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Imagine a cake in the shape of a giant rectangle.

It consists entirely of vanilla quick bread except for a single six-foot-thick layer of chocolate fudge in the middle.

Your assignment is to burrow into that layer of chocolate fudge and extract as much of it as you can without triggering a collapse of the cake over your head.

If you were a coal miner in possession of a longwall mining machine,
you would drill several narrow tunnels into either end of the exposed side of the cake,
directly through the chocolate fudge,
all the way to the other side.

To prevent the cake from caving in, you wouldn’t carve out all the chocolate;

you’d leave pillars of it inside the tunnels.

As these pillars are made from chocolate, and your goal is to extract as much chocolate as possible,
you face constant pressure to take more of them than is safe.

Then, facing the direction you came from, you’d use the longwall mining machine to cut your way through the chocolate and back to where you started.

That’s when the job becomes especially treacherous.

You are now moving back and forth across the entire cake,
removing essentially the entire layer of chocolate fudge
— which also happens to be the sturdiest layer of the cake.

The machine starts on one end of the wall and slices all the way across to the other
and feeds the chocolate it removes onto a conveyor belt that carries it out through the original entry tunnels.

The more chocolate fudge you remove, the more vanilla cake you’ve left over your head without anything to support it.

You’re now effectively carving out a roof over your head.

Lacking support, that roof will eventually collapse.

Happily, the mining machine has a makeshift roof on it
— a giant metal plate capable of supporting a lot of cake
— and so you are protected from any cake that falls from immediately overhead.

But if that happens, you’ll probably need to stop mining and flee.

Meanwhile, behind you, the cake is collapsing in on itself.

That’s expected.

If anything unexpected happens, you can scurry into your original entry tunnels and out of the mine

This is the first sort of disaster that Chris set out to prevent.

“Pillar Design For Longwall Mining”
would be the subject of his PhD thesis
and the title of his first paper.

Bad pillar design was killing longwall coal miners.

It’s what killed 27 people in the Wilberg Mine.

It had killed miners since longwall mining had been invented in the 1940s.

It had also cost the coal industry money.

By the time Chris turned his attention to its roofs,
the longwall coal mining industry was out of pocket $200 for every minute its mines were shut down by some roof collapse
— and a single roof fall could shut a mine for days.

“The same roof fall that can kill miners can also cost a lot of money,” Chris said.

And yet even though the coal mine industry had a huge financial incentive to figure out how to solve the problem, it hadn’t solved it.

But the problem was complicated.

It didn’t frame itself as a single problem but thousands of smaller ones.

Each mine was sufficiently different from every other mine that regulators felt compelled to devise rules specific to it,
almost as if each mine were its own little industry.

The deeper the mine, for example, the heavier the weight over its roof,
and the more support it would require.

Rock itself differed from mine to mine in diabolical ways, so there was no reliable way to measure the load the pillars needed to support.

“A mine is unlike any man-made structure,” said Chris.

“It’s not a designed environment.

Most of the material the structure is made from is kind of unknown.

With rock you don’t know what the engineering properties are
— what the loads are.

You have a problem that is really not an engineering problem,
but people were insisting on using an engineering mindset to solve it.”

There was a reason no one could agree on coal pillar formulas:

No one could agree how to measure the rock the pillars needed to support.

Preventing the roof from collapsing inside a coal mine was less like analyzing the stresses inside a Gothic cathedral than building one from scratch.

There was only one way to do it: trial and error.

“The science wasn’t there,” said Chris.

“It didn’t have a clear mathematical solution or a way to get one.”