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Old 02-13-2011, 11:19 PM   #1
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Retaining Wall / French Drain Question


I am going to be building a gravity retaining wall about 3 to 3 and a half feet tall and 30 feet wide with the blocks that have the lip on the back of them. So I am going to be digging a ditch approximately six inches deep then putting wall rock about two inches thick and then compacting it then level it out before I put out my first layer. Then add my second third layer etc.

My question is on the french drain do I put the piping for the french drain right behind the 1st or second row of stones? Then put crushed rock over the french drain for the next one or two feet until the surface?

Any help would be appreciated.

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Old 02-14-2011, 07:12 AM   #2
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Retaining Wall / French Drain Question


Ayuh,... The dranage tile should be behind, or below the lowest point of the wall...

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Old 02-14-2011, 10:55 AM   #3
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Retaining Wall / French Drain Question


Dig a "ditch" about 12 inches deep, fill 10 inches of CRUSHED gravel compacting every 3 inches or so. then add 2 inches of screening to level out the foundation. Lay your first row and second row, your first row should be atleast half buried. From the back of the stone leave about a foot to back fill with CLEAR gravel. Using crushed stone will not allow for good water drainage as it will compact. Run a perforated hard pipe and run it as close to the bottom of the wall as possible. 3 ft high isn't too bad, make sure to route all your water as far as possible from the foundation of the wall.

Hope that helps
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Old 02-14-2011, 12:26 PM   #4
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Retaining Wall / French Drain Question


um, Im not sure you want to build any retaining wall without a proper foundation. I guess I'd have to see the conditions to say for sure. Even a short height wall can be holding a heck of a load in certain soil conditions.

I'd check the codes in your area, because I believe any retaining wall over 42" requires an engineer's approval. You may raise a few eyebrows should your friendly code inspector drive by that big shiny new wall you just put up.

I'd put the drain behind the crushed stone so that the bottom of the drain pipe is at the bottom level of the stone

Make sure you use the french drain with the screen on it too, the others will just clog with rain and time.
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Old 02-15-2011, 09:29 AM   #5
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Retaining Wall / French Drain Question


Bonez - When you say add 2 inches of screening to level out the foundation. Do you mean geogrid?

Steeler99 - I am going to make the wall under 42". I don't want to mess with any permits.
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Old 02-15-2011, 10:09 AM   #6
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Retaining Wall / French Drain Question


When using segmental retaining wall block (SRWs) and concrete footing is not recommended or required. This applies even to 40' high engineered walls.

Should be no problem using SRWs for under 4' high and even 5' in some jurisdictions with no engineering. For over 5' the product licensors and manufacturers say the wall should be engineered, but concrete footings are not recommended or required.

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Old 02-16-2011, 10:54 AM   #7
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Retaining Wall / French Drain Question


This isn't always understood very clearly, so I've attached a digram from Versa-lok. Notice that the pipe should be above the finished grade on the front side of the wall, NOT sitting on the footing. Impervious soil under the pipe rather than a clear draining stone. The concept here is to get the water to exit the face of the wall rather than saturate the footing & soil the wall bears on.


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Old 02-16-2011, 08:40 PM   #8
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Retaining Wall / French Drain Question


I would go at least 4" compacted rock for the base unless you are sure the ground under is very solid. If one block sinks the whole wall is going to look bad so I would start with the proper base. I normally don't worry too much about the drain pipe if there is no sign of drainage problems on smaller walls. I agree that you need impervious below finished grade behind the wall. You don't want water trapped behind the wall where it can soften the ground below your wall. You can use crushed behind the wall for drain rock as long as it doesn't have the minus in it. So 5/8" clean or clear crushed instead of 5/8" minus. What I normally use