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.

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.

http://www.manta.com/c/mthg8jf/bryan-rock-products-inc

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.

He said, providing no hard data whatsoever.

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?

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 :-)

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.

#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.

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 :-)

https://www.youtube.com/watch?v=J4Unt7MRsV4&index=56&list=PL...

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.

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.

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.

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.

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?

http://www.edinburghgeolsoc.org/r_sites_cra.html

https://en.wikipedia.org/wiki/Concrete_recycling

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.

That's never stopped us before!

This study addresses the issue but is behind a paywall: http://www.scientific.net/AMM.174-177.604

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'.