“engineering” for 18-inch wall, DIY help
b3lbittner
| Posted in Construction Techniques on
I’d like to build a raised flower bed. It would be two 18″ high concrete walls, with 2′ of dirt between them. I’d like to make the walls 4″ thick, if possible.
I’d appreciate input on a couple of questions. Can I make the walls 4″ thick? Do I need any rebar? Do I need a real footer, or can I just place directly on compacted gravel?
Please see attached picture (if it works) for what I’d like to build.
Thanks for the help.
Replies
That's why they make retaining wall blocks.
Your project specifically.
Well, this is true. I thought about using them. But, I figure it will about 1/2 the cost (probably more like 1/3) to use concrete. And I want to learn how to do forms and I thought this would be a relatively small project to get my feet wet on.Thanks for the comment.
Then plan on rebar and footings, as well as some form of face detailing to cover up the bare ceement - unless you incorporate designs in the forms and color in the ceement.
Methinks you've not considered the labor and time factor.
Retaining wall blocks will have that project done in a few hours. And allow you to change design should you change your mind.
Don't forget to include irrigation and perhaps a landscape lite or two.
We just over the weekend went to a wedding at a 1930's concrete church. They appear to have formed it with rough sawn one by, and used the rough wood surface transferred to the concrete as a design element.
-- J.S.
you can make the walls as thick as you like really...if 4" is what u want then go for it...thinking about the size of the project, i would think that a nice level bed of gravel would be sufficient to support the small walls...footings would be overkill in my opinion...as far as the rebar, i would probably put some in it to strengthen it a little...nothing too serious...stick some vertical rods in every 3 or 4 ft, then i'd probably tie 2 horizontal horizontal rods to the verticals...maybe one at 6" above the ground and the other 3/4" below the top...i dont know how familiar you are with tying rebar...all u need is a little roll of tie wire...cut up a bunch of 3" long pieces with pliers...where the vertical and horizontal rods meet, u wrap the wire around both together, and then pinching the ends of the wire together with the pliers, twist several times around...as u twist the wire gets tighter...when you're done the horizontal 'hangs' on the verticals...for the rounds i'd use some 1/4" plywood, or really any other sheet material that bends...as long as you have some rebar floating in the forms when you pour the concrete, you'll be alright...every rod u put in will only help more...
as long as the form material is relatively smooth, and you make sure to compact the concrete sufficiently, it will turn out nicely...just using a stick, move it up and down in the concrete to help it settle and remove any air pockets...if there are any bad spots u can always patch them up later...it's a nice little project to practice some form work on...
good luck with it!
We've done exactly that for customers. Blocks, however, were 6" thick and were dry stacked on concrete footings with vertical rebar every few feet. We used open-ended bond beam blocks that are made specifically to be filled with concrete and with ledges that horizontal rebar rests on. They look like an "H" when viewed from above, with the middle/horizontal part of the "H" cut down depthwise to make a ledge for rebar to rest on.
We dry stacked the block; filling them with concrete acts like mortar anyway. The rough exterior was stuccoed and the whole thing was capped with bullnose brick. We even did a planter in the shape of a "U"; curves like you diagrammed are easy. Just cut a little off one leg of the "H" block. And stagger your joints as you build the walls.
I bet this costs more than retaining block type walls/planters, but I don't find them aesthetically pleasing to begin with. Not a huge fan of their cap systems either; just don't trust PL400 in the long run to hold that cap on.
Matasky's reply is also what I would suggest..I would only add some drainage behind the wall with some sort of weep holes thru the wall..Insert pices of 1/2" or 3/4" PVC piping into the forms every couple of feet. Backfill the planter with some gravel..cover that with filter fabric and then your soil..
Can I make the walls 4" thick?
Sure, concrete is an "elegant" material, capable of many, many things--jsut as brownbag <g>.
Do I need any rebar?
Possibly. As previously posted, 18" long #3 at 24" OC verticals and a continuous #3 8" up and 8" down would probably be enough while still leaving room to properly compact the pour. Bear in mind, I'm naught but a voice on the 'net, and no where near as qualified as a local engineer.
Do I need a real footer, or can I just place directly on compacted gravel?
That is the question that has been nagging me since last night when the connection crashed on me. Concrete is 140#/cf, you have 2 x 0.3 x (about) 170', or 102 CF; that's 14,280# all told. The bottom of your wall is 0.3 x 170. or 51 sf. Dividing into the 7 tons of wall, that's 280 #/sf. Will your soil support 250-300psf? I don't know.
You're buying/making close to 4 yards of concrete (102 / 27 = 3.77777), a 12" x 4" footer is only another 51 cf, a skosh under two more yards of concrete. That 12" 'foot' triples the contact area, which lowers the ground pressure a bunch.
But, that's jsut me, others differ.
Of course, we have no idea where he lives nor the effect of the freeze-thaw cycle.
But your post is elegant in how it describes the need for a footer.
And how perhaps the retaining wall block alternative could look better...
Of course, we have no idea where he lives nor the effect of the freeze-thaw cycle.
Too true (I want to remember OP is a "she" though--could be wrong, it's happened before).
Since it is just a low planter bed, it really may not need to be below the frost line--the footing can "float" where it is. Which is helped in that there's another foot-and-a-half or well-watered soil inside to help "even" any "heave" out.
And how perhaps the retaining wall block alternative could look better
I'd user a footing, but I'd use a mix of concrete block & brick; 4x8x8 & 4x8x4 to form that curves i nthe design, then parge it smooth. But, that's me, if I'm DIYing, I'd rather lay block & brick over lashing up single-use concrete forms.Occupational hazard of my occupation not being around (sorry Bubba)
i thought it was pretty obvious they WANT to do a little form work...they're not getting paid as far as i know, it's just a home project...
anyways, if you're familiar with form work you'd realize you wouldn't save much time by laying blocks one at a time...
thanks for all the input, everyone.trout, that book is on my wishlist, just haven't gotten around to buying it yet. i did pick up the concrete book that vatom is always talking about and another one by taunton. so, i've got some reading to do. thanks again.
if you're familiar with form work you'd realize you wouldn't save much time by laying blocks one at a time
I'm far too familiar with formwork <g>
And, there's no time saving, per se, in using block. For me, if I were doing it for myself, I'd rather invest my time in laying the block which becomes the finished product, versus making formwork which would be single-use.
That's my bias, were it my work, for my own place. If I were building a boat, I'd likely "glass" a wooden shell versus making a mould, too--for similar, if different reasons <g>.Occupational hazard of my occupation not being around (sorry Bubba)
You might like to check out the "concrete at home" book by Cheng. He talks about floors and counter tops as well as anything else you can possibly use decorative concrete for.
Don't be afraid of using color in the concrete as that can really add a nice touch if it helps fit the overall look of the area it's going into.
Have fun and just be careful of your forms--the concrete will exactly follow what you make.
Oh yeah, use rebar. For this type of project you'll find more of the 3/8" rebar is easier to work with than the 1/2". You might also want to use a water reducer and fiber reinforcement depending on what the final finish is going to look like.
Does it freeze where you live?
If so, don't do it. Water in the flower bed will shove the walls apart and the corners will crack. There are 10 or so of these around town & they are all cracked. The only way to avoid this would be lots of drainage, all gravel inside and flower pots sitting in the gravel.
Belgium block or some other attractive dry stack material would look better over time.