Best Depth For In-Ground Return Lines?

[hlwimmer]

hello: the progress of our semi-in-ground steel-walled, vinyl lined pool is coming along. the 4ft deep pool is 3ft in the ground with 1ft exposed -- we're building a low platform/deck around it all.

we have the structure assembled and i'm plumbing it now. due to the way everything is braced and laid out, i have two options for the return lines -- the first being way easier (and how i have it set up now). i should mention, we're in northern california. it rarely dips below 30deg and the ground has never frozen.

1: the method with the fewest bends and least amount of level changes will put the 1-1/2 rigid SCH40 PVC return pipes only about 6in under ground -- this area will be under the deck, but there won't be a lot of earth surrounding them. does this matter?

2: i can lower the pipes about 18in. this will put them much deeper in the ground, but necessitate more level changes. it also means that if there's a problem, i'll have to dig. the benefit to deeper pipes, i'm guessing, would be stability (less movement = encased in dirt) plus a little insulation for sound and temps.

the suction line, due to the layout (and skimmer height), has the pipes about 12-14in under ground.

wisdom for the return lines would be much appreciated. if having the pipes securely in the ground is a good tradeoff for fewer bends and level changes, i can modify the layout.

here's a link to a blog post with photos. in the latest photos, the top of the sono-tubes will be ground level... scroll down to the most recent bits.

http://redneckmodern...-pool-plan.html

[hlwimmer]

anyone?

[jkusmier]

If you truly never experience a hard freeze in your locale then periodic plunges to 30F should not be an issue, so long as you install some sort of electronic control panel (e.g. Pentair EasyTouch) that has a native freeze protection feature that will automatically start the pump when temps approach freezing (else watch the weather and run the pump manually). You can eliminate most worries if you winterize your pool for the coldest stretch of the year by blowing all near surface plumbing, like the returns and skimmer. My nagging concern is the possibility of upheavel/movement near the soil surface. This could definitely cause problems. If sub-freezing temps last no more than a few days then you are probably OK. I assume there is an overdig around the perimeter of your pool that was filled with backfill, usually 3/4" limestone. If it was compacted and if most of your plumbing runs in this backfill you might be OK. But you should ask yourself how much of the plumbing runs outside of this backfill. That is where you are most likely to see problems, if any. If you're really talking about a handful of sub-freezing nights during the winter season, it's entirely possible that you won't have any issues whatsoever - but I'd still make sure the pump is running during those cold days/nights.

[hlwimmer]

thanks for the input.

re: freezing: true, we almost never freeze (concord, CA: bit.ly/MAi7YG ). another bonus is that the pump and filter will be in an interior utility room shared by 2 other heat-producing devices (radiant heat boiler and hot-water heater) so the room stays quite warm in the winter... so 15% of the piping will be very-well protected. it'll also be a salt system which will drop the freeze point a slight bit... so, i'm not worried about freezing as much (and can winterize). i'm more worried about movement and potential for leakage. i do plan on horizontally supporting the longest bits with "legs", too, by using 2in Ts slipped around the 1.5in pipe with drop down legs to the solid DG... the backfill process might get "jiggly" and these could help stabilize things a bit.

re: backfill: yes, there is an overdig and we're not yet to backfill. we'll be pouring a concrete collar for sure (required per instructions), but there was no mention of backfill material. we were going to use a good bit of the excess soil, but can put gravel/limestone closer to the walls themselves. what's the reason for this? drainage? we'll have a deck/platform around the pool, so we'll need to put the posts in something relatively solid. i have sonotubes down to the compacted DG most adjacent to the pool. the other posts will be 2-3 feet out.

thanks again for you input... i hope to really get the project going today as soon as my laborers show up (literally momentarily).

