There are lots of different kinds of sand, some pretty specialized.
In Minnesota, we're losing our beloved Renaissance Festival grounds to sand mining. The sand is uniquely useful for the kinds of drilling and fracking techniques used in the North Dakota oilfields, so it's worth more to strip-mine than to support a multi-decade business that generates millions in income every year. Sigh.
An interesting article to say the least, seems that former areas exploited once by Western powers are they themselves doing the same to countries that are in a position they once were.
With regards to Minnesota, there are a number of active "sand mines" down here in Georgia, mostly exploiting old deposits when the ocean was much further inland. I remember visiting one while going for Pecans. They are used for everything from providing construction sand, braking sand (think trains), to extracting other valuable minerals like zirconium and titanium.
It's the same principle. Sand in MN and WI is from prehistoric shorelines when the inland US and Canada were covered by a shallow sea. Ironically the same sea that produced the shales that the fracking sand deposits are being mined to exploit.
This sand in particular is from a flood-prone river valley (the Minnesota River), so speaking as IANAG (I am not a geologist), I suspect it was brought there from elsewhere upstream and deposited over thousands of years.
Do they not have reclamation requirements once they are done mining? Most strip mining outfits are required to do that after they take what they want from below.
I don't blame the mine owners - they'll make more money exploiting the land than continuing to rent it to the Ren Fest. But I'm not convinced that it's a net benefit for the community.
I'm wondering if it's possible to turn this problem into a solution. I don't know the numbers, so I'm probably off by orders of magnitude, and maybe it's a losing energy proposition, but:
One of the biggest harms of global warming/climate change/whatever you want to call whats happening is the sea level rising. Is it possible we could use enough sand collected from the seabed to offset this sea rise?
This is a fun math problem to use as an interview question, it can expose how people think.
The interesting trick is to assume 75% of the earth is covered in ocean, And sea levels are expected to rise 20 meters (or more). Since you can meet "half way" between the rise in the sea level vs the lowering of the sea floor, you remove 10M of sea floor, and re-distribute it over the 25% of the "land", that means that all of the land goes up by 10M/.25 (the ratio of land to sea) or 40 meters.
So burying the continents under 40M of sand would counteract a 20M rise in sea level, sort of :-)
Moving that much of the crust around would likely to create fairly massive earthquakes and the surface of the earth is effectively floating so you are limited on how much you can change things. Still if you’re willing to move a lot of rock you could more or less get it to work.
But, your solution would raise the land and lower the sea levels at the same time. You want the difference (added land) + (lower ocean) = 20M. (.25 land : .75 ocean = 1:3) Removing 5M of sea floor would add 5 * 3 = 15M of land 5 + 15 = 20.
Of course there is little need to raise all land just the low lowing areas would be enough. Or you could also make new land vs messing with any of the existing land.
Bonus question #1: If sea level rise is an exponential with respect to time, at what rate would we have to extract sea floor material to keep the level at a current coastline constant?
Bonus question #2: What is the minimum sea wall you would need along the coast of all continents in order to hold back sea level rise?
The point being such questions are always useful for eliciting the interviewee's response who questions they might consider to be "unknowable" when they start the interview.
I have no idea why people like to use HS math problems for interviews.
#1 If the ratio is A:B, then you need to remove A/(A+B) * f(x). So, using your numbers it's 1/(1+3) * f(x) = 1/4 f(x). #2 = rise + safety factor. But, safety factor is going to be some BS number. Area wise it's got to deal with rivers not just coast lines, but for a minimum wall you could in theory just use pumps and ridiculous amounts of energy.
Honestly, Google actually tracked this stuff and these types of problems are very poor indicators of workplace performance.
PS: For more fun with exponential functions grub starts a reaction that produces 1 atom the first day 2 atoms the second day... 2 ^n. Now your blast away from the area in a space ship at .9c, approximately how long do you get to live? Hint the earth has around 10^50 atoms it's radios is ~4,000 miles and the speed of light is 1.6 * 10^10 miles per day.
> I have no idea why people like to use HS math problems
> for interviews.
Did you go through Google's hiring class? I did. There is a lot going on in an interview, and much of it has to do with how people respond to stressful situations. That is especially true of people interviewing at a place like Google because they often feel like it is their "dream job" and if they blow it they will end up homeless or something.
