In November 1970, the Oregon Department of Transportation
blew up this dead whale using 20 cases of dynamite. They were hoping that the whale
would explode into chunks small enough for seagulls to eat, but–
…that did not happen. In fact, as you can see, pretty much the opposite happened.
One chunk destroyed a car a quarter mile away;
and this substantial piece of the whale [see video] basically didn’t move at all.
What went wrong? The engineer responsible for planning the blast used the wrong amount of
dynamite. 20 cases of dynamite blew the whale into chunks too big for birds to eat but small
enough to fly all over the place and hit cars and people. If he had wanted to explode the whale
into a few large chunks that could be bulldozed away, he should have used far less dynamite– say,
20 sticks instead of 20 cases. If he had wanted to completely vaporize the whale,
he should spread the 20 cases of dynamite throughout the carcass
instead of concentrating it in one area. And there are other complications to consider,
like the difference between how bones react to explosions versus soft tissue.
The art of blowing stuff up, it turns out, is harder than it looks.
But we’ve gotten a lot better at it over the years, despite the odd mishap
In fact, we’ve gotten so good at it, that we can now use explosives to do this:
This doesn’t even really look like an explosion. It looks like the
building, just… kinda… turns into a liquid and splashes down to Earth.
This is called explosive demolition, and to understand how we do it today,
you have to understand how we used to do it 350 years ago.
Let’s set the scene. The year is 1666; the Great London fire has been raging for two days,
and people are desperate for a quick way to create a firebreak– spaces between houses that the fire
can’t cross. Enter gunpowder; specifically black powder. Black powder is a mixture of
three chemicals: potassium nitrate, charcoal (which is basically carbon), and sulfur, which,
when it’s struck or heated, react to form a couple salts, lots of carbon dioxide and nitrogen gasses,
and lots of energy. The gasses expand, and boom– you have an explosion. People would literally
plop a barrel of black powder on the ground floor, set a fuse, and blow the joint. The house would
“pop” a meter off the ground, come back down, and the force of its landing would splinter the
wooden frame and reduce it to rubble in a matter of seconds, creating the fire break they needed.
Then, more than 100 years later, in what is now Waterford, Ireland, demolitionists brought down
the Holy Trinity Cathedral using 150 pounds of gunpowder. Funny story, the internet is a bit
split on whether it’s 150 lbs of black powder or 150 British pounds sterling worth of black powder.
Either way, 150 pounds of black powder is actually not the best way to bring
down any large building, especially if you want to do it artfully, like this.
The cathedral demolition, on the other hand, probably would have looked something like this
What explains the difference? BRISANCE!
Is my mustache coming off?
BRISANCE is actually a word that comes from the French brise which means to break or
shatter. A high BRISANCE explosive will shatter something into millions of pieces
whereas a low BRISANCE explosive will break it into a few large pieces.
Black powder is a low BRISANCE explosive, which is OK when you’re trying to take down a small house
built of wood in 1666, but not OK when you’re trying to bring down a massive stone cathedral.
So, if you’re trying to do this, you can’t use black powder. You need a highly,
highly BRISANT explosive. Why? Well, let’s look at a concrete column on the ground floor of a
typical concrete building. It’s supporting the weight of all the floors above it,
and you want to bring all those floors down in as straight a line as possible,
so, if you could, you would make this column magically disappear instantly – you can’t,
but the next best thing would be to shatter it into as many pieces as possible.
And in 1866, Alfred Nobel (yes, THAT Nobel) made that shattering possible
with the first major advance in explosive technology since the
Chinese invented black powder in the 9th century: dynamite.
To see why dynamite is so different – and so much more BRISANT than black powder,
let’s look at what it is and how it works.
In Nobel’s dynamite, the explosive compound is a single chemical:
nitroglycerin. Here’s its chemical structure.
With black powder, two or three chemicals need to come together for the reaction to happen,
and that takes time; whereas with nitroglycerin, all the atoms are right
there in the same molecule, so the reaction is much faster. This is a general rule in
chemistry – reactions that take place within the same molecule, especially if it’s a small one,
are generally faster than reactions that happen between two or more molecules.
How much faster? Let’s look at what happens when a stick of dynamite explodes.
First, the blasting cap explodes – this is a relatively small explosion
at one end of the stick of dynamite. That small explosion generates a shockwave
which compresses the surrounding nitroglycerin enough that it heats up, and starts to react,
which generates more heat along with gasses that start to expand. This pushes the shockwave to the
next little section of the stick of dynamite. This keeps happening, so what you end up with
is a convex-shaped shockwave and reaction front that travel from the blasting cap to the other
end of the stick, leaving the reaction products in their wake. Temperatures in the reaction
front can get up to 2700 degrees Celsius, and pressure can get up to 300 times atmospheric
pressure. The shockwave travels through the stick of dynamite at more than 7 kilometers per second.
This is about 10 times faster than black powder,
and it’s also faster than the speed of sound in dynamite. When a shockwave travels through the
material faster than the speed of sound would – that’s detonation. When it travels slower,
that’s deflagration. Detonation is much, much more destructive.
The other major difference between black powder and dynamite is how much energy
they release. Black powder releases about 3000 kJ per kg; dynamite releases twice that. Oh,
and dynamite also releases three times the volume of gas per kg as black powder.
One stick of dynamite produces 70-ish 2L bottles worth of gas at standard temperature and pressure.
So, dynamite releases three times as much gas and twice as much energy in 1/10th the
time as black powder. These differences in the reactions make a huge difference
in how each explosive behaves. Black powder is low BRISANCE; dynamite is high BRISANCE.
So for example, you’d never put dynamite in a gun barrel to try and propel a bullet–
But if you’re trying to completely shatter, destroy, even pulverize a material,
then high BRISANCE is exactly what you’re looking for. And that brings us back to…
Let’s say you wanted to take down this 8-story building
with concrete column supports, which is right next to a couple other buildings. So you
want to make absolutely sure the building kind of implodes, falling inwards and not
outwards. What you absolutely cannot do is just pile a bunch of explosives in the center of the
ground floor, because that would send concrete chunks flying right into nearby buildings.
What you need to do is use as little dynamite as possible, and place it strategically: here,
here, here, and here, in holes drilled directly into the concrete columns.
Then, you detonate everything– but not exactly at the same time. You start with this column,
Then, maybe 25 milliseconds later, you blow these columns. Then, 25 milliseconds later,
these. Then these. And you keep going till you’ve blown them all.
Because you started in the middle,, and because you’re blowing the columns
sequentially, the building collapses inwards.
You’re essentially pulverizing the building’s
essential support structures so that gravity can gently pull it all down.
If everything goes right, the whole thing comes down in a matter of seconds and the
nearby buildings are spared – a little dusty, sure, but spared.
These days, buildings are a lot taller than they used to be, partially because we switched to steel
cores for high-rise buildings. That presents a bit of a problem if you want to take them down: steel
is a lot more difficult to shatter than concrete. So you need an explosive with even more BRISANCE
than dynamite. Enter RDX, which is brisant enough to shatter steel. But you can’t drill a hole in a
steel support and throw a stick of RDX in there, because steel supports are usually only 50-ish
millimeters thick. So you have to use RDX’s explosive energy to cut through the steel.
And the way you do that is with linear shaped charges. A linear shaped charge looks like this:
it’s sort of a V-shaped metal cylinder full of explosive. You put the open part of the V against
the thing you want to cut, and when the explosive detonates, it pushes the metal outwards, forming
what’s essentially an explosively propelled knife that slices right through the steel (which,
by the way, has ideally been scored or cut to weaken it) and helps to bring the building down.