# Thread: Empty pools & hydrostatic pressure?

1. ## Empty pools & hydrostatic pressure?

Because of significant scale issues I have to drain and acid wash my new (5 months old) pool.
In short; After all of the cash & effort that has gone into completing it, I am terrified that when emptied it will pop out of the ground and be rendered useless. (~\$25k to repair a popped pool)

Everyone says - this is a risk with in ground pools. I am trying to better understand and quantify that risk.

I have searched for a few days for some decent info on Hydrostatic pressure and the chances of a pool popping out of the ground.
I've not found anything as detailed as I would like so I have drawn some pictures and am going to post my assumptions here for comments.

Again, I stress, what is written below is my assumption, if it happens to be factually accurate that is a chance combination of a basic physics education and luck!

When an in ground pool is full of water it exerts a force downwards due to gravity (weight) that is larger than that of the hydrostatic pressure, if there is any, that is exerted upwards on it by the water table (Figure 1).

When that pool is emptied, the force exerted downwards by the pool is less. I envisage three potential scenarios depending on the height of the water table:

Figure 2: The water table is lower than the bottom of the pool. No hydrostatic pressure exists so the pool does not move!

Figure 3: The water table is so high that the force exerted by the hydrostatic pressure exceeds that exerted by the weight of the pool and the pool pops!

Figure 4: There exists a point of equilibrium where the upwards force exerted by the hydrostatic pressure equals that of the weight of the pool. As long as the water table is lower than this point then the pool will not move

Making the bold assumption that the above theory is correct I tried to calculate where this point might be for my pool. I went to school in Europe so my calculations are metric (apologies)
The pool is an irregular shape and the equations for volume and surface area get pretty complex pretty quickly so I have used the following assumptions in this calc:
The pool is now rectangular with straight sides, an average depth and a flat bottom. (30' x 19' x 4' = 9.1m x 5.8m x 1.2m)
Air has no density/mass
Shotcrete/gunite made with normal weight aggregates will have a density of approximately 2323 kg/m3 (145 lb/ft3)
Shotcrete/gunite average thickness 20cm (8")

Originally Posted by Archimedes
If an object is less dense than water (if it floats on water), it displaces a weight of water equal to the weight of the object
So for the pool to "pop" (or float) it needs to be displaced by a volume of water that weighs as much as the empty pool.

To calculate the weight of the empty pool. We find the surface area of the gunite: (2 x L x D) + (2 x W x D) + (L x W) = That gives about 90 square meters
We then acknowledge the fact that the pool's not square and we want to be cautious and so knock it down to 80m2
We multiply the surface area by the thickness of the gunite (80m2 x 0.2m) and get a volume of gunite equal to 16m3
We multiply this volume by the density of gunite (16m3 x 2323 kg/m3) and get a mass of just over 37,000kg

What we really want to know is how high does the water have to be in the water table around the pool to pop it.
Mass of the empty pool = Mass of water = Density of water x volume of water

Splitting that down:
Mass of the empty pool = Density of water x length of pool x width of pool x height of water table above bottom of pool

Rearranging :
Mass of the empty pool / (Density of water x length of pool x width of pool ) = height of water table above bottom of pool

Putting the numbers in:
37000 / ( 1000 x 9.1 x 5.8 ) =~ 0.7m (2' 4")

So if the water table around my pool is higher than 2' 4" from the bottom of my pool it may pop!

Am I wildly off the mark here and how do I find out where the water table is?
Does it really matter? Am I making a mountain out of a mole hill?

2. My take on the subject is when they built the pool it was empty and the same ground conditions existed... Prior to pebbletec and water fill, it was a completed shell and it wasn't popping out of the ground then right? If they weren't fighting the water table during build I don't know why you would have a problem 5 months later. Prior to gunite, was the dirt at the bottom of your pool staying dry or was it wet and muddy?

3. Or you can poke a hole in the bottom of the pool so it will sink. This is what opening the hydrostatic relief valve in the main drain assembly does.

