pressure wave back to the surface. This makes for a big jump of the entire surface. Smaller rocks of a couple cubic feet volume (but still big by human standards) literally jump into the sky as if the law of gravity were suddenly repealed.

One point that I haven't heard explained is whether the blaster was using metallic leads with an electric charge to set off the explosives, or detcord, which does NOT use an electrical charge to detonate the explosives. If detcord was used, the whole lightening, storm, stray current hypothesis goes out the window. I have tried to shy away from electric systems for the past 7 to 10 years because of the inherent danger associated with them.

Just to offer an opinion from my very non-exisitant experience with mining blasts (but I do work in the mining industry and pal around with an experienced blaster). I wonder if it wasn't so much a 'mistake' or 'overloaded' pattern that caused the event you're describing or if it was just a different type of blast in the first place.

As often occurs, the rock fragmentation isn't as complete as it was thought it would be after a blast, and sometimes 'small' areas are not affected at all. This leaves the odd motorhome sized boulder in the way of equipment ready to move the blasted rock. To deal with a situation like this, a blaster is sometimes called into 'rectify' the initial oversight and blast the boulder into more equipment friendly pieces. Depending on the size of the boulder, explosive used and the judgement of the blaster (not always a precise science involved), sometimes too much explosive is used. Unlike a drilled pattern explosion where the depth of the hole , amount/type of explosive and nature of the surrounding rock helps 'contain' the blast to a well guess-timated area, 'open air' blasts can be much more volitile and have been known to be good at tossing rocks far and wide.

A situation like this may have been the cause of the rock in the parking lot, but there could also be more scenarios. If you find out more about the incident, please let us know. Thanks!