Water Behind Liner

[cschnurr]

This is the 3rd season since we've had our inground pool installed.

We have an SWG but once or twice a year have had cloudy water without any combined chlorine and chlorine has always held overnight first time, so I'm not even certain I had algae. Each time I've treated as algae with the shock as prescribed on this forum.

Additionally once or twice a year, I've noticed water behind the liner. Our pool is about 8ft deep in the deep end and the water behind the liner forms in the deep end just above where the sloped side wall rounds out to the flat bottom of the pool. (I don't understand why the water doesn't simply flow down to the pool floor) The affected area is about 18" high and up to 6'-8'long. When I push on the liner it feels like about 1/2" -1"deep and usually goes away within a week.

Each of the three years we've had the pool in early August, both cloudy water and water behind the liner occurred simultaneously and correspondingly disappear around the same time. The first year we had an incredible rain storm where the water went as high as the coping, so I thought the water could have went over top of liner, but that's not the case for the 2nd and 3rd year. We live on top of a hill, and I've never heard of any drainage / water table issues.

I usually notice the water behind the liner I vacuum after an algae shock. The vacuum stalls over the spot in the pool as the liner gets sucked closer to the vacuum opening, and the pressure on the filter drops.

Should I be concerned about occasional water behind the liner?

I'm wondering if there is a relationship between cloudy water, and water behind the liner?
Is it possible that one causes the other, or they have both have the same common cause.

 

[duraleigh]

I can not think of any circumstances where one is related to the other. IMHO, they are entirely independant and seperate issues.

 

[JasonLion]

They could be related if there was a leak. Ground water could bring in contaminants. But it would have to be a small leak or you would have noticed it lowering the water level when the ground water was low.

 

[waste]

Hi

Just a shot in the dark, but the 2 may be related to heavy rainfall - I know you said that the first time it happened was after a 'drencher', but was there heavy rainfall the other 2 years just prior to the cloudiness and liner floating?

 

[cschnurr]

Sorry I haven't been back to this forum for awhile.

"was there heavy rainfall the other 2 years just prior to the cloudiness and liner floating"
To be honest, I can't recall. I'll keep a better eye on it next year.
I know the first year, the rain rose above the liner seam in the coping.
The other two years it definitely wasn't so high.

What is your shot in the dark?

I checked for leaks using a pail on the steps. Over a two week period the drop in water level relative to the pail is less than 1/8". I wouldn't even notice this amount during the year, and don't know if this is a sign of a small leak or not. Should I worry about this before I close?

 

[waste]

Glad to see you back :-D

The 'shot in the dark' was that you'd had enough rain to both wash stuff across the deck, thereby clouding the pool and be enough to temperarilly raise the ground water level enough to lift the liner if the drainage in the pool area was insufficient to allow the water to pass in a timely manner.

The < 1/8" of difference betwixt bucket and pool over a 2 week period would not have me concerned

 

[cschnurr]

Waste, Thanks for your reply

I would be surprised if the ground water was high. I've never had any problem with surface drainage in the yard.
I live in town on street with storm sewers, about 1km away from a river on top of a hill likely about 100ft elevation above river.

My theory is that I have a small leak somewhere. Water enters through the leak behind liner to form a water pocket between the liner and the concrete. Before the pocket drains naturally through concrete, it is in an area without any circulation and therefore prone to algae. Once the algae takes hold in the pocket, the algae enters the pool through the leak.

Does this sound feasible, or likely?
How exactly does water drain from behind the liner through the concrete? Will the concrete at this location be damaged?

Is there a way to determine whether ground water or pool water is the source of the pocket?

I'm getting into science experiment mode...gives me something to think about over the winter.

I've thought about taking a syringe through the liner into the water pocket to draw a sample then patch the hole.
Testing the water for CYA might give me some idea as to whether its ground water or pool water.

 

[JasonLion]

What you describe is possible but I don't think it fits exactly. Concrete is very slightly porus, water will flow through it very slowly. If there was a leak enough water would go through the concrete to maintain a small pressure difference, so the liner would stay tight to the concrete but water might be visible behind the liner between the concrete and the liner. Flow would always be out of the pool and it is unlikely that algae would make it into the pool against that flow.

