I'm working on a proposal that will require us to build a retaining wall from concrete masonry units, and I'm hoping I can get some help from the bright minds of the GTX membership on a few questions.
Here they are:
With no setback, I'm concerned about the mass of the wall being able to hold back earth. With that in mind, what size block should I use? Also, is there something I can use as a tieback for this kind of wall, similar to deadmen for a timber wall or geogrid for an SRW wall?
For reference, there will be two tiered walls, 3-4' in height each, with at least 10' separating them.
I have not built cmu retaining walls, but I have built quite a few poured in place concrete walls. The mechanism for both of these wall types to be successful is different than that of an srw type wall.
An srw is typically a "gravity" wall. That means that the mass of the wall (including the back fill added to it with geogrid) has to exceed the force acting upon it.
A cmu or poured wall is typically a "cantelever" wall where the wall works with its footing (with the help of rebar) to form an"L". The mass of the backfill sitting on the footing, or bottom of the "L", has to exceed the force acting on the wall, or vertical part of the "L". The taller the wall, the wider the footing (with the "L" facing the uphill portion, or course). The idea is that the wall can not fall over if the footing can not lift, so you know that you have to rebar that puppy in pretty good. The footing should be below the frost line.
However, you could use geogrid behind these walls just like you do with an srw. Geogrid does bind mass even if it is not "tied" to the wall material itself.
I don't like cmu retaining walls because I see so many fail. They are usually pretty old and have inadequate footings.
If you are only going 4' high, I would use 12" hollows, and pour the wall solid. You will not need any additional tieback on a wall that small. Use rebar in the walls in two directions and that wall will be solid. We are actually doing a wall similar in size to both retain a bank and act as a back wall to a new garage. Should be fun.
Excavate for a concrete footer to local specs regarding depth. In front of the wall excavate an additional width of 2'. Weave normally your spec'd rebar the 2 horizontal layers onto your spaced verticals.
The front excavation is for rebar bent into an L shape that goes from the front of the additional excavation, bends to the vertical plane with the other verticals, and is wired in. Concrete will have greater strength on the compression than on trying to hold something on top of itself. Your footer in cross section will be in a squat 'L' shape but with the leg to the front of the wall not back under the fill of the wall. Any slip pressure will be compressing forward onto the reinforcement rather than pulling at the attachment point and cracking.
I fill the voids of the cmu's with mortar or cement, but also backfill the wall similarly to an SRW, using clean 57's and also the drain pipe to daylight. PM me if you need any photos or drawings.
Here is a classic example of a failed wall. Cinder block on a good footer with brick facing. No rebar found in the mess. Also no stone behind the wall except for approx. 4" on top of a clogged 4" drainline. Wall was 7' tall.
The pool was put in 3 months earlier and as settling occured, it looks like the wall suffered a bad fate, just like the 2 sets of patio furniture and the gas grill.
Pool company says it was builders fault, builder says it was the pool company's fault and his homeowners insurance wouldn't cover it. He contracted us to fix his mess and we installed Keystone Compacs and used all stone behind the wall with a working drain system!
Wow - Lots of great information. Thank you for providing it. These folks are very particular about their new home and the materials used - they hate all SRW's, and find limestone outcropping walls to be a bit passé. Because they want an outdoor kitchen integrated with the walls, given the previously stated tastes, I can't really do drylaid, so CMU's, concrete and mortar are going to have to be the way we get this one done.
Treedoc, good to see you around.
Is everyone in agreement with Agla that the footing should be below the frost line? Seems like a lot of extra materials (42" is the frost line here), but I'll defer to those who know more about this than I do.
Also - would anyone recommend throwing some weep holes in the wall every X face feet? I'm thinking something simple like a cut piece of 1/2" pvc snuggled into a mortar joint. These would be in addition to clear, crushed stone and drain tile.
Treedoc - any drawings you'd care to share would of course be appreciated and valued. You can send them to jeff(at)groundtradesxchange.com.
This is a link for our county's 'typicals' diagrams for retaining walls. I've been told that we have some of the most stringent requirements so this might help as a guideline.
I've done a poured wall FPSF style without going to below the frostline. Used (IIRC) 2" or 3" polysterene insulation to insulate the earth around the footing.
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