The important thing about interview success is getting an understanding of three things, can the person listen, can they reason under stress, and can they get stuff done. The candidates always over think the questions, it is the nature of things. Locking someone up mentally because you've over stressed them a question on tensor physics really doesn't help determine if they would be a good candidate or not. If you start with the HS math and they blow through it, you can work your way up to multi-variable calculus :-)
Maybe, maybe not. But I saw an awesome video from Standford on Youtube about a process to scrub C02 out of power plants and turn it into carbonate based sand, aggregate, and cement. This could offset our C02 production while providing more sand that we'd know what to do with.
Is it possible we could use enough sand collected from the seabed to offset this sea rise?
Even current levels of seabed mining are considered deeply problematic for the oceans' ecosystems (already under heavy attack from overfishing, ship traffic, waste dispersal, and species transportation). And any effort that would make a dent in rising sea levels would be orders of magnitude greater.
Is there much research being done on alternate polymers or binders which can be produced to be used as an alternative on a large scale to sand? I remember reading about mushrooms/fungi being grown for use in a cement-like material.
Its troubling to see these issues across every resource we have and need in the world. Off the coast of florida here we have sandmines dredging in the gulfstream. They look like wider versions of oil rigs.
That's my new pet peeve -- using "page down" on an article only to have some of the content I want to read hide under an idiotic header floating over the content.
In the article, the guy that dived for sand bringing it up in a bucket made $16 dollars a day. He had to dive about 40 feet down. Someone else hauled the sand up that he dove for. This person made less.
So lets say between the two and maybe a boat, $25-35 a day. I looked up how much concrete sand costs... in the US at least I found it for $7 a ton. Are they able to bring up 3 or 4 tons a day per two man team diving down 40 feet? I am skeptical, but maybe there is something I don't understand.
They would only need a 2.7 gallon (USG) bucket to bring up 4 tons per day.
But please check my estimate: A cubic yard of sand can weigh approx 3000 lbs, so that's 2/3 of a cubic yard per ton. 27 cubic feet per cubic yard, so 18 per ton, which is 135 USG. 4 tons (from your question) is then 540 gallons, and at 200 dives per day that is 2.7 gallons per dive.
I'd bet they do more like 8 tons per day per diver.
200 dives a day, lets say they are super and work a 10 hour day (for easy math).
That's 20 dives an hour or one every three minutes.
Diving down 40 feet and bringing up a 3 gallon bucket of sand, once every three minutes for 10 hours straight again seems... very difficult (or maybe a bit improbable).
Perhaps it's a special kind of sand and they get more money for it so they don't have to do quite that much?
Concrete is already recycled - although it is mostly used for subgrade (the aggregate underneath a concrete slab) and is considerably cheaper than crushed rock.
The Sahara is a complex ecosystem, like everywhere else (actually, it's a network of several deserts with different geographies, climates, and ecosystems). It's not a dead place, and can't just be harmlessly stripped.
That said, it's easier to make square sand round than round sand square. It's probably easier to crush rock into concrete-appropriate sand than to turn desert sand into concrete sand.
It helps to know why it is currently not used:
http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech...
(The grains are too smooth and too small)
I wonder what the magnitude of the resulting loss of strength is and if it can be compensated for by making concrete structures built with desert sand much thicker (though this might not be economical)
For the Sahara example, I fear that part of 'strength' here is brittleness. The concrete may be strong enough, as long as you don't drop heavy objects onto it or hit it with a hammer.
Ignoring that, thicker walls in the Sahara probably are a good idea to keep rooms cooler, but can be problematic in high-rise buildings, as they weigh more, and thus require even thicker walls at lower levels. Floors are a problem, even in single-floor buildings.
[1] Actually, it is not just creation. Age plays a role, too. For example, you have to replace sandbox sand once the smaller grains have been rained out of it because it stops 'sticking'.
In Minnesota, we're losing our beloved Renaissance Festival grounds to sand mining. The sand is uniquely useful for the kinds of drilling and fracking techniques used in the North Dakota oilfields, so it's worth more to strip-mine than to support a multi-decade business that generates millions in income every year. Sigh.