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Old 08-04-2010, 06:55 PM   #16
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Dry-stack concrete block retaining wall?


Rusty -

If they had up a sign and warning, they were aware they were not a good block producer and should have known that could make a good block.

If they used the real Haydite aggregate since many producers have used the real Haydite making units that meet all standards (ASTM is the main one) including being listed in the UL standards. There are many othe good manufacturers of lightweight aggregates meeting the ASTM requirements (ASTM C331) that are also used in premium masonry units that are used because of the benefits. While with a MN block producer, we used another manufactured lightweight aggregate that was shipped from Louisiana up the Mississippi because of the benefits.

Dick

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Old 08-04-2010, 09:05 PM   #17
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Dry-stack concrete block retaining wall?


Concretemasonry is correct in pointing out the critical difference between a rigid, gravity retaining wall and a segmental block retaining wall. You would be building a rigid wall if you mortared the blocks together and used reinforcing rod to keep them together. You would be building a segmental wall if you stacked blocks on top of each other without use of mortar or any other reinforcing.

Here is the critical difference. A rigid retaining wall fails either by sliding, tipping, or by foundation failure. It acts as a unit. A typical rule of thumb for a rigid wall is that the base should be approximately 40 percent as wide as the retained depth of soil behind the wall. If you place 4 feet of soil behind the wall, you should have approximately a 16 inch wide wall at the bottom. Your 8 inch block is not wide enough.

The actual minimum width depends on the unit weight of the soil you are placing behind the wall, the water table behind the wall, the face angle of the wall, the slope of the soil behind the wall, and the friction angle of the soil. It gets complicated very quickly for rigid walls. A rigid wall absolutely needs a strong footing, typicall concrete.

Segmental block walls operate on a completely different principal. The block itself is designed to prevent fill behind the block from coming loose. The block in actuality has no significant structural role, the soil holds itself up due to its internal friction. When a segmental block wall gets more than about 4 feet tall, the backfill is strengthened using geogrid, typicall fabric or occasionally steel grid.

Segmental walls do not require a rigid footing, in fact typically the first course is placed on a 6 or 12 inch thick layer of crushed stone or occasionally even sand. The backfill behind a segmental wall MUST be free draining material such as crushed stone or sand for a distance specified by the manufacturer. Segmental walls require good drainage at the base of the wall, typically supplied by drain pipe.

The advantages of segmental walls are lower cost, faster construction, and good appearance. Concrete walls and rigid block walls are typically used where it is impossible to place geogrid behind the wall to strengthen the soil, or in applications where it is impossible to drain the backfill, making a segmental wall impractical.
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Old 08-06-2010, 07:53 AM   #18
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Dry-stack concrete block retaining wall?


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Originally Posted by Daniel Holzman View Post
A typical rule of thumb for a rigid wall is that the base should be approximately 40 percent as wide as the retained depth of soil behind the wall. If you place 4 feet of soil behind the wall, you should have approximately a 16 inch wide wall at the bottom. Your 8 inch block is not wide enough.
Thanks for the info Daniel. I'm not putting 4 feet of soil behind my wall, only 16 to 24 inches. The grade behind the wall will actually slope down and away from the wall. So I would think the 8" wide footing might be enough, although just barely. Also, it's not soil behind the wall, it's crusher run, which does not slump as badly as most soils.

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