Building retaining wall next to existing slab

Old 02-22-15, 10:42 PM
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Building retaining wall next to existing slab

I have a 25'x23' rectangular area that is sloped (lower end is ~6' below the upper end), and I'd like to level it to build a play area. My idea was to put two 3' retaining walls, one at the upper end, one at the lower end (and some at the sides).

The lower end (and the sides) have good access, I should be able to build the wall there. But the upper end of the existing slope continues into a large concrete slab. So how do I build the upper retaining wall?

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If I build the wall so that the top is right next to the slab, then I won't be able to insert drain rock and drain pipe, or backfill behind the wall because I'll have no access from the top (the slab is blocking me). So I have to leave some space. How much do I need to be able to backfill and sufficiently compact the dirt, so that the support of the slab is not compromised? I don't want to lose too much space from the play area, but I don't want to have the slab crack and start shifting, either.

I had a wild idea on how to avoid having to leave any space at all. Can I just dig a trench right next to the slab, put in a drainpipe with some drain gravel, then pour in a gravity wall so that the back of the gravity wall is the trench itself? Is this going to support the slab well enough? Is this going to be able to drain the water away, given that I won't have the full height drain rock channel next to the wall?

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Some real photos below.

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Old 02-23-15, 06:33 AM
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What do you plan to do with the substantial trees in new "play area"?

The root and trees will only get bigger. The roots will especially be a problem initially for wall construction, but in the future they will continue to grow and try to change the wall (crack). It looks like the existing slab has already started going through the beginning of the distortion. - Each wall would only be 3' high so no engineering is required and soil reinforcement is NOT necessary behind the walls.

You might want to consider segmental interlocking retaining wall block the are never laid on a concrete footing (compacted soil only) since they are meant to create a "flexible" wall. It is not allowed or recommended to use any mortar. The open joints in the wall allow some moisture to naturally be released ocontinuously.

Old 02-27-15, 08:41 PM
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Concretemasonary has a great idea about using these segmental interlocking retaining wall blocks. It will make constructing the wall much easier and also provide some flexibility as the trees grow etc. What kind of railings are you planning on putting on top of the retaining walls? Code will require some kind of railings for that much of a drop.
Old 02-28-15, 06:19 AM
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Check with your local codes but you will have a patio right next to a 3' cliff. In many areas that will require a railing. In my area 24" is the limit so if your lower grade is 23" below the patio you can get away without a railing.

I think the tree is going to be an issue with any wall. Excavating for the wall and it's footer is going to get into a major portion of the trees roots. Is that species very tolerant to being disrupted? Is there anything that would be damaged when the tree came down in heavy winds since you'll be removing most of the roots on one side.

A interlocking block wall is a great fit especially since it looks like you will have limited access to the area, making pouring concrete difficult or expensive. In general I think of a wall taking a 3' wide trench/work area for the footer, wall and backfill. Since you'll be working close to your existing slab picking the correct season will be critical but plan on working fast while you have a sheer face at the edge of your patio. If it collapses there goes the support for the concrete.

Another option would be to cut back your concrete pad and locate your retaining wall further away from the tree. Then after the wall is built you can pour to concrete the area between the pad and wall, then place your plastic decking back to conceal the different appearing concrete. You could preserve the tree but you'll have to keep in mind that trees are pretty but are great a destroying man made structures like footers, sidewalks, retaining walls and patios.
Old 02-28-15, 08:31 AM
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If you place any amount of soil around the tree trunk the tree will die. Seen it happen many times. I think Pilot Dane has the best solution. read what he wrote and follow his advice!

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