Assuming it has a good foundation, would a retaining wall built out of concrete block only 3 or 4 courses high need to reinforced or are the weight of the blocks and mortar enough to hold the wall together. Would this wall be easily knocked down by an earthquake. At what height does reinforcing become neccesary.
What are you retaining, what shape is the wall, and how long do you want it to last? Does the ground ever freeze in your location, and if so, how many freeze-thaw cycles do you typically get per year?
How big of an earthquake? …
Leaving the steel reinforcement out would be flirting with false economy … re-bar is cheap and placing it is quick and easy … and if nothing else, it will hold your cinder blocks in place … all lined up and even …
I haven’t pour a lot of concrete, but I’ve poured some … I just can’t remember ever not using steel … even stoop pads I’d throw a bit of steel mesh in … think of it as dirt cheap insurance …
Reinforcing for earthquakes is a whole 'nother kittle of fish … this is done typically by cladding the concrete … encasing the wall in steel … because during the earthquake, the concrete will crumble and fall away, thus bringing down whatever the concrete was holding up … by encasing it all the crumbles are held in place giving anyone above a chance to escape before the structure falls …
Unless you live in the Pacific Northwest … then don’t worry about earthquakes … when she comes everything man-made will be completely destroyed … your three foot retaining wall will be the leastest of your concerns …
Why on earth would you want to use standard CMU block designed for building construction in a retaining wall ? It is not the use they are designed for.
All decent masonry supply places will have the types of block specifically designed for retaining walls. Many different types, styles and colors. They all have a locking system to tie the blocks together and some do not need re-bar.
The best thing to do is go to the masonry supply place in your area and talk to the counter people and they will help you out immensely. Note I said Masonry Supply, that does not mean Home Depot or Lowes.
For a retaining wall, I would definitely use steel (both rebar in the voids and railroad track between courses) and core-fill with concrete. It’s not unusual for CMU walls to also have a concrete bond beam every four courses or so, but that’s usually for structural walls.
You typically want a few inches above and below the rebar, so I would say anything 6" or more you might as well reinforce. Why risk crumbling concrete unnecessarily.
I know about interlockers, but im not looking to spend a lot of money. Something cheap and effective would be fine.
But if you choose the cheap solution and the retaining wall falls after a couple of years and it has to be rebuilt, are you really saving money?
If he core-fills the webbing, it’s unlikely to fail, even in the long term. This is only at 3 or 4 course wall, which means at the most it will be 3’ tall, using 8" blocks.
I’ve seen these blocks used for 8 foot basement walls before … I have no idea why anyone would think it’s “not the use they are designed for” … they’re cheap and effective, just they look cheap but effective …
The designer blocks do tend to the expensive side, and though they look better … they’re not really better at the job at hand … if looks are that important one could always come back later and face the blocks with brick …
Re-bar and core fill … the wall will last a lifetime …
Sure, if he reinforces the concrete block wall as you suggest, he will probably be OK. But when he says he wants cheap and effective, that suggests to me that he’s not willing to do that. And reinforcing the wall properly is quite a bit of work, especially compared to using mortar-less interlocking concrete blocks designed for retaining walls. So even if the interlocking blocks cost a bit more, he might save time and money over the reinforced concrete block wall.
Depending on the length of the wall, of course. Sakcrete is cheap, and can be mixed in a wheelbarrow by anybody with half a brain. I’d venture to say that a wall that short would likely not even require rebar, which is the real expense in all this, not to mention getting it cut to length.
Has re-bar gone up in price, it used to be dirt cheap many years ago … meh … but typically where they sell re-bar, they’ll have a cutter and bender there and they’ll let you use it once you pay for the re-bar …
I would at a minimum have some re-bar sticking up out of the footing into the block voids … then plop enough Sakrete* down the hole to hold the bottom in place … but overkill is part of my game as it were …
- = Pronounced “secrete”
Regardless of the construction type you choose, and especially if you go something thats on the cheap and nasty side of the equation like you are proposing, its critical to provide appropriate treatment to the infill side of the retaining wall to prevent failure.
