Draining a concrete shell swimming pool

[miamicuse]

Not sure of the best forum for this, a bit of structure a bit of drainage and a bit of soil...

On 4/12/2023, a historic flash flood happened in south Florida, where I live, 26 inches of rain within 12 hours, and most of it within 6 hours. Water so deep cars floated away, I live next to a river and the river water surface rose up to join the rain in the streets and became one. I had 22" of water inside my garage, and on the outside, the fresh rain water mixed with the brackish water in the river, debris everywhere. My pool was flooded over, became a swamp, and there is an inch of black silt at the bottom.

My questions are related to the cleanup process, which involves scooping up all the debris, applying chemical treatment etc...

However, because of the extent of the contamination, I would like to do as extensive a water change as possible. I know this pose a risk since now the soil is saturated with water, and the pool shell is concrete, so draining it fully may result in the pool cracking or popping out due to the hydrostatic pressure acting on the shell.

So the question is, how much water can I drain out of the pool before I need to worry about possible damages? Can I drain it down to zero? I assume not. Can I drain say 75%? 50%?

Some parameters for the pool, total volume ~18000 gallons, depth varies from 43" to 75".

We have a very high water table, even without this rainfall, when I dig around for holes to set fence posts, after 28" below grade I start to see water. I know the water table is not supposed to be this high, but we are next to a river. With the rain I suspect the soil is much more saturated than usual.

 

[HTURKAK]

Do you have as built data ? You should check for the risk of buoyancy and and pool cracking. If you can not do , ask the service of an experienced structural engineer..

In case of the risk , my suggestion would be , use ballast material during draining and cleaning of the pool to avoid flotation.
The ballast material could be concrete blocks, aggregate bags ...








I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[miamicuse]

I do not have as built data, the original plans filed by the owners then to the city for permit processing in 1992 was a one sheet plan related to the location, size of the pool/spa and the piping schematic of the pump/filter equipment. I have no idea how thick the concrete shell is, or how it was built. I have seen pools being built around here and typically it's sprayed gunite.

If I use some sort of a ballast, which the dirty water in the pool is acting as one right now. I still need to calculate how much ballast is needed to keep the pool stabilized, that calculation I believe is the same as how much water I can safely drain off the pool, thus the remaining water is acting as the ballast without risk of damage. If whatever calculations suggest I can drain it down to 20% by volume, that means I need the equivalence of 3600 gallons of water by weight, or 30,000 pounds in weight, that means 600 fifty pound sand bags. Not sure this is practical in any case because these bags will just be in the way of cleaning and hosing off the settled sediment, although much of those will be vacuumed off by then.

 

[phamENG]

Ballast won't be that much of a help. The water isn't just acting as ballast like stone would - it's applying an equal but opposite pressure on the shell.

Best bet is probably to get an auger and drill a hole next to the pool (away from any buried pipes, etc.) to check current ground water level. You can probably pump down to that level safely. Beyond, that, I would not remove any more water than the weight of the pool shell. So you'd have to estimate the weight of the shell, then figure out how much water that would be (surface area x depth x 62.4 or 64 or whatever it is depending on the chemistry of the water). I would probably only take half of that amount of water out to leave some wiggle room in case I overestimated the weight of the shell.

If that's not enough, then drive a well and pump the ground water down to a manageable level while the pool is drained and cleaned.

 

[LittleInch]

Could you get some big bags and just fill them with water to displace the shitty water volume?

That would do the weight and then prop the side walls as you drain down?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SWComposites]

probably better to find some sort of pump / filtration system to clean the water rather than risking emptying the pool.

 

[SwinnyGG]

Am I missing something here?

That pool wasn't filled with water when it was constructed. It should survive with no water in it just fine. IF you're that close to a river, the water table is likely very close to where it's been for the last 20 years or whatever.

If you're that concerned you could rent some post shoring and shore the walls up both directions. These are available in pretty long lengths, spanning the full width of the pool should not be difficult.

Assuming you can't just pump water back into the river with a 2" trash pump, which could pump the whole pool out in a couple of hours, the quick route: hire a vac truck to suck it dry, it'll be done quickly, at the cost of a grand or two. And it'll be powerwashed and ready for you to disinfect and refill.

 

[3DDave]

I'd start with a flocculant and call a pool service to perform the vacuuming process to clear the debris from the bottom unless you want a lot of work to do. See

https://www.swimuniversity.com/pool-flocculant/

for example.

It may be worthwhile to set up a small above ground temporary pool to act as a settling tank for the muck that is vacuumed out to allow recovered water to be recycled back to the main pool or just direct a fresh-water supply onto a floating board to minimize mixing and let the fresh water float on top of the brackish water while the brackish water is pulled by the vacuum and discarded back to the river.

Also, check for gators. They can plug the vacuum.

Also:

https://www.mrpoolman.com.au/blogs/news/how-to-clean-swimming-pools-after-a-storm-or-flood

 

[phamENG]

That pool wasn't filled with water when it was constructed. It should survive with no water in it just fine.

Not in all cases. I live in an area with high ground water, but lots of in ground pools. For vinyl liners, they'll pump down the ground water to put it in. So if you don't have the pump going when you drain the pool, POP! Fiberglass and concrete pools are similar, but not as common around here.

 

[1503-44]

If you are worried about it, install some well points and dewater the soil around the pool, just like they probably did when it was constructed.

Alternatively, you might try a vacuum pump disposal service, filling with clean water from the opposite end with clean water at the same rate they are sucking it out. Eventually it will clean up to where the rest can be filtered.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[DAVIDSTECKER]

Maybe just clean up the water you have.
Drilling rigs use what's called a Mud Pit that has a series of baffle's that allow solids to settle out in the different chambers.
Use a pump to vacuum the pool like you normally would and let the water recycle back into the pool after it goes through the pit.
Hopefully you can shovel out the solids and dispose of them properly.
Once the pit has done what it can, then come up with a sand filter for the finer stuff until your regular filter can handle what's left.

 

[epoxybot]

If all there is at the Building Dept. is a basic plan, it was probably filed by the pool contractor. Whose name is on the submittal or filing and does it appear in the state contractor's license board?