# Dry-stack concrete block retaining wall?



## BRO931 (Jul 12, 2010)

I want to build a small retaining wall maybe two or three blocks high (16-24 inches). It seems to me that I could just stack regular gray blocks and fill them with concrete to keep the whole assembly together. The wall would have two 90-degree bends with about 30 feet in between, so the concrete should be strong enough to prevent tipping at the center of the wall, no?

Anybody ever try to do something like this?


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## Scuba_Dave (Jan 16, 2009)

Where are you located ?


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## Gary in WA (Mar 11, 2009)

No mortar? Only for college dorm bookshelves. 

Be safe, Gary


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## Daniel Holzman (Mar 10, 2009)

If you use unit masonry blocks designed for walls, you can build up to four feet high with no mortar, provided you use appropriate granular backfill and make sure you have a drain pipe at approximately footing level. You can get away with ordinary concrete block, however they typically have more issues with drainage, and you have to be more careful about installation since they lack an index point that automatically sets the angle of the block slightly less than vertical.


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## Bud Cline (Mar 12, 2006)

Gonna need a footing.


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## msk (Aug 1, 2010)

look into segmental retaining wall block. Anchor diamond pro is a good 8" block. Till you pay for the block and concrete, they wont be much more. They'll save some labor and mess as well. At 30' (+ whatever distance on the other side of the 90s) you only need 60-90 linear feet. Talk to a local landscaper that does that sort of thing. They're likely to have a bunch of partial pallets they'd be glad to sell you at a discount. Regardless, make sure you install proper drainage behind he wall if you want it to last.


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## concretemasonry (Oct 10, 2006)

Do not put a use mortar or a concrete footing under a segmental retaining wall system. They can go 40' high with no concrete footing because of the concept and unit design. A concrete footing severely limits the wall strength.

Dick


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## BRO931 (Jul 12, 2010)

Scuba_Dave said:


> Where are you located ?


Sorry about that. I've added my location (North Carolina) to my profile.

Seems to be a difference of opinion about the need for a footer. I'm planning to use regular 8x8x16 hollow blocks, but I figured I'd use solid 4x8x16 caps at the base so that the concrete (or morter/grout) fill can bond to that and the hollow blocks above. I realize a vertical stack with no gradual set-back will put some lateral force on the wall, but I only plan to have a maximum of 24 inches of crusher-run behind this wall. For most of the wall, it will be 16 inches or less.

Good point about the need for drainage. Slotted septic drain line should do fine here. I think I'll also drop some #4 rebar into the holes for good measure.


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## epson (Jul 28, 2010)

Bud Cline said:


> Gonna need a footing.


and some re-bar...


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## concretemasonry (Oct 10, 2006)

Since you are building a rigid retaining wall, you will need to have a footing.

A rigid wall and a segmented wall are two different animals. Mortar, grout and rebars are needed to try and prevent cracks or uncontrolled leakage. A segmented wall is designed to move slightly and then return as the weather cycles and usually has a textured architectural face. One requires some skill with concrete and mortar, while the other requires a compacted soil base and a level first course and can be straight or curved.

Dick


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## rusty baker (Feb 13, 2009)

Make sure that they are concrete blocks. Do not use hadite. Hadite will not last if it is in contact with dirt.


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## BRO931 (Jul 12, 2010)

rusty baker said:


> Make sure that they are concrete blocks. Do not use hadite. Hadite will not last if it is in contact with dirt.


Haydite is used in the more expensive lightweight blocks, right? I recall that my local Lowes store used to have a lightweight concrete blocks at a higher cost, but now they just have what they call standard gray block. 

When I was a kid, we used to call everything "cinder blocks". Anyone care to educate us on what's what?


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## concretemasonry (Oct 10, 2006)

BRO & Rusty -

If the block are referred to as "cinder block" you either have to be older than me or from the eastern U.S. (PA, NJ or near a train line) where cinders were a variable waste product from the inefficient eastern coal-burning plants that were given away or cost money to dispose of. The slag (dross) from steel mills was a decent product that was sold for a better price. The term cinder block is a short term description of block in some places or a term used for "junk" block in other areas depending on the person's local lore.

Haydite is a manufactured aggregate that is produced for applications such as better insulation and fire resistance. It cost more than the typical heavier aggregates used in all types of concrete because of the value and manufacturing costs. This is similar to other brands of lightweight aggregate that can acheive very high strengths Some are also used for highway bridge decks or fireproofing of taller steel structures or just where a higher fire resistance is used to separate spaces in schools, hospitals, etc. that are closely specified.

Some block produce unknown quality block and give them the label of "Haydite" when the may not use the real aggregate but a cheap substitute such as bottom ash from power plants or other waste unrefined or processed waste materials. - It is just a short catch-all term for block that are lighter than normal weight block.

Lightweight aggregates used in block can create many advantages but few people are willing to pay the extra cost ($0.10 - $0.20 per unit) for the benefits, so the producer supplies cheaper normal weight aggregates.

Retailers, like Lowes sell on price and generally are not concerned with quality or properties or particular products because the floor space dictates the amount of selection/inventory (or knowledge) the buyer may have about what he is shopping for.

Dick


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## rusty baker (Feb 13, 2009)

We had a plant here that made hadite blocks. They had signs up and on their receipts, it said they were not to be used as foundation or anywhere they would have direct contact with soil. When asked why, they said the blocks would deteriorate if in direct contact with soil.


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## Bud Cline (Mar 12, 2006)

You want a substantial footing, preferably below the frost line. Trust me grasshopper.


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## concretemasonry (Oct 10, 2006)

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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## Daniel Holzman (Mar 10, 2009)

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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## BRO931 (Jul 12, 2010)

Daniel Holzman said:


> 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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