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I am building a playground area, and as part of having the ground level, I ended up building a retaining wall, and I have a question about drainage. Some data on the project:

  1. The wall will be retaining 12 inches of soil in a sloped area.
  2. The lateral sides of the area, start at 12 inches, and slowly gets reduced until it meets the leveled ground (as you can see in the picture, 12 inches, 10 inches, 8 inches...)
  3. I am planning to build the wall with landscape timber.
  4. The wall will be acting as part of the border of a playground. It will be a rectangle of landscape timber.
  5. The playground area will be filled with cedar wood chips.
  6. The playground area is leveled. That's why I did the retaining wall to start with. So, the playground area doesn't have a slope (we are installing a swing playset).
  7. I live in Seattle. Not a lot of storms, but constant rain.

I have a couple of questions:

  1. Is it recommended to add any type of drainage behind the wall given the height/context of the project? And if so, what type? Gravel, Gravel+Pipe? If I do piping, where should the pipe output go to? At the end of the slope I have my neighbors fence.
  2. Should the drainage also be added on the lateral sides of the playground?
  3. Would it be enough with just covering the soil with some landscape fabric, put the timbers on top, and then just covering with the cedar wood chips?

Here is a picture showing a part of the area. The other lateral side looks just like the one shown on the picture.

enter image description here

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  • Is your soil fairly free-flowing (ie granular) or is it something that holds/impairs water, like clay?
    – Greg Hill
    Oct 10, 2022 at 15:46
  • Greg Hill, is hard for me to tell. Do you know of a good exhaustive test to figure it out?
    – Nobita
    Oct 10, 2022 at 15:55
  • A "percolation test" is done for applications that really matter (drain field of a septic sewer system for instance). It's overkill for your short wall though. Here a simple "it's pretty sandy" or "the clay is awful, it sticks to my shoe and I get 3 inches taller when I walk on that stuff while it's wet!" will do.
    – Greg Hill
    Oct 10, 2022 at 18:57
  • Greg, is mainly clay.
    – Nobita
    Oct 10, 2022 at 21:53
  • The easy solution (if you have room, and it appears that you do) is don't build a retaining wall, just build a slope instead (the technical term for this slope is a batter). the only downside it you'll have to mow this awkward slope.
    – Jasen
    Oct 11, 2022 at 3:38

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If you want a sharply walled area, presenting a clear divide between the lawn and the cedar chip play area go ahead and make your wall, but give some consideration to what it is holding back, and what is going to keep it in place.

  1. No. You could add drainage, but the main purposes of drainage in a retaining wall are to reduce pressure on the wall due to the mass of water pushing on the wall, reduce soil movement (erosion) that could undermine the wall, and soil movement (liquefaction) that could compromise the structure. For a 12" wall the pressure from the mass of water and soil is not large, and with clay soil the water is simply going to flow over your wall rather than under it. If you do add drainage, it comes out under/through your wall, generally to the flattened area but can be anywhere conveniently farther down slope (note if there are city ordinances about directing water onto neighbouring properties - you may have to be a certain distance within the property line).
  2. No. You could extend the drains, but it would be even more overkill than point 1. The shorter slopes will have even less pressure.
  3. No. Here we get to the real crux of the matter. If you just lay the timbers on the dirt and against the dirt wall, what is going to hold them in place? Nothing, so they will be free to move, tip over, in colder climates they would heave in the winter, etc. so you must give consideration to what this means for your project. In points 1 and 2 we talked about the water pushing on the timbers, but also the dirt itself will be pushing on them, people walking on them, etc. that will tend to push them up (dirt settling below them) and down slope (dirt settling behind them). You will want to look at pinning them into place (basically wood or metal driven deeper so that the timbers cannot move without the entire pin also moving). My suggestion would be an earth cut, which just means you dig a trench deeper than the wall itself so that soil on the lower side holds the timbers in place against the pressure from the upper side (e.g. a 24" tall timber with 24" of dirt for your lawn, and 12" of dirt for the cedar chips). There are many many variations on how to pin the wall in place so that it does not move.

But in the end, it's just a 12" wall for a play area. You don't need to overthink it or overdesign it. A few timbers without drainage and with minimal pinning may start to move over time, but your house isn't falling into a ravine... as the kids get older the playground might no longer be a playground, or you may need to dig back the wall to reposition the timbers and refresh the chips. It's not a landslide.

The simplest option... grass makes for a great erosion control system to stop soil movement. Cut back that slope a bit so that it's mowable, plant some grass or sod, and call it done.

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  • Hey Doug! Thanks for the answer. I was planning on pinning the timber down the ground with rebar. I have a question: with your solution, do you suggest that the timber is touching the ground and the dirt wall directly? I was planning on setting them on top of some crushed stone, and leave a space between wall and timber and fill it with round rock. You are suggesting to just put the timber against the dirt?
    – Nobita
    Oct 21, 2022 at 21:58
  • Rebar will work, but it is thin so it is easier to push through the dirt than say a 4x4. Basically you are pinning the timbers into the dirt to stop them from easily being pushed down slope, so you want whatever you use as pins to be as deep, thick, and solid as practical. A 4x4 in concrete is great! But you don't need to overthink it... you aren't holding back a mountain and the effort to readjust in say 5 years is minor anyway.
    – Doug
    Oct 22, 2022 at 19:37
  • I assumed you were putting the timbers directly on dirt. Which works fine, just use pressure treated timbers, preferably ground contact rated. Adding the gravel for drainage is better to make those timbers last even longer. Combining that with landscape fabric or drainage sock is even better so that the dirt doesn't migrate into the gravel. It's really about planning your quality and effort to fit your desired goals. E.g. are you aiming for 5 years, or 20 years, then build to those goals. For a kids play area I would do less, for a garden shed I'd do more.
    – Doug
    Oct 22, 2022 at 19:48

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