Enclosed Retaining Wall Question

[Mike Goetzke]

Built this two-step retaining wall for my wife this weekend (two of the hottest days on record in the Chicago area). She wants plants in the front area and mostly decorative things in the taller back. I'm wondering what to fill the back area with. I'm afraid if I fill it with topsoil only it will settle quite a bit - maybe half gravel / half topsoil (back section is about 20" tall)?

386717

Thanks

[Jim Koepke]

You only need soil as deep as the roots of what will be planted in the deeper area.

Gravel and sand will provide drainage. Then use a bit more soil than the depth of the roots. If it settles, add more the next year. Does your wife like to cook? Herbs can be decorative. Some are perennial some are annual.

Go deep with the soil if you want to grow tomatoes or squash.

jtk

[Lee Schierer]

If you put gravel on the bottom, be sure to put landscape cloth under the gravel and als on top. This will prevent the gravel from settling into the soil beneath and it will keep your new soil fill on top from working down into the gravel.

I saw a show on TV where they filled the bottom half of a planter like that with the styrafoam packing pellets and then covered those with landscape cloth and soil. Their thought was the pellets would last for decades.

[Bill Dufour]

Forget the rocks and just use dirt all the way. Gravel has no advantage over good soil. yes it will settle just add more soil each spring. If she changes her mind in a year do you want to have to dig out all the dirt, remove the gravel and pay to get rid of it, then add more soil? If you are really cheap add empty soda bottles as fill. Just not the glass kind like the DPO of the house I grew up in.
years ago I read about a retired nurse who died. The new home owners found out she brought home medical waste like plastic tubing, syringes and empty plasma bags to fill up behind her retaining walls.
Bill D.

on edit: I have never lived where I had to worry about ground freezing.

[Mike Goetzke]

Thanks for the hints/suggestions. (Oh boy, looking at that photo I really need to power wash that fence!)

[Mark Bolton]

If the retainers on the taller section are just lip stacked or glue bonded I wouldnt fill the entire thing with soil. Id be filling all but the top 18" or less with rubble of some sort (coarse interlocking), a couple layers of fabric, and then soil. Id be too concerned with freeze/thaw with soil alone and additionally if you are not able to retain a good bit of moisture in the soil in that high area your plants may need to be heavily watered or very drought tolerant up there. The water will drain out of that high portion like a sieve and then in the winter the lower will freeze and trap moisture in the high soil area which could give you some issues with the block.

[Mark Bolton]

In additon, remember that lip stacked blocks used in curved/radius situations get tricky in that the curves create a situation with the lip is only engaged on one end of the block. You can see this in the back wall. This means that internal pressure (weight of soil/freeze thaw) acts on each block making it "pivot" or "twist". The lipped blocks usually have a maximum allowed concave radius which is a radius small enough to keep the lip engaged with the courses below but this means cutting the blocks on install. If you look in your photo there is a diagonal block stagger on the back side that is only engaged with the lip on the left side in the photo.

If the rear lip is not fully engaged with the course below the strength is limited to the friction of the faces of the block from course to course. This is why I wouldnt fill completely with dirt/soil.

Sadly the home centers and even block yards dont really pay attention to much. If you look into any of the lipped block retainers your maximum height without tie-back mesh or some other solution is often 2'-2.5'. While this isnt an application holding back a home or a pool its still a lot of work that you hope will last.