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Old 05-17-2011, 07:00 AM   #1
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Retaining Wall Questions


What's up everyone.

I am finishing up digging out an old and falling retaining wall and getting ready to put a new one up. The wall will be going along one side of my driveway, the driveway slopes towards the house and has a drain at the bottom of it.

The block are 85lb interlocking block, no pins or mortar will be used. They are 12 inches wide. I believe that the wall will be 5 block high at it's highest point. I have a few questions:

1. how far back behind the block should be dug out for gravel? I am at around 2 ft right now, which will leave 1 ft for gravel behind the block.

2. I was not planning on putting in a french drain or weep hole. I figured since the block are interlocking that they will get enough weepage through them. Am I correct on this thinking?

3. how deep should the gravel bed be underneath the 1st course of block be? I was thinking 3-4" should be plenty. Is sand needed?

4. how deep into the ground should the first course be? I was thinking 6-8 inches. Is this enough? too much?

5. Should I use landscaping fabric between the gravel and the dirt/clay?

Thanks

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Old 05-17-2011, 07:27 AM   #2
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Retaining Wall Questions


Put the block at least 2-3 inches underground. You definitely need landscaping cloth to keep the dirt from getting into and clogging the gravel. I would start it under the first course of block, lining the bottom and going up the slope behind the wall. Leave enough so you can wrap it back so it goes under the last course of block. This way the topsoil on top doesn't infiltrate down into the gravel.
Water can move a lot of weight. I would put a drainage pipe behind the wall.
If you're in a freeze zone, I'd put at least 18" of gravel behind the wall.
Ron

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Old 05-17-2011, 07:42 AM   #3
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Retaining Wall Questions


Ron,

Thanks for the reply. One additional question for you or anyone else:

If I was planning on somehow adhering the top course of the block, as well as the caps that I am going to use, what would be a good adhesive to use in this situation? Liquid nails?
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Old 05-17-2011, 07:48 AM   #4
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Retaining Wall Questions


A few globs of silicaone adhesive is probably the best rout to hold the caps in place. They are thick enough and remainflexible to allow the caps to move with the rest of the wall since the wall is not built with rgid attachment.

Dick
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Old 05-17-2011, 08:04 AM   #5
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Retaining Wall Questions


Take a look at this site. http://www.allanblock.com/retainingW...ntro.aspx?ta=1

I like filter fabric and use it a lot. It not only filters the soil materials and prevents them from staining the wall front, but will reinforce the soils behind the wall preventing creep. You mentioned clay. This soil if very tricky to build on. It expands a lot when wet, and shrinks when dry. Around here, it is slippery than snot when wet, so things move when placed on clay. So, I would place a premium on drainage.

I'm not sure burying only a half of a block is a good idea. That bottom layer is the only thing keeping the wall from sliding. Depending on what is in front of the wall, I would go deeper.

Good luck and keep us informed as you proceed with this project. Take LOTS of pictures. I for one would love to see them!
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Old 05-17-2011, 08:17 AM   #6
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BigD, the clay is mostly behind the wall, not actually under it. I'm sure its down there if I dig deep enough, but as of now I'm still into the slag that used to be used around Pittsburgh a lot under concrete and for fill. It's a byproduct of the mill so it was plentiful around here back in the day.

concretemasonry,
Thanks for the tip on the silicon adhesive.
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Old 05-17-2011, 09:13 AM   #7
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Retaining Wall Questions


"I'm not sure burying only a half of a block is a good idea. That bottom layer is the only thing keeping the wall from sliding. Depending on what is in front of the wall, I would go deeper."
My suggestion of 2-3" was based on Brandens statement the wall was, " going along one side of my driveway". If the wall wasn't right against the driveway, I'd drop it down as BigD9 mentioned.
And with the clay, I'd make sure the base under the wall was substantial.
Ron
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Old 05-17-2011, 10:38 AM   #8
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"I'm still into the slag that used to be used around Pittsburgh a lot under concrete and for fill."

Slag can be an excellent fill material. It is usually porous, angular and lightweight. I built a road to bridge tieback retaining wall once where slag from a power plant was specked as the backfill material because of its light weight. The slag was new, and hadn't weathered at all. It held the tiebacks so well the straps broke before the straps pulled out in a test! Problem cropped up about a year later when the wall started to fall down and we discovered the slag was extremely alkaline (Ph>12) and the galvanize steel tie back straps were compromised and breaking. We had to remove the entire 28 foot tall, 200 foot long fill and retaining walls, and replace the slag with limestone fill and new straps.
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Old 05-17-2011, 05:04 PM   #9
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Retaining Wall Questions


Just look at the installation instructions on line for the products you are using. Your supplier can tell you which one of the 4 major systems (Allan. Anchor, Keystone or Versalok) it is. For such a low wall the instruactions for all systems are about the same since there a many millions (billions?) of sf installed around the world. The importanthing is that the compacted base is leve with no lazy cheating by using sand to smoth out the base surface. If it is "knock-off" product, call the manufacturer to see if there are instruction - don't depend on the guy in the "big box" store.

Dick

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