4. Matj,
You are going to fit in great here with the rest of us pool nerds. Is that you in the bushes?

5. For 3 yrs we had a small crack in our fiberglass inground pool, right by the main drain....it was costing a fortune adding water, 2-3 xs a day.

When we decided to drain the pool and repair the crack, we read all about the problems of the pool "popping" out of the ground. We called several companies about repairing and they said they would not work on the pool as they could not guarantee the pool would stay in the ground.

I was especially worried as I knew the water table was very high and we could see dirt and mud "coming up into the pool" through the crack.

We decided NOT to drain and repair but to "live with it." Long story short, we went away for the weekend and forgot about our pool. We were horrifed upon our return to find that from Sat. A.M. until Sunday P.M. our 16,000 gal pool was totally empty.

We could not believe it had drained that quickly! We went to Lowes or Home Depot the next morning, bought a fiberglass repair kit......patched the crack, and 1 year later it's still holding water perfectly. It doesn't look too good, but hopefully next year we can afford to have it resurfaced and repainted.

Of course, through all this, I prayed constantly! Don't know if this is offending anyone....not meant to.....but w/water table high and 16,000 gal draining through the crack (right by the main pool drain)...if our pool didn't pop out....well,...I don't know......good luck whatever you decide.

6. Mat,

If this pool is only 5 months old, is your Pool Builder taking any responsibility on this one? I see you are in the DFW, TX area. In our situations in this area (TX region), most inground/ gunite pools are fabricated with vents (holes) in the bottom of the pool during construction to prevent "Pop-outs". These holes are filled during plastering process and then the polls are immediately filled. I would consult with your pool builder and get his opinion. I am sure he is familiar with the water tables in the area and should be able to shed some light on the subject. Good luck and keep us posted.

7. In some places the water table is always high. In other places it is always low. And in yet other places it is high at some times of year and low at others. To find out where the water table is around your pool you can dig a hole and see if it fills up with water. Digging a deep enough hole can be a pain, so it is difficult to know if you will have an issue without talking to someone familiar with your area.

There isn't any one answer for all pools. Even though the risk is usually low, it can be teribly expensive to repair if your pool floats. Some pools have special drains under the pool that can be used with a pump to get the water level down around the pool. It is also possible to do most treatments with the water in the pool. There is no exact equivalent to an acid wash when the water is left in, but something similar can be done by bypassing the heater, if any, and lowering the PH of the water.

8. Thanks for all the replies folks:
After reading what's written here, speaking with a few local pool companies and weighing up my options, I'm going to do it this weekend. Fingers crossed - I'll update you on Sunday!

@ EskimoPie - It rained through out the entire construction process. Everything was wet and muddy. Prior to gunite there was a small amount, less than 6" of water in the hole. It has been a lot dryer of late. I'm hoping that will work in my favor.

@ JohnT - Both main drains have hydrostatic relief valves - I will use them!

@ vejadu - Thanks, yes I love it here on this forum! No, that's my kids in the bushes laughing at their old man for being such a nerd.

@ DRIVEN7690 - Thanks for sharing. Here's hoping for the same luck!

@ divnkd101 - No they are not. Yes we did have "vents" in the gunite. Long story short (there's more on this thread) I am brand spanking new to pool owenership & care and my pb went through a staffing crisis right as my pool was being started up. Something was put in the water which jacked the CH up from 150ppm to 650ppm where it remains today. They will not admit liability (though I know it has to have been them) but are acid washing the pool as a good will gesture. I knew I would have to drain most of the pool to lower the CH so this seemed to be the quickest and most amicable way out. The other alternative was fight them all the way with no guarantee of winning. My pb are happy to drain the pool and acid wash it, as were 3 other local reputable companies I contacted, all of them saying this is the best time of year to do it.

@ Jason - Thanks (again!) The locals I have contacted seem to be happy to do it. Though obviously nobody will give a guarantee that the pool will not pop. I had the pH of the pool lower than 6.8 for a week and brushed so hard I bent my handle! Am resorting to acid wash to get rid of this stuff.

