Settlement around pool - help requested

[danvines]

I'm going to post this in both my favorite home improvement forum and in my favorite engineering forum because at least 5 of you guys are going to come up with a better idea than I've though of. This is my house, so it's my nickel.

See the problem.

[image]

https://www.dropbox.com/s/kc1fj0im871ip03/2013-03-19 16.50.56.jpg

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This all started as a remove and replace concrete decking around my pool at 3 adjacent, severely cracked sections. I figured I had some pretty good settlement. The whole pool is on fill, and just beyond this edge, the property falls away into a small, but continuous creek. That's reasonable - let's fix the bad area and go on. I should also be able to afford a fancy topping this year as well.

Then we saw the chasm under the slab on the other side of the joint. I immediately did what any respectable engineer would do: picked up a hammer and sounded the whole slab. It's hollow at least 3 feet back from the edge all the way around.

I've got a call into one of the more reputable pressure grouting companies. I'm just scared about the possibility of blowing out the pool sidewall with too much pressure.

My other thought was to core the slab at some spacing (4 feet?) around the perimeter and put in flowable fill. I like this idea, but it seems as though it would be somehow better if that could be placed under a small amount of head. I'm not sure if that is really feasible.

Oh yeah. It gets worse. At the corner right behind where the first photo was taken, I have this.

[image]

https://www.dropbox.com/s/q5zic91nbn3tw0z/2013-03-19 16.46.13.jpg

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I pumped it out down to a level even with the bottom of the steps before I went to dinner. When we got home, it was back to the same level, approximately, 10" below top of pool. To my knowledge, we have NOT been experiencing leakage from the pool, though admittedly, it's been quite wet lately. We certainly aren't losing enough water to refill that much so quickly. And the water levels don't match.

Anyhow, I throw myself upon the mercy of the collective wisdom of the Internet.

 

[hokie66]

I can't open your photographs. All I get is a red X. Do I not have the right software?

 

[GeoPaveTraffic]

I have the same problem with the photographs.

Mike Lambert

 

[danvines]

Sorry about the issue with the photos. I would edit the original post, but I don't see that I can.

Here they are in order.

 

[GeoPaveTraffic]

The void below the slab doesn't surprise me. In my experience even the best contractors don't want to compact right against a wall, and most pool contractors are far from the best.

As for the water, hard to say without a lot more information. I would start by trying to repeatedly pump it down and see if the recharge stays constant or slows. If it stays contant, or nearly so, then I expect you have a leak in the pool.

Good luck.

Mike Lambert

 

[danvines]

Thanks Mike. I agree about the compaction - especially against a vinyl pool.

My intent this morning, to try and get a flow rate on the water. I just don't see how it could be coming from the pool. I think I would have noticed that much water going out.

 

[hokie66]

There are a few types of construction used in these vinyl liner pools, and it is hard to tell which type you have, other than it looks like some type of metal prefabricated units. I think the backfill was intentionally loose, with the deck supposed to span over the void. The corrosion in that deck attachment is a worry.

A good way to check on leakage if you have a shallow flat area (step) in the pool is to fill a container like a plastic bucket to the same level as the pool, monitor the pool and bucket level. If the two stay the same, no leak. If the pool level lowers more than the bucket, then you are losing water. There are also ways of testing with dye, but you really have to do some diving to be sure using dye.

 

[danvines]

Thanks for the input guys. We had some good solutions/ideas from both forums.

I had a guy w/ a pressure grouting company come out this afternoon to give me an opinion. He agreed with my fears of using the kind of pressures typically associated with real "slab jacking" techniques that close to a metal-walled, vinyl-lined pool. Best solution from his end was a really neat aerated/foamed concrete that was feather light (I held a sample), around 300 or so compressive strength, and supposedly flowed like Elmer's glue. I really want to use it just because I think its cool.

For the areas where we've already cut the slab out, I'm letting the situation dictate the response. I think we've located one slow leak at the interface between the swim platform and the liner. My go-to pool troubleshooter will be here in the morning to confirm. It's an easy fix. We also may have a plumbing leak, but a pressure test tomorrow will verify or rule out. As per above, once identified, a relatively easy fix since most of the concrete is already out.

Then we come to the problem of the saturated, uncompacted, clayey soil (this is a southern suburb of Atlanta) that is within about 6' of the pool edge. We can bring this up to where subgrade should be w/ #57 stone, but there's little chance it will always remain there. So, I'm going to make my contractor put some actual reinforcing in it, and make it span between the pool wall and the portion of fill that was actually well compacted about 6' from the edge. I actually pity the guy a little bit placing honest-to-goodness structural concrete FOR a an honest-to-goodness structural engineer.

Anyhow, given the above solution for the areas already torn out, we're going to play the waiting game with the rest of it. There's a reason I didn't know it had voids until we started this process. There's every likelihood that it will span, unreinforced (courtesy of MR) forever. Then again, maybe it won't. If nothing happens in a year, maybe I'll rethink things.

Thanks again.

 

[Ron]

Is the water level in the pool the same as the water level in the void? If so, you likely have a leak. Notice that the water outside the pool is nearly as clear as the water in the pool...another indication of leakage since groundwater would not likely be so clear.

 

[shobroco]

I am slightly familiar with cellular foam grouts for old masonry walls, and from what I know, I'm not sure I'd want to use it in what is basically an exterior application. You don't have frost to contend with like we do (Canada) but I'm not sure that the grout can handle the large moisture changes and stresses that you might experience. I work with a consultant who has very extensive experience with foamed grouts (he has developed his own formulae & has used this technique across the country) so I can ask his opinion if you want another oar in here. He's not an engineer, but he's a kindred spirit.

 

[shobroco]

I was speaking to my foam grout guy yesterday on another matter, & I asked him about your pool & the exterior application. This is not a definitive statement but he said he has done various adhoc tests on samples for moisture absorption and freeze-thaw resistance & they all exceeded his requirements and expectations, so you should be golden.