Questions about Bracing, Backfill and Retaining Wall

[robamato]

Hello I'm new to this site and planning to to a 100% DIY small oval IG Vinyl pool (~15'x30'). This info on site is amazing. I initially had truckload of questions, but just reading the various posts answered all but a few.

My yard is hilly, so I'll need a retaining wall on the low end - probably as high as the pool wall itself. My first question is do the wall bracing and concrete bond beam provide all wall strength without back pressure from the backfill? If not, I guess I have to be carefull as to how close I can put my retaining wall to the backfilled wall shelf, and that I'd need to build it before putting water in the pool. Is that right?

Also, about the backfill, I'd really like to use the excavated dirt rather than buy crushed stone. I'm in central NC and mostly have red clay. Is it OK to backfill with dirt?

Thanks in advance.
-Rob

 

[bk406]

My first question is do the wall bracing and concrete bond beam provide all wall strength without back pressure from the backfill? If not, I guess I have to be carefull as to how close I can put my retaining wall to the backfilled wall shelf, and that I'd need to build it before putting water in the pool.

Copule points to clarify. Not sure what you mean by a few of the terms here. By wall bracing, i assume you mean the steel supports the brace the pool walls. True? Next, Not sure what you mean by concrete bond beam. Inground vinyl pool packages, as far as i know, do not use a concrete bond beam, it's steel.
How hilly is your lot where you are putting the pool? Reason I ask is that will dicatate how high a retaining wall you will need. In order to hold up the low side, the wall will need to be as high as the top of the pool. For mine, i needed a 5 foot wall at the highest point. Now, the retaining wall is not something that can be thrown together from materials from Lowes. Depending on the height, you will need some heavy materials. A stone wall would have to be about 3-4 feet thick to hold the weight. Best material is Versa Lok block. But, if it's over 3 feet or so, it will need geogrid and most likely need to be engineered.
I'm not trying to discourage you from a DYI job. If you have a level lot, DIY of an inground vinyl with a pool package can be done. My uncle did one and it looks great. But, having a retaining wall on a slope to hold up the pool is a different animal. This takes quite a bit of engineering know how and an experienced landscaper who specializes in hardscape to build a proper wall. The weight here could be well over 100,000 pounds (due to water) so you need someome that knows what they are doing.

I know a little about building a pool on a slope. Take a look at my pool build.

my-pool-build-t18966.html

 

[duraleigh]

Hi, Rob,

Using the excavated dirt is fine unless you are going to pour concrete over it. It simply is too difficult to compact to make a substrate for concrete.

I bit the bullet and did #57 rock for mine. I hated the extra cost but, seven years later, I have ZERO cracks in my concrete!.

 

[robamato]

Thanks a lot for the quick replies! BK406, your assumptions are correct on my lingo. Sorry for the confusion. My slope isn't as severe as yours. If i was to set the pool elevation where one corner was at grade level, the grade at the diagonally opposite corner would be at the bottom of the pool wall. I've attached a (very rough) shot of this from Google Sketchup. I'm starting to think that I should try to lower the grade a bit at the high end so I can lower the pool and allow for a smaller retaining wall at the lower end. Regarding distance, would the "45 degree rule" apply here? In other words, keep a 2' high retaining wall at least 2' away from the overdig area?

Dave, not having to backfill with stone is good news for me. Thanks for the tip on settling. That brings up another point of confusion for me - I see that some pool wall braces have a deck support attached. Are they really meant to support a deck? It seems they are too light to support a concrete slab. If that is indeed how they are used, I'll go with dirt backfill and let the deck supports prevent the deck from moving as the dirt settles.

Thanks again!

 

[BobinBaltimore]

Robamato, can you provide a quick drawing of what you are intending to do? I think that would help a good bit in getting well-targeted advise. I agree with bk406's comments....retaining walls of any size or complexity really need some experienced eyes. Even if you do choose to built it yourself, having a plan and construction details reviewed and blessed by an engineer (or at least a very competent hardscaper) will save a lot of trouble.

For our gunite pool, we had to install 110 foot retaining wall which ranges from 12 to 36 inches finished height. Feel free to click the link in my signature for some photos. We chose an EP Hardy system that looks similar to stone, has multiple different block sizes, is installed slightly battered for strength (each row is recessed from the one below it just a bit) and is also pinned. We didn't want to rely on just gravity or adhesives for stability over 20 years, so a pinned system became a requirement for us. We chose a hardscaper that we had worked with before. Because our wall was just barely under 36" at top finished height, our county did not require engineering drawings. The wall sits on 6 - 8" bed of compacted crusher run over fabric on virgin soil. Our landscape contractor also put in 2 - 2.5 feet of 3/4 stone compacted in 6 inch flights behind the wall, and he used geogrid to tie the wall back in to the stone and undisturbed soil. (Other contractors we talked to were offering more like 1 foot of stone behind the wall, and no geogrid.) He compacted in layers as manufacturers require, but too many contractors ignore. A french drain is down there, too, along the base of the wall. This drain coupled with the french drain installed under the perimeter of the deck gives us just excellent drainage.

Hope this description helps. Looking forward to hearing more about your project.

 

[BobinBaltimore]

Robamato, thanks for the drawing. You posted it just as I posted my comments. Now I understand that you are looking for a retaining wall to hold in the pool, not hold back dirt. Totally agree with bk406...this is a different beast than a typical retaining wall. I'd definitely recommend getting some engineering advice before you execute.

 

[bk406]

Yes, it's hard to see what needs to be done without some actual pictures to see the slope. You also need someone to actually shoot a grade and draw up some plans, as bob said.

Bob, sounds like a similar system to versa lok. It's pinned as well, with a 3/4 inch offset as they are stacked. I have 3 layers of geogrid in the back and side walls.
Same as bob, my walls sit on a 2.5 foot deep footing filled with 3/4 stone. It has a french drain in the bottom wrapped in filter fabric with a pipe out of the bottom of the wall at the lowest point. The wall is then backed with filter fabric, and backfilled with 3/4 inch stone up to the 1st layer of geogrid. then, processed gravel was layered in and plate compacted after the pool walls were up. Each layer of geogrid is packed in level with the processed gravel and compacted as it was stacked in. That wall will be there longer than the house!
When it come to building a retaining wall for a pool like this, a pinned wall system with geogrid is really the proper way to do it, even if the wall is only 36 inches high.

You need an engineer to look at this and draw up some plans. The excavation will be critical too. The wall, excavation and back fill is not dyi. Puttitng up the steel walls, plumbing, equipment set, etc can be. But the initial constuction, no way. This will require professional and experienced people.

 

[cobra46]

Another option is that the wall can be made from CMU's (cinder blocks) that does not require a geogrid. This would require a key at the footing. Of course, as others have said, you need an engineer to design this wall no matter what construction technique you decide to use.

 

[bk406]

While CMU's have their use, I would not use them as a retaining wall in this application. I have seen CMUs TRYING to hold back just earth at ~ 36-40 inches tall. The back pressure buckled the wall after 2 years. For this type of wall, you need a pinned wall system with geogrid. No way around it.

 

[robamato]

Thanks for the replies everyone. Based on this feedback, I think I'll be targeting the other said of my backyard where it is much more level!