Depending on the soil type and climate this will differ. but assuming a moderately free-draining soil and no aggressive freeze-thaw cycles, directly behind the wall use a sheet of geotextile (or equivalent) and a geopipe type product (slotted tube) that will allow water that collects behind the wall to be funneled out. Also use some form of roadbase or gravel substrate behind the wall and surrounding the geopipe, to facilitate drainage and smooth out the angle of repose issues from the base soil. Collecting water behind the wall increases the stress on the retaining wall significantly if it is allowed to build up and is not removed.
if you just google image “retaining wall drainage” or similar there are hundreds of engineering type drawings. Also nearly every local council, roads/highway department and the like will have published typical standards for this stuff.
There are other key steps, like correct compaction of the substrate under your wall (whatever the type), including a slight backwards slope to the wall, the multiplying effect of terracing of retaining walls and the anticipated load on the top side of the wall - is it a driveway, person access, no loads at all, etc, that all come into play and affect how strong the design needs to be.
the cheapest walls tend to be precast “panel and post” retaining systems, and they are pre-engineered to 3-4’ off the shelf, so I’d look into them if i was you. even accounting for digging footings, the labour and materials cost will be far less than even an not-reinforced CMU wall.
if you live in an area where limestone is cheap, a bulk mass retaining system using 30kg/65lb limestone blocks can also be braindead to make, and very cheap.
Agree with Richbox, having built a few retaining walls over the years. With block, you’ll need to leave some channels so the water can get out the front- it would be easy to seal it up and trap the water that way.
My experience is that at 3 feet you should be fine with almost any construction, as long as you include a little lean in the wall (back, against the infill side)
Another cheap and easy construction that looks pretty nice is PT timber. Google for the tips and tricks, but it certainly doesn’t take an engineer. The only downside is that in my experience some PT lasts forever and some rots- I don’t know why. When I have done a PT wall I supplement with 2-3 foot rebar. I drill holes through the timbers and hammer in the rebar with a 4lb hammer, like mega-nails. I also like overkill
Concerning cheap you got two considerations here.
One is the purchase of standard CMU units, plus rebar, plus the mortar. You got the tools like a mortar trowel ? Add that in. How you going to bend and cut the rebar ? Then add in your time to do all that extra work.
Compare that to the cost of a lower price point block built to be interlocked for retaining walls and dry set with pins. There are basic ones built for this type of situation that are not a lot more than CMU block. It is the ones built for the look that are expensive.
I am thinking the 2nd option is the least expensive, but that is me looking at it.
Also make sure the base is level and is best to have a few inches of crushed rock.
Then advice by another poster of taking care of the back side is good, ya gotta have a way to let the water out from behind the wall.
Either way, we’ll still need a footing, and that may well be a considerable part of the cost of this retaining wall … do you live in Minnesota? … if so I hope you’re not afraid of a shovel …
I think OldOlds has your best economy … stack up four courses of railroad ties … get a four foot long drill bit and drill vertical holes every couple feet … then drive a six foot length of re-bar down … no footing, no weep holes … and no mixing up 90# sacks of Sakrete for hours and hours and hours and hours … (I wouldn’t do this, I’d call for a mixer and hire a grout pump before I’d mix that much Sakrete … experience talking here)
This will last a minimum of twenty years … longer depending on climate and soil type … so don’t forget to include replacing the wall in your cost/benefit calculations … and try to get either new or slightly used railroad ties … just don’t pick through the railroad company’s rejects …
For a footing on a short PT, I’d probably still dig down a couple feet, drop in some gravel, and make sure the first course was partially buried. I’d feel better about drainage, though with timbers freeze/thaw shouldn’t be a big deal. But then, as I’m not a pro, I like to be conservative