If, on the other hand, ground water pressure exceeded pool water pressure the net flow could be into the pool. Water would accumulate behind the liner, pushing the liner out from the concrete in places and also enter the pool through any holes. That water could easily carry dirt and algae into the pool.

What you describe doesn't exactly fit either of those cases. It is more like there is one particular spot where ground water pressure is high and it is low elsewhere. I suppose a leak in a drainage pipe near that spot or an artesian spring could cuase such a thing, but it is difficult to imagine the placement being exactly right.

Water flowing through the concrete will eventually cause damage, but this will normally be a very very slow process.

If you want to experiement you need to dig a dry well somewhere around the spot where the liner pushes out. This must be done with care to not hit any pool structure or plumbing. Such a hole is normally lined with a pipe full of holes. You can then watch the water level in the dry well to gauge the ground water level, or drop a sump pump in to lower the ground water level and see what effect that has on the pool.

 

[waste]

The trouble I see with the 'small leak letting water accumulate' theory is that the pressure on the supposed leak and the liner around it is equal - therefore even as the water is trying to puddle under the liner through the hole equal pressure is being exerted on the liner around the hole and therefore no 'pocket' of water. I'd say it's gotta be either the water table rising (which I doubt) or something (hardpan, bedrock, clay ...) holding the water in the surrounds of the pool. Also, because the pressure of the pool water must be taken into account, the water outside the pool would have to be higher than the pool water's level :? But you say this happens in the deep end, where the pressure is greatest from the pool water - the only thing I can think of, and it's not very good , is that they had to blast out bedrock when the pool was built and filled the blast area of the floor with the cement - basicly creating a 'well' for excessive water to fill, the slow seepage that Jason mentioned would then allow for the slow decline in the 'bubble'. (something about this doesn't sit right with me... but I'll give it some more thought).

Do you have a MD with a hydrostatic valve?

 

[cschnurr]

The whole thing doesn't sit right with me either.
I'm at a loss too, thanks for the input.

I do not have a main drain in the pool, and I saw them dig the hole, and witnessed most of the build.
There was no blasting only a dig simple dig with a shovel.

I did not see the PB actually concrete the bottom of the pool.

I don't really feel like digging a 9ft deep hole to see where the water table is while experimenting.

Any other ideas would be more than welcome.
Like I say, I have the winter to think about it.
I haven't talked to the PB about the problem since the first year which I attributed to rainwater over the liner seam

 

[duraleigh]

Well, I'm curious about this now, too. Most vinyl pools do not have concrete behind the liner. Rather a steel or fibreglas wall down to about the 4 foot level and then poolkrete below that.

Is there reason to believe your pool does have concrete walls and/or floors? I am still thinking ground water is the issue regardless but I'd like to clear up what type pool you have first.

 

[kirbinster]

What is "Poolkrete" Maybe that is what I have in my pool but do not know the name. The concrete below the metal walls is a low density concrete that I believe is a mixture of concrete and vermiculite. What my pool builder did because we have a high water table is build a drywell under the deep end of the pool. I can tie this into the main circulating pump and valve it to waste to pump the sump dry after heavy rains if there is any water behind the liner.

 

[duraleigh]

is a mixture of concrete and vermiculite

Yep, that's poolkrete.

 

[cschnurr]

I closed my pool this weekend.
One of the difficulties I've had is that due to the liner pattern, water behind liner is very difficult to discern without actually diving in and looking.

I previously said my bottom was concrete, without knowing the difference between poolkrete and concrete.
Why pookrete? Is it more porous, less prone to cracking or something else?

We took photos during the build which have been amazingly handy on several occasions.
When I looked through the photos I found one of the crew putting in the concrete/poolkrete.

I've attached a couple of photos showing the crew putting in the pool bottom.
Can you visible tell the difference between concrete and poolkrete?
I've marked up the area where the bubble forms. No sign of bedrock or anything else.
The 2nd photo of the bottom complete.

If the leak theory doesn't make sense, I guess the next most likely contributor has to be outside water pressure.
There are a couple other pools in the neighbourhood, that I can ask if they have similiar issues.
I also have a friend that digs wells typically in rural areas out of town, but I can ask him if he knows anything about the water table depth in my neighbourhood.

Should I ask the builder if poolkrete or concrete was used?

 

[duraleigh]

Why pookrete? Is it more porous, less prone to cracking or something else?

