Question on a retaining wall: I want to build a 6' retaining wall poss. of cinder block. How deep? Tips?
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Doesn't this also depend on the amount of weight the retaining wall is supporting?
Retaining walls which exceed four feet in height often are engineered. They require special footers, specific wall thickness, and the prolific use of steel reinforcing. Many residential basement walls often have seven feet of backfill against them! However, there is a difference between a free standing retaining wall and a residential foundation wall. A residential foundation wall has two things that help it structurally. The basement slab and the wood subfloor assembly which is bolted to the top of foundation walls are important structural elements. The basement slab stops the bottom of the wall from sliding inwards, while the subfloor assembly helps to keep the top of the wall from tilting inwards.
However, these two elements are not enough to totally withstand the forces of soil and water against a poured concrete or foundation wall. These walls must be constructed in such a way as to resist the forces which will be placed against them. To further complicate the issue, no two building sites are exactly the same. For example, a foundation constructed at the bottom of a hillside will have greater forces against it than a foundation built at the top of the hill. A foundation which will last 100 years in areas of little or no seismic activity might crumble in a moderate earthquake in southern California. All foundation system plans should be checked for the particular building site by a competent structural engineer!
18"
Retaining walls are tough items to get right. Concrete block can work but you need a good concrete foundation, vertical reinforcement probably in every other block core, and a bond beam on top.j I would go with reinforced concrete with a foundation the meets the overturning forces, and make sure water is not held behind the wall. If all this sounds expensive and difficult, it is.
a six foot retaining wall requires 50% or three feet below the soil line and dont forget to use the proper retention bars, and extras for good measure.
What's at issue here is what you're trying to retain, a main concern being water in the ground. Also does six feet take you just PART way up a higher slope -or does it get you basically to the top? If you were to use massive landscape block or precast concrete sections -installed by a crane of with the bucket of a front end loader (used as a crane) is would not matter -the mass of the wall being adequate to hold the fort, as it were.
It sounds like you are opting for block because it is manageable by one guy -light enough to lift, place and mortar together. If so, use the heaviest block you can manage, and:
1. Start the wall in a trench, dug down to very firm soil -at least six inches, and put a concrete footer on top of gravel (washed stone). Depth of footer should be at least 6", so the bottomyour first block will be sonewhat BELOW the soil at grade (level).
2. The wall should be at least a foot in front of the point where the slope meets level ground.
3. Place landscape fabric behind the wall, and on top of that lay perforated drain field pipe (4" black plastic) sloped right or left to carry ground water to some discharge point.
4. Over top the pipe, and in layers as you go build up, back fill with gravel. If you looked at the construction "edge-on" you would see a V shape, narrow at the bottom and wider at the top, filled with gravel between the back of the wall and existing slope. At 5 feet or so, another layer of landscape fabric, then dirt so you have a place to plant flowers, grass or whatever.
5. Tilt the wall slightly toward the slope so the top of the wall is perhaps 6" or a foot more toward the slope then the bottom -like a dam. It is, after all, a dam. You can do this by making the mortar under the first layer of block somewhat thicker in front then in back.
6. Put mortar in the cells of the block to give them additional mass and a more solid connection from layer to layer. You may also drop in rebar, through the cells.
7. At the top, cap off the wall with solid cinderblock caps or stone, secured with construction adhesive to the top row of block.
Sounds like a project to me!
I would definitly pour a footing below the frost line. Make sure it is nice and level. It will make placing your retaining wall blocks a breeze. I hope your using the stackable kind. I wouldnt suggest mortaring in cinder blocks. I have seen people really mess that up. The dry stack ones are really easy to do. put your first row on the footing then you put these pins in each block. once you get to the top fill the cores with pea gravel and behind the wall with pea gravel also. That locks everything in. then you glue your caps on with construction adhesive. It goes pretty quick. good luck
You probably want to go 3 feet into the ground to hold back 3 feet of earth above, but you have to put enough re-bar in to keep the wall from deflecting. Also of great importance is making sure that water drains from behind the wall so that it can't build up pressure. Back filling against the wall with gravel and putting a french drain or drains through the wall at regular intervals will help.
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