[jkusmier]

You do not want to backfill an overdig with dirt. Why? Because it takes time, gravity, rainfall and sometimes some mechanical assistance (like tampers) to properly compact the dirt. To illustrate, go find an undisturbed area of your yard and excavate just a small clod of dirt with a good trowel. Then pulverize that clod by any means available - chop with an old knife, break into smaller pieces and place in a food processor (that's a stretch), etc. Then take the remains of the clod back to the hole and pour it in. It won't all fit into the hole, will it? You'll find that you have to tamp it in with some device, like an old broom handle, etc. Now imagine that happening on a larger scale, like the overdig around your pool. Even mindful tamping won't do the trick. We built our home some 12 years ago, and some 6 or so years ago I arrived home after work one winter evening and noticed that my headlights cast a strange shadow in the yard alongside our driveway. I realized that the backfill over our main sanitary sewer line had finally collapsed - a full foot in places. Thankfully, the line was not damaged, but my point is that we took considerable pains when backfilling the trench when we installed the line through a repitive cycle of adding dirt, a judicious amount of water and tamping. It still collapsed because of the reasons mentioned above.

There are at least 3 certainties in this life: death, taxes and concrete will crack. Probably the most important step in the process of pouring a durable concrete deck/collar/driveway/sidewalk is a deep (3-5"), well-compacted bed of 3/4" clean limestone chips. The bed is is essentially the foundation for the deck, but there is another foundation beneath that - the underlying soil. In the case of your pool overdig, it is an earthen backfill that will inevitably subside and lead to major cracking and collapse.

I highly recommend that you backfill the entire overdig around the pool with 3/4" clean limestone. The limestone will settle much more nicely upon installation due its weight and the fact that it is impermeable, and the water that will later and inevitably infiltrate under the deck/collar and into the backfill will simply flow down through the impermeable rock instead of dissolving and compacting it and causing its eventual subsidence.

It will cost more to have the dirt from the backfill hauled away, and to have limestone purchased, transported and installed, but it is more cost-effective in the long term. If you backfill with dirt, expect periodic subsidence, with resulting damage to the collar and the pool plumbing.

The sonotubes are a great idea and I applaud you for using them. We used - I can't remember, some 30+ under the deck surrounding our 16x32 pool. They add tremendous support to the deck but will not carry the weight of the deck as an earthen backfill collapses. (Hint: sonotubes are expensive, you can use less expensive 4" schedule 30 PVC, just cover the tops with duct tape before backfilling, then remove the tape and have your concrete guy fill them with concrete before he pours the deck/collar). Although nearly industry standard, you'll want make sure your concrete guy installs rebar and uses a fiber mix (meaning that hairlike plastic fibers are mixed into the concrete before pouring). Fiber adds tremendous strength and crack resistance to the pour. Also make sure the concrete guy trowels joints into the collar to allow relief from natural expansion and contraction.

Mind you, it will crack. But you can live with that. Major collapse, sawing or jackhammering out entire sections of the collar, re-plumbing cracked return lines, etc - you can live without that.

[hlwimmer]

thanks again for the wisdom.

re: backfill: limestone isn't as available in my area, but the crew brought over a few yards of crushed granite which should serve the purpose well for the immediate backfill. we will need to use /some/ dirt to fill in the ramp created by the bobcat, but the bulk of the backfill will be stone. if for some reason crushed granite would be a terrible idea, please let me know... too, because the are surrounding the pool will be under a wooden deck (and the earth beneath that covered in pea gravel, some settling won't be too troublesome. the piers for the deck will be sunk in solid. the existing sonotubes are resting son mechanically compacted DG with a ton of stabilizer mixed in... on top of undisturbed earth. the next few sonotubes/PVC will be sunk in a hole probably dug by an auger. you can see the rough post layout in the drawing in the blog post linked above.

re: concrete: fortunately, the "beam" will be below the backfill, so any cracks -- as long as still somewhat secure -- will be fine. i think the beam is just to help lock everything in together and with a pool as small as mine, i'd wager even fencepost concrete would suffice, but we'll bring in a truck and some wheel barrows.

re: sonotubes... thanks. they weren't that much ($6?) but i can see how a 4in tube might work well and use less concrete, too. might think about that for the remaining posts.

thanks again... much appreciated.