Some of my first experiences as an engineer came digging holes in sand at the beach.
I remember being so upset that as I dug deeper and deeper the sides of the slope would collapse
in on themselves.
When slopes here at the Kalgoorlie Super Pit collapse, we lose a lot more than a few minutes
of progress.
Our story begins in 1893 when prospector Paddy Hannan discovered 100 ounces of alluvial gold
in what became known as the Golden Mile.
The mile is actually a 5km long, 2km wide and 1km deep stretch of a thousand particularly
rich ore seams located about 600 km from Perth.
Soon tents and rough sheds grew into the booming gold-rush town that is Kalgoorlie-Boulder.
It wasn’t long before surface deposits were spent and miners needed to head underground
in order to access those particularly rich streams of gold.
By 1903, 49 operating mines existed in the Golden Mile with tunnels extending more than
3500 kilometres.
Tunnels are a good way of getting you to the deposit and extracting ore, but unfortunately
leave a lot of potential gold in the surrounding rock.
This was fine in the early years, but after nearly a century of tunneling was becoming
too difficult, dangerous, and expensive to continue.
It took mining visionary, patron of the Australia II racing yacht, and reasonably corrupt businessman
Alan Bond to realise how to make the Golden Mile profitable one more.
He started buying up all of the old mining leases, consolidating them together into what
became known as the Kalgoorlie Super Pit.
Peering into the pit, you can start to appreciate what a truly mammoth undertaking creating
it would have been.
The stepped sides fall steeply to a depth of more than 600 metres.
Trucks worth $4 million dollars roll past with their 200 tonne payloads slowly making
the mine even deeper.
In May 2018 two mammoth rockslides occurred over a 12 hour window which temporarily suspended
all work in the mine.
You can see the slips on the eastern wall, spilling over the roadway.
Even today there is an exclusion zone around the collapse, reducing operation and leaving
thousands of kilograms of gold inaccessible.
While there are plans to remediate the problem over the next few years, the long-term solution
is to stop future rock falls occurring in the first place.
One cause of 2018’s collapse was the presence of a pre-existing fault which runs the width
of the mine.
Old mine shafts too need to be accounted for and you can see some of these as dark patches
on the walls.
Another factor we need to account for is the interplay between soil cohesion and slope
steepness.
Soils with really low cohesion are those we find around the rim; sandy red stuff which
falls apart really easily.
That's why the slope around the top has to be quite shallow with an angle of about 45
degrees.
On the other hand, as we go deeper, sand turns into rock and this rock has a much higher
cohesion.
Near the bottom we can get angles of close to 90 degrees.
You might think the best option is to go for really shallow sides, making the mine as safe
as possible.
But, as it turns out, as you get deeper and deeper that's a lot of wasted material, which
comes at extensive cost to the operator.
A much better idea is to make the slopes as steep as possible given the circumstances.
But how steep is too steep?
In the past the solution would have been to take a sneaky look at your neighbour’s mine,
and assume that you can get away with doing something similar.
But today's engineers have a much better solution using computer simulation and the finite element
method.
Here is a model some friends and I made for a unit last year.
It’s a 2D cross section of a slope.
3D would also work as well, but the computers we’re running it on are a little bit slow.
We begin with geologists taking core samples of the area that we’re trying to mine.
We then simulate this in the computer program.
For example, here we have a fault line and here is some soft sandy soil.
Taking these we can then cut in some different slopes.
This one is 30 degrees, but we can also try out with different angles; 45 degrees if the
30 is proven safe, or 20 degrees if it's too steep already.
How it works is we take a really big complex object and break it up into thousands of individual
finite elements.
These are simple shapes like quadrilaterals which we can easily convert into a matrix
form and analyze how they interact.
For example, we can include properties like stress-strain and weight characteristics,
performing some really basic matrix operations to work out how they interact.
The more elements we have the better our results, however the simulations also take much longer
to run.
So, I set up the entire library computer room running different iterations of our code,
each with a slightly different slope angle.
On a lonely Friday evening I spent fourteen hours setting up different simulations, working
on our report, and drinking an unhealthy amount of coffee.
In the morning these were some of our results.
We could analyze them to work out when and where the failure occurred.
For example, on this 43 degree slope we have failure at the base; the bottom bit would
have just gone to mush.
On the other hand, at 44 degrees we have a face failure; the whole front of the slope
would have slid into the bottom.
Finally, we then calculated a Factor of Safety; essentially how safe each of the slopes is
going to be.
A low Factor of Safety of one of less means that the slope is very likely to fail.
If you have a high risk tolerance and don’t really care about your work force you can
get away with quite a steep slope angle, say close to 50 degrees.
On the other hand, if you have a low risk tolerance then you’re going to want a shallower
slope.
It’s more expensive but you’re also less likely to kill your workers.
You might want a slope angle of 30 degrees.
Unfortunately even our most detailed models can’t predict everything, so in addition
to running some really advanced simulations, operators perform constant monitoring of their
slopes.
Lasers let them know of any millimetre-scale shifts which could warn of an impending collapse.
Stepped sides help catch any small rock falls before they turn into a full-on avalanche.
It's also possible to shore up weak zones using concrete retaining walls and rebar supports.
For more than 130 years, Kalgoorlie’s Golden Mile has been turning children digging holes
at the beach into engineers searching for buried treasure.
But how do we turn these mineral ores into the metals which run the modern world?
It’s an amazing story of engineering, but one which will need to wait for another episode.
Until then, this has been James Dingley from the Atomic Frontier.
Keep Looking Up.