can i build a block retaining wall on top of a concrete slab?

I've searched around a bit, but couldn't find an answer to this one. My back yard is essentially a concrete jungle with a pool in the middle and it sorely needs some greenery. I have a large back left corner (probably 20x30ft) that I'd like to be grass and plants. Demo'ing the concrete is quite expensive, one quote (on the high side) was $2,500!
A friend mentioned putting in a large planter box ON TOP of the concrete, and that this would be sufficient for grass and light plants (no large trees or anything). Of course I'd want to put in some drainage and even automatic sprinklers. I figured I'd use some nice retaining wall blocks (e.g. Allan Block), perhaps 3 high (or ~12" in height) so that it looks nice and finished. The back of the planter box would be against a typical wooden fence that separates the yard from the neighbors. I also thought I'd install some nice pressure treated planks of wood along the bottom of the existing fence to the same height as the planter box to protect the fence against moisture, rot, etc. and make it a more permanent installation.
So that's my plan in a nutshell. I think with 12" of soil grass/sod will be no problem. Small plants should be fine too. I believe the retaining wall blocks are meant to be used on dirt, and that the bottom of the first row is usually partially (a few inches?) below the dirt, naturally this keeps the block in place. However on concrete this is not an option. I'm concerned about the bottom row sliding out since it would just be sitting on top of the concrete. I've entertained ideas of drilling holes in the concrete and hammering in some kind of anchor, such as rebar in the middle of the blocks to keep them secured. Seems like it would be a lot of work though 'cause I'll probably have 30-40ft of wall.
So, can I use retaining wall blocks on concrete? Do I need to secure them, or will the weight of the blocks plus gravel inside them keep them in place?
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Where are you? Do you have frost and freezing conditions in winter? This can make a big difference as frost can move anything that is not anchored. If no frost I cannot imagine a 1 foot high wall will move much, but I would use the approved adhesive to bond the bottom row to the concrete.
Also do not hold up the dirt with a fence. Even with some additional pressure treated wood added, it will speed up the deterioration of the fence causing it to decay. Any pressure from the dirt can start the fence to lean away from the garden especially if the wind adds pressure and the posts are not anchored deeply. Install more blocks at the back of the garden. Use cheaper ones as they will not be seen.
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California bay area, so no frost issues to speak of. Do the block companies typically recommend an approved adhesive for this application?
Thanks for the info.
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. Sometimes referred to as mortar. Think about putting down an occasional block at right angle to the others rather than all in a row. This would be a buttress to gain vertical stability. HTH
Joe
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Demo'ing a concrete slab is not that difficult. Labor intensive yes. I removed a schoolhouse footing/kneewall foundation (20'x50') using nothing but a sledge hammer and a 8' iron 'digging' bar. Took me one summer. Footing was 6" x 14" verty solid, kneewal 8" x 3' also very solid.
Drillign the holes for rebar is the way to go and is not difficult or labor intensive. Rent an electric rotohammer - the big pro job. One afternoon should do the entire job. Laying out the location of the holes so they will line up with the block holes is the critical part.
"gluing the blocks down" using some type of epoxy or just mortar is dubious as the existing slab will have to be almost surgical clean for it to adhere well.
Harry K
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The simple foot high block wall that OP wants doesn't need all that reinforcement. Back in Physics 101 we learned that hydrostatic pressure is related to depth, etc., etc. so the lateral forces are actually rather modest. Keeping it uncomplicated will get it done much faster..

Any mason will confirm that mortar works just fine on weathered concrete. After all, the cement blocks that you buy have probably been out in the weather for many months and they have no mortar adhesion problems. Cementing with epoxy is certain;y a robust bond, but in this case an expensive overkill.
Joe
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A few comments / questions
you'd be creating a very large "flower pot"...where's the water going to drain to? puttting soil against a fence, even just 12", is a bad idea. The treated timber, if placed against the fence, will retain mositure & rot the fence.....you'll need another run of blocks along the fence but spaced off a bit.
The blocks & the soil you need to add are not free. Get another demo / removal quote, $2500 is nuts. Dont you have a concrete recycling operation nearby? You have about 13 tons. Even going with two lowboys (the most expensive disposal means) I think a number closer to 1/2 of the first quote is more likely (but still higher than a remove & recycle quote).
If you really want to "cheap out / poor boy" the job........ break up the slab by hand & dump it in the trash 250 lbs per bin (if you have hydraulic lift arm trash trucks). Ask you all neighbors to use their unused trash capacity and get rid of it a lot faster. Or just pile it up & toss over the next year or so.
Use a cheaper (cheaper than epoxy) waterproof "concrete" adhesive to bond the first course if it would make you feel better. But IMO its not needed, the friction between the blocks & the slab is sufficient to keep them from sliding.
My suggestion is demo / remove the slab and be done with it.
cheers Bob
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Thanks for all the input guys. Did get another quote for about $1,100 - so I will probably go with that.
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Just rent an F-ing hammer drill, drill the holes, and anchor with rebar. It's not that much work, especially with a hammer drill. Two holes per block, even 40ft, might take you two hours tops. The retaining blocks already have holes for fiberglass rods that they normally use to tie the block layers together.
Otherwise the blocks are just going to slide across the concrete.
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