Pool on slope; water no longer level

[miuwtant]

Hi,

I just noticed something that is very troubling. We live in Northern California in the Sierra Nevada foothills. We have a 20-year-old pool built by a leading local company. The pool was here when we bought the house ten years ago.

The pool is on a slope. Our ground here is like iron in the summer and it's August. Yet the shallow end of the pool appears to have settled recently so that it is about an inch lower than the deep end. Before, the waterline was even with the tile line all around the tiled portion of the pool. So it is easy to tell if the pool is not level. The length of the pool runs parallel with the slope (meaning the shallow end and deep end are both set in about the same slope).

The redwood deck around the shallow end has settled about 1-1/2 inches below the edge of the pool but that happened slowly over many years, and the deck structure is on a separate foundation from the pool. The concrete slab that butts into the deck cracked 5-6 years ago but the crack has never widened. The change in the water level is very recent; I look at it often and didn't notice it until today.

You can see the side of the pool from under the deck. There is a concrete buttress in one area below the skimmer. The ground has eroded around its base and there is moisture at the base and water is dripping slowly from the concrete at the bottom. This is the only sign of a leak I can find.

We had a big hidden leak in the waterfall bowl opposite the buttress on the uphill side that I repaired a few weeks ago. We don't use the falls much and I didn't realize it was leaking until I left the falls on overnight and the pool was down 3" of water by the morning. I think the waterfall leak (about 3,000 gallons) may have undercut the pool and caused one end to settle. I never found one drop of that 3,000 gallons--it just disappeared into the ground under the pool, apparently. There are no visible cracks or bubbles in the pool wall anywhere except as noted with the spa, below, and no other sign of water besides the wet buttress at the base of the pool wall on the downhill side. I think the buttress water may be coming from a small leak in the skimmer above it; I once saw a few tiny bubbles coming out of the skimmer but didn't see any today even though there is fresh water on the buttress.

For the last year or two I have noticed that the top two rows of tiles around the spa on the pool side are not solidly attached to the wall. Just a few weeks ago one popped out and I pulled three more out before they fell. The spa wall has had a vertical hairline crack for several years that does seem to have gotten very slightly worse lately. There is about a 1/8" crack across the piece of slate that forms the spillway bed. This crack is in line with the crack in the spa wall below. Before there was just a crack with no gap in the slate.

Please see attached pictures.

Any recommendations?
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20,000 gallon free-form custom pool. Plaster lining. Attached 1100 gallon spa. 2 skimmers, deep-end and side drains. Polaris 280. IntelliFlow pump w/ Compool controller and Pentair DE filter. Pentair heater, copper-and-aluminum solar heating.

 

[JasonLion]

Very gradual settling is normal, sudden changes are not. I recommend you get a soil/structural engineer out to look at it.

Most of what you describe is normal as a pool ages. Tiles come off sometimes, small cracks form, and there are occasionally leaks. But having the pool shift like that is not normal.

 

[miuwtant]

Jason,

Sigh. That was what I expected but not what I wanted to hear, obviously. I talked to a friend tonight who is an architect and he said basically the same thing. He did ask me a question I couldn't answer, though, about how the pool was built. Was there sand or fill under the gunnite or solid, virgin earth? He said usually there is little to no fill and that it would be odd for a structure like a pool which is more or less like a tea cup to settle so much without cracking. He even said to be wary of having an engineer come out because to protect themselves they often order up a battery of tests, maybe more than they really need, and it can get expensive in a hurry.

We had experience with a slab failure for a house on a slope and have been through the loop with soil engineers. It is expensive. But I would hate for the pool to fail if there is something we can do to ensure the settling doesn't get any worse. Right now the pool is totally functional and the cosmetic issue very minor. Maybe we need to pump some concrete up underneath or something. Two kids in college...

This was my worst nightmare with this pool having gone through a slope problem with the other house. But when we had the home and pool inspected before we bought in 1999 we were told (and have heard this repeatedly since) that foundation failures in our area - where most homes are on a slope - are extremely rare. My friend told me he thought it was really weird that water - even 3,000 gallons - running at the equivalent of water coming from a garden hose would carve out enough earth without a trace to cause the entire end of a pool to sink, especially the shallow end. 3,000 gallons / (twelve hours * 60 minutes) = about 4.2 gpm.

I'll come back with a report on this later. I'm going to see if the builder can give me any details or photos of the construction (they are still in business) to help with the detective work. Hopefully this story will be of interest or use to others on this site.

Thank you very much for taking the time to read my sob story and to post your recommendation.


Bob