9. A cautionary tale. My parents pool had a hydrostatic valve that they depended on one year when painting the pool. They happened to get a very strong rain when the pool was empty and the hydrostatic valve failed to open and the pool popped up out of the ground.

I have since learned from several pool builders that they do not like to rely on these valves and simply have an extra relief pipe in the ground that they open when the pool is emptied so they know for sure that there is a way for the water to escape.

So even if you have an hydrostatic relief valve, you have to exercise the same amount of caution when you empty the pool.

10. I have a mysterious piece of PVC sticking up right next to my diving board. There is a threaded fitting on the end but the pipe is hollow. Could this be a "stand pipe" for draining water from under the pool in the event I need to drain it? My pool was built about 30 years ago so I have no way of knowing why this pipe exists.

11. nocaster, How deep does the hole/pipe go? If it's as deep as your pool, then possibly. If it's anywhere from 8"-18" deep then maybe just a pool umbrella sleve?

Thanks Mark. Am not planning on leaving the pool empty more than a few hours... as long as it takes to acid wash.My pb said we start draining at 8am and will be refilling by 8pm. We shall see!

12. A local PB just told me a story of a popped pool in our area. The company was going to install Pebbletec so they drained the pool. In doing so, they drilled some relief holes in the bottom as rains were coming and they were concerned about a rising water table. The rains came, so the company didn't go to the site as they couldn't do the work in the pouring rain. Meanwhile, the homeowners noticed muddy water burbling up into their pool through the holes drilled in the bottom and they panicked. They plugged the holes with sections of broom handle pounded in with a hammer. The pool then popped.

Oops.

13. 400 gal/min, 3" Trash pump for the day - \$48.....

Good will acid wash from your friendly, local (but in no way responsible for how we got into this situation) pool builder - \$0...

.... Getting most of the scale off the pool, refilling it with water that has CH < 650 without the thing popping out of the ground and all in less than 24 hours.....

... PRICELESS!!!

Pool was empty in about 40 mins with that pump, acid washed in about 2.5 hours and then refilled in about 14 hours. We did open the hydrostatic valves in both main drains and notice a little bit of ground water seep up into the pool, less than 1" though.

14. Whew! I bet you're glad that's over!

15. Yeah... kinda feel like I made a whole loada fuss 'bout nuffink!

16. ## Re: Empty pools & hydrostatic pressure?

Excellent topic as I was looking into empty my pool, chlorine and acid wash it.

The water table level is a problem when the pool is in a flat land. For instance, my in-ground pool is not on a level surface. The long end that is close to the house is about 2-3 feet higher than the other long end and the soil continues to get lower on that side as you get further away from the pool. I think it is safe to say there is no water under my pool. Would you agree? There is a relief valve in the main drain as well and I suspect it pops open sometimes to balance the pressure which might contradict with what I just stated above though.

17. ## Re: Empty pools & hydrostatic pressure?

Groundwater level can slope across a site depending on gradient, flow characteristics of the soil, sources of recharge, etc. The best thing to do is to look for some indication of the groundwater level nearby. If you have sloping ground, is there an area downslope where the ground surface is lower than the bottom of your pool? If there is and there is no standing water you are probably ok. Also, if there is a house next door on the low side and their basement is dry, you should be ok. When a pool is built with high groundwater, they use a pump to lower the level until the pool is complete and filled.

Another way to check the groundwater level below your pool is to pull the cover off the bottom return, unthread the extra plug in the drain (where the hydrostatic valve is or would be if it was installed), and thread on a length of pipe that will extend above the water level in the pool. After waiting a little while for any water that got into the ground while you were connecting the pipe, you can measure the water level inside this standpipe. If it is dry, then your groundwater elevation is below your pool. If there is a valve, it will open and you will be able to read the groundwater level. Unless it is stuck then maybe you can try poking at it to see if it comes loose.

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