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made-in-america 08-12-2012 07:56 AM

Retaining Wall/Shed
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I am looking to build a concrete block retaining wall perpendicular to my home. I have a walk in basement with a slope that is graded to the basement door. Extremely heavy rains bring the muddy water through my door into my basement. I have knocked the hill down and back about 6 feet from my door and have a 4' high "cliff" of dirt/ I want to put a concrete block retaining wall here now, BUT was thinking of pouring a 6'x12'x6"thick floating slab and building concrete block walls on the 12' and the 6' edge to make a small generator shed and wood storage area. (dual purpose retaining wall). With the frost line at 36" depth in Connecticut...Is a footing recommended for this "small" structure...or can the walls be built on the floating slab. There is a large deck over this, hence the 10" diameter sonotube in the corner. I am planning on perforated pvc tubes behind the 12' edge of the wall and plenty of pea stone (about 1' wide and 3 feet height). Any help/ideas would be greatly appreciated.

Daniel Holzman 08-12-2012 08:18 AM

I built a similar segmental block retaining wall under the deck in my backyard in front of a basement door. Mine was a little shorter (three feet), but somewhat longer (about 20 feet).

Couple of things you should consider. First, you need to use segmental block, ordinary cinder concrete block does not work as well because it typically lacks a system for locking the blocks together. Segmental block walls use one of several different methods,including concrete splines, pins, or steel dowels. The system aligns the blocks as well as restraining their movement.

Second, the backfill against the wall must be granular for at least half the height of the wall. In other words, if the wall is 4 feet tall, you need at least two feet of granular (sand, gravel) backfill against the wall. If you use ordinary soil that is high in silt or clay, you will get high pressure against the wall, and the wall will bow and could fail totally.

As to the foundation, segmental block retaining walls typically do not need a concrete foundation, usually they just go on natural mineral soil.

KevinPh 08-12-2012 10:08 AM

In an area prone to winter freezing conditions, the wall is usually poured separately from the concrete slab, with an expansion joint in between the two, filled with bituminous material. This will allow the wall and slab to expand and contract with the changing temperatures separately. You don't want movement from the wall to crack your concrete slab.

Walls that are retaining soil and are more than 2 ft. in height require footings. Make sure to backfill with 6 " width of clear aggregate up to 4 inches from the top of the wall. Since your wall will be 4 ft. in height, I would go down to the frostline. Make sure you install a PVC permeable pipe along the length of the base of the wall that outlets to a clear area, and don't forget weeping holes. Clay soils retain water and put a lot of pressure on a retaining wall, especially one as high as 4 ft.

made-in-america 08-12-2012 10:32 AM

Thanks for the quick response...Looks like I will be going 3 more feet down for the footings(arrrrgh)...Gonna turn into a longer and costlier project than I had hoped BUT...I hate rework...gotta do it right the first time:yes:

concretemasonry 08-12-2012 10:52 AM

I don't know where the arbitrary 2' requiring a footing comes from. It is poured concrete or reinforced block, a footing is required.

Segmental retaining wall block (SRWs) do NOT use mortar and NEVER use a concrete foundation. In fact the manufacturers state this in their literature and installtion instructions. Usually, if the are higher that 4' or 5' they are usually required to be engineered. The first course of block (usually 8") is halfway buried and set on a compacted gravel base. I have at least 10 SRW walls within 2 miles of me that between 1' and 20' high and none were built on concrete footings and most are curved and varying heights to match the contours of the surrounding soil.


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