Cutting into 3-1 slope for AGP

durtynacho

Active member
Aug 4, 2015
37
Ellis County, TX
Hey all, we’re looking to cut into the slope of a big hill by our home for a large AGP (18FT x 33FT). Never done a slope before, hoping to run my plans by the group and see if anyone with experience can identify anything I might be missing, or any gotchas. I think the slope is about a 3-1 or maybe a 4-1 pitch at most. We live on a hilly, rocky area of Tennessee.

I’m hiring an excavator to dig down to undisturbed soil (mostly clay) a few feet wider than the pool in all directions, and create a “retaining” wall to support the raw exposed earth that was once the high point of the slope. I put retaining wall in quotes because the retaining wall will be a 45-degree pitch of packed dirt, probably 4FT tall so not too crazy. Below is a picture that sort of represents what this would look like. I think it will do fine. The brown area is the dirt retaining wall.

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Going to create a berm to divert water on the upper slope before it gets to the retaining wall, and then a french drain around the pool for any water that makes it near. The berm will be as close to the wall as possible as to eliminate as much water from landing behind it and trickling down the slope. But some will get by, which is what the french drain around the pool is for. The pool will also have a deck around it that will cover most of the gap between the wall and pool, so water landing behind the wall and running toward the pool should be limited to whatever seeps through the decking.

Any guidance would be most appreciated!
 
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Rand K

Active member
Jun 15, 2014
27
Perkiomenville, PA
My pool is round and about half the overall size of yours, but I'm also on a steep slope. I built a retaining wall that goes all of the way around the pool. My circular retaining wall is backfilled about 2' deep, so that the pool is sitting on a 2' high raised crushed stone platform. The retaining wall is bigger around than the pool too giving me a 2' wide solid walkway all of the way around, and the 2' of backfilled crushed stone provides drainage under the pool.
 

durtynacho

Active member
Aug 4, 2015
37
Ellis County, TX
@Rand K, thank you for the reply. Are you saying there is 2FT deep of stone behind the retaining wall, as in, on the slope side? That’s a good idea, I hadn’t thought of that but it would def provide excellent drainage.

What is the material you used between your pool liner and the stone? I never heard of using rock under the pool but that also a good idea for drainage. I imagine stone and extruded foam or a gorilla pad would work well and prevent washouts long term.

I used extruded foam under my last pool on sand that had some rocks in it and never had a problem.
 

Rich D

Bronze Supporter
Aug 3, 2018
742
MA
You might want to get a builder's level and check your elevations before committing to a plan. If you cut into a 3/1 slope 20' (18' pool +2' (2' would be the absolute minimum , you will most likely need more)) you will be left with a 6' vertical drop. Then to achieve a 45 degree slope (1/1) you need a additional 6' cut into the bank. So you will be starting with a 8' vertical drop.

Typically you would do a cut and fill operation. This uses all of the excavated material on the lower part of the slope then you put additional retainage on the lower slope so you end up with two 3' retaining walls instead of one 6' retaining wall.

If you are going yo have a excavator on site and are looking for the lowest cost retainage, it would be to make 1/1 slopes and cover with filter fabric and 3-6" trap rock. This takes up much more area than retaining walls and is not as aesthetically pleasing and adds a certain amount of maintenance. You do not need any dams to divert water you just add drainage to the bottom of the rip rap slopes.
 

Rich D

Bronze Supporter
Aug 3, 2018
742
MA
Typically you would do a cut and fill operation.
I should mention with a cut and fill you will need to be absolutely sure you get 95% compaction under where the pool will be. This can be difficult especially on a slope. You need a experienced excavation contractor, correct material at the correct moisture level along with the correct compactor for your material type. To avoid any possible settling issues the entire area under the pool could be brought up with compacted 3/4 - 1" clean stone starting from a solid base of virgin soil. Just be sure there is drainage from the lowest part of the stone out to daylight so water does not collect there. This will also give you more dirt which can reduce the slope on the downhill side.

Do you have any pics of the entire area?
 

durtynacho

Active member
Aug 4, 2015
37
Ellis County, TX
@Rand K I didn’t realize you had built up. So you’ve got 3/4” stone built up to level, and are retaining it all around the pool, and all that’s needed after it’s compacted is strong enough foam board?

@Rich D Yeah my contractor stated it might be best to build up with some of the material he excavates, and he may do that if needed. I’m prepared for ditching the 45 degree dirt wall and to have him build up and compact, with two smaller sloped walls with drainage rock, as you stated. I’ll try to draw up what I think this would look like so that I can share with you and you can confirm that I understood your recommendation.

I don’t have any pics, it’s all woods right now anyway, so I need to clear the trees and roots first (excavator also doing that).

Thanks for all the replies and guidance, I cannot tell you how much I appreciate it.
 

durtynacho

Active member
Aug 4, 2015
37
Ellis County, TX
Thanks again for all the replies, and for your patience. It has taken me a few days to get back to this thread.

My excavator came back out and he agreed with you in that he should build up the pad some so that we end up with a more conservative "retaining wall". Our soil is mostly clay and not a lot of squishy top soil, so it will pack really well. He is going to build it up and then water/compact over and over until its solid, and then let it sit and gel for a while before we put anything on it. That will ensure we end up with a smaller "wall", and we can likely cover it in rip-rap and do a french drain at the bottom.

Here is an updated drawing to depict what I am envisioning. I understand your recommendation to do another retaining slope on the low side of the hill, @Rich D, but unless I use a gravel base, I probably don't need to do that, at least I don't think. I think I will end up with at least 2-4 feet of clearance and as long as my drainage is superb, I don't expect that clearance to decrease over time. If you disagree, please let me know as I can easily add a low-side retaining slope and implement some drainage to ensure water doesn't run toward the pool.

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