Putting a drain pipe through a retaining wall

[plowhog]

The fabric should go against the dirt behind the wall, not around the pipe. And then you don't really need the pipe.

Thanks.

Is there a recommendation on a type of fabric that is less prone to clogging?

 

[dstig1]

Any decent landscape fabric will work. You don't really count on it filtering water too much, though some will work through it. The main purpose is to keep dirt from plugging up your rock bed behind the wall. If that remains open, water cannot accumulate and freeze and expand. So it is more of a barrier to the soil than anything.

Also I should note, you will want to put the fabric down early and then backfill with rock for each layer as you go. Otherwise it is harder to stack blocks and then come back and fill at the end. Plus if the wall is tall enough, the higher sections will need to sit on that backfill, at least partially.

 

[plowhog]

fabric down early and then backfill with rock for each layer as you go.

Regarding backfill, a couple of questions.

- My first course is below grade. What material should I use to fill after this first (below grade) course is added? Only for this first course, I have to add material both behind and in front of the blocks. What material do I use? Choices:

- Gravel. Seems no, as having gravel below grade would seem to defeat my drain pipe behind the wall (which will be sitting on grade.)

- Natural soil, then compacted. The drain pipe lays on top of soil backfill and is then covered with gravel.

- Base material, then compacted. The drain pipe lays on top of the compacted base and is then covered with gravel.

I believe doing backfill along the sub-grade first course "locks in" the block positions. So is it better to use soil for that, base material, or does it matter?

Thanks!

 

[orezok]

I don’t know exactly what you mean by stacking stones, but a possible alternate if the stones will allow is to provide weep holes. At the first course above grade leave a 3/4” space between “stones” every 10’ or so. It allows the water to drain through the wall.

 

[plowhog]

I don’t know exactly what you mean by stacking stones, but a possible alternate if the stones will allow is to provide weep holes. At the first course above grade leave a 3/4” space between “stones” every 10’ or so. It allows the water to drain through the wall.

They are manufactured stones designed to stack atop one another. One version has an added lip in the back, and as you add each course the wall moves slowly back toward the dirt or hill behind it.

My version has no lip and they stack vertically. There are two pins near the center of each stone that allows connection from the block above to the block below.

These stones are designed to allow weeping of water, which I am trying to minimize as it would be weeping across a gravel driveway. That is the reason for a pipe behind the wall to catch most of it and pipe it away.

 

[2manyrocks]

IMO, pipe is cheap relative to the cost of labor and other materials for this project. In our clay soil, it was important not to excavate too deeply and disturb the soil. It was better to dig only deep enough to lay down the filter fabric, put down some gravel in which to embed the drain pipe, add more gravel to cover, and then wrap the whole gravel/pipe with more filter fabric to keep sediment out of it. The best description was that we were making a burrito.

Also, very important to make sure that the pipe is laid with the proper pitch, avoid sharp bends, and be sure that there is an adequate outlet for the water discharge. There is actually some precision required for a good installation if you want it to work well.

FWIW, the so called filter fabric I see at home centers is nothing near the strength of the filter fabric used for road bed construction. That stuff is so tough it's hard to cut. I bought some from a local road building contractor the last time I needed some.

 

[dstig1]

Fill gravel at the bottom too. If you use expansive soil and it freezes, it may move your foundation and you don't want that. The space the stone fill creates is your freeze buffer, so you want it all the way down. And compact it as you go. A jumping jack works well for this. The stone will fill in up to the soil behind at the back of your trench and support the wall from moving just like soil would but without the freeze expansion risk.

 

[Streetcar]

You want a non woven filter fabric for the wall. They make various types and grades of fabric. Make sure the fabric you buy is water permeable. Backfill entire depth of wall with uniform sized gravel

 

[Rockbadchild]

You want the water to run away from the wall and a place for the water to flow too, the illustration is how I would personally do it, now I am not sure if you want pure stone below the wall or not but I would think it be a good idea although I am not sure if I would or not. Compacted gravel like 3/4 minus is pretty impermeable so the pipe can be on that (actually better to be on gravel) but the grade should go up from there to the base of the wall like the illustration below. Now only pure stone with a permeable textile would be fine as well as long as you have a permeable textile on top... pure stone has 30% void within it but flow grade is paramount.

 

[dougtrr2]

I built one small wall using fabric. Then I saw a video by a landscape contractor vehemently criticizing the use of fabric. It got be to rethinking the use on my second larger walls. I think landscape fabric is designed to lay on top of the dirt and let water through. If it is under the dirt I think it might clog up and act as a barrier and not pass any water.

And that is the problem with the internet. You can find someone to justify whatever you decide to do.

On my second wall, no landscape fabric, back filled with 2 feet of gravel, sock covered pipe at the bottom, several levels of geo grid. After a rain I evidence of water weeping out of all levels of the wall. I think that is a good thing, to me it means the water is not trapped behind the wall.

Doug in SW IA