Inground Pool Excavation Question

[ProDIYer]

Hello, I have a general question concerning my excavation. I will be installing a Vinyl Liner , steel walled, in ground pool. The site I have picked out in my yard will need to be graded up and raised approx 3'. Since most of this "new" dirt will just be dug back out, my question is on what is the preferred method:

Should I grade the entire area, and use a soil compacter at various stages of the build up to firmly compact the new soil and then excavate my pool as normal.

OR

Should I just level the existing area, set my pool walls (since the top of the pool walls will be at the correct finished grade depth) and then back-fill everything back to the desired finish grade?

The reason I ask, is that I keep reading that "over digging" the over dig area is a bad thing. Is this just because it creates more work and material to backfill with non expanding material in the end. Or is it because the newly disturbed (or moved) dirt will not be able to withstand the weight and pressure of the walls and filled pool?

 

[duraleigh]

Welcome to the forum. :lol: That is an excellent question and I'm not sure of the answer. My initial reaction is to level, set your walls, and then dig and backfill.....your second option.

The primary reason for not overdigging is so the concrete decking can be poured over "virgin" soil. Backfilled soil settle badly and makes for cracks in the concrete.

In your case, your gonna have to backfill anyway but you will do a lot less the second way.

Others will chime in soon and offer advice.

Where will you get enough fill dirt to bring your surrounding area up to grade? What you dig from the pool will not be bearly enough.

 

[bk406]

Agree with Dave. Second option. Never try and "build up" anything like that. Level the pad, then dig it out. The walls go in after you dig it out, generally. You can dig the deep end and hopper, set the back walls and back out and set the rear walls last. But, you may have to over dig more than you think. With a steel wall pool, there are struts that come out from the walls to sturdy up the sides and give it support. You are going to need to over dig enough to properly pour a concrete collar around the perimeter. Also, you will need to shoot grades on each seam where the walls meet. That means you need enough room to level the walls using some type of blocks so the top of the pool, all the way around is level. Generally, you need about a 4 foot over dig, I would think. With a steel wall pool with an over dig, you will not be able to pour a deck on virgin soil. The back fill material needs to be good stuff; generally NOT what is excavated out. Do you have experience shooting grades with a transit? You need to get everything correct, including the hopper, or the liner will be a tough fit. What kind of excavator are you going to use?

 

[ProDIYer]

Thanks for the quick reply. I am fortunate to live on a 3 acre lot in the country, so I have lots of dirt that I can move around on the property. In fact the back third of my lot is a good 10 feet higher than the front third so that will not be an issue.

I am kicking myself now because I actually shaved down the area that I want to use for my pool site a few years ago by a good 3'. The site is behind and slightly beside the house. At the time, I was bringing it down to the grade level of the front yard and was going to extend the driveway back there but I went with a different plan. Now as luck would have it, it seems to be the perfect spot for the pool.

Since I am doing the bulk of everything myself, I am not near as concerned about the extra time and labor of the extra back-filling as I am about possibly weakening the structural integrity of the pool walls if I do not grade everything first, then starting the dig.

Would love to hear more opinions.

 

[ProDIYer]

Sorry BK406 I was typing my reply when you posted. I will sub out the actual dig, I was only going to do the grading to level ground myself and leave the precise digging to the pros.

My real concern now (if building the site up is out) is that the existing grade it too low to just start digging on virgin soil. The site is a good 4 feet lower than the finished grade needs to end up. It would be the equivalent of setting all the shallow end walls directly on the existing grade and then only digging out for the deep end. This scenario leaves absolutely no over-dig on the shallow end to contain the concrete collar pour.

How would I handle that? Would I just have to use a concrete form of some type to form the over dig area for the concrete rim around the shallow end?

 

[bk406]

Without seeing the site, it's tough to tell you what to do. I see what your saying, though. Your ground slopes about 4 feet from one end to the other? So...if you dig the deep end, the walls on the shallow end would sit on top of the ground?

If thats the case, put the deep end on the high side, and the shallow end on the low. You would dig down the 3 feet (or what ever the shallow end will be) then dig the deep end. The deep end will be dug out the depth to make the top of the pool level, then dug down to the bottom of the hopper (i.e if the pool is 8 feet deep, your deep end dig willl be 8+4 or 12 feet). This means that you will need a retaining wall on the deep end. Presumably you will have a deck on that end that extends some distance out. The wall would then be on the edge of the deck holding up 4 feet (or so) of earth.
Thats how to do it, but it's hard to get across just reading here. You might get an architech to look at it.

 

[Guest]

Whichever road you take, you need to be either in all native (virgin) material, or all in compacted fill. Do not transition soils (part in native and part in compacted fill). Soils expand and contract differently, and transitioning different soils (and compactions) is a sure failure.

 

[ProDIYer]

BK406, you kinda have the right idea but a little off. Picture two perfectly level areas but one of those level areas is 4 feet lower than the other. They both sit side by side and if you were walking across the higher section toward the lower section you would have a 4' 90 degree vertical drop down to the lower section, but again its level once you get on the lower section. I will try try to do a simple diagram and attach it. I don't think I will need any type of retaining walls, since both sections are level with no rolling slopes of any kind.

 

[bk406]

OK, i think i see what you want to do. You want to butt the shallow end up to the high side of the 4 foot drop. That would leave the walls on the shalow end sititng on the existing grade. So your question was/is, how can you backfill the walls sititng on top of the ground and no down in a hole?
The ansewer is you can't. What you want to do is not feasible. My original suggestion still stands.
You will need to "sink" the pool so that the top of the pool is on grade with the low end. That means a retaining wall to hold back the earth on the deep end (high side). A package vinyl pool needs to have the steel walls buried in an excavated hole so that a concrete collar can be poured and proper backfill used. Those steel walls have to have backfill material against them so they wont buckle.
You should really have an architect look at it. They would know how to do it and explain it.

 

[ProDIYer]

Gotcha, thanks bk406..............not that it matters but the pool would actually run the other direction than what you describe. My idea was to have one of the long sides of the pool butt up against the existing higher grade, not just the shallow end, but I suppose this would still require a retaining wall (in fact that would require a much "longer" retaining wall going down the entire other long side of the pool correct?)

 

[ProDIYer]

If I choose to proceed with my plan and just raise the grade up by 4 feet prior to starting the pool excavation, is there a preferred method and material for building up the site and compacting it properly? We have soil that is probably 75% clay, 5-6% sand and the rest other material, so it will compact very well, but with the high clay content is there a concern about the clay expanding as it absorbs ground water and cause me some problems later on?

If I went this route, I would back fill the over dig area with gravel to support the final concrte deck around the pool.

 

[bk406]

If I choose to proceed with my plan and just raise the grade up by 4 feet prior to starting the pool excavation, is there a preferred method and material for building up the site and compacting it properly? .

That wont work. You can't build it up like that, it wont be stable enough. There is nothing you can use to make that work.

 

[Guest]

You want at least 91% compaction. If you "benched" down into native material (at least 12") and then came up in 6" "lifts", compacting and testing as you come up, you can get stable material.

I'm not addressing any of the retaining wall stuff as it looks like you have a pretty good handle on that! Just trying to help you understand what you need for ground conditions. The clay is not a good thing........

 

[ProDIYer]

Thanks for the info guys, it's time to rethink my site location.......(that's code for) I have to convince my wife that we need to choose a different location for the pool.............that might be harder than all the labor involved