All of those plus one big extra....it should be soft enough that it will actually dent if something heavy is thrown into the pool....frequently preventing a liner puncture.

If that were concrete, you would have some very hefty cracks by now and most likely, tears in the vinyl.

Now, if your liner is floating off the wall at the point you indicate, that means the ground outside the pool is saturated to at least that level or maybe even a little higher.

I don't understand why the water doesn't simply flow down to the pool floor

Because the ground is saturated with water below that point.

There is nothing in the photograph that would indicate that the soil isn't porpous enough to carry away any leaking water, thereby leaving your liner snug to the poolkrete so I don't think a leak is likely.. I'm almost certain you have very high ground water from time to time and the only way to cure it is with wellpoints around the perimeter of the pool or simply live with it.

 

[cschnurr]

Depth of Well Points

I'm starting to think about spring and the pool.
I am resurrecting this thread, as I still had some unanswered questions that were interupted by winter.
Let me know if you think it's been too long to resurrect this thread, or should I post it as a new thread.

I have no rationale to explain why my water table could be so high as my liner hasn't floated in the spring when pool water is low, and ground water is highest. The bubble has occurred in late summer when water table is lowest.
I need to understand how deep to my well points should be before I take on this endeavour.
I have two conflicting rationales.

Duraleigh commented that the ground water is saturated to at least the height of the bubble.
Does this imply the well points need to be dug at least down to the depth of the bubble?

My rationale is that if the bubble is caused from a high ground water table, the water pressure at the bubble depth behind the liner must be greater or equal to the water pressure in the pool. Since the water pressure in the pool is the depth of the water, the water table on the back side of the liner must be above the waterline in the pool.
Therefore the well points need to be dug below the water line the water given a path to drain.

This year, I will wait until I get a bubble, then dig a hole below the water line. I'm expecting the hole to fill with ground water. When the waterlevel in the hole subsides the bubble should also.

If this works out, I'll make the drainage more permanent with well points.
I don't think I'd bother with wellpoints if they need to go down to the depth of the bubble which is about 8' deep but if the well points only need to be 2 ft deep, it doesn't seem too bad.

 

[cschnurr]

Could someone provide some advice on the depth of the well points?
Thanks

 

[duraleigh]

They need to be dug down to the depth of the bubble. The water behind the bubble must be removed so the water in the pool can push the bubble back flat against the pool wall.

Importantly, that water may need to be continually removed. If the wellpoint is allowed to refill, the liner will again float.

 

[cschnurr]

Re: Depth of Well Points

Thanks Duraleigh,
Can you help me understand this problem as it will determine whether I live with it or fix it.

My understanding is that the bubble is formed when the pressure behind the liner is greater than the pressure in the pool at the same depth. The water pressure in the pool is the depth of the water, the water pressure on the back side is the pressure of the soil and water.

If I assume water saturated soil is around the same density as water, then the water table would have to be at or above the waterline of the pool. To decrease the pressure on the backside of the bubble, I would only need to dig down ~1-2 foot deeper than the waterline and pump out the water

The other way I've rationalized this assumption is that the pool walls held without any problem during construction the walls support themselves even without water pressure.

Thanks for you help.

 

[cschnurr]

Could Water behind liner cause pump failure

After 3 years use, my pump is making a lot of noise, I suspect the bearings. Often when my Hayward Ultra passed over the area with water behind the liner, it stalled, the filter pressure dropped and I had occasionally noticed cavitation in the return lines. I suspect this was due to the liner being sucked into the vacuum and restricting the flow. I imagine this could lead to premature failure of the pump.

If so, my motivation for putting in a well point may have increased considerably.
How do I go about getting well points installed.
(Start digging, rent a post hole digger, contact a pool contractor, or a well driller?)
Any guesstimate as to the cost?
Is there another brand of vacuum that would be better suited?

Due to the location of the affected area, and the pattern in my liner the only way I can notice the floating liner is my stalled vacuum. I have to take off my glasses to swim, so I can't even notice the problem when I dive in unless I touch the area. So I don't actually know if my liner is floating, as water is still far too cold.

(See http://www.troublefreepool.com/posting. ... ply&t=4911 for my thread on premature pump failure)
Should I keep these as separate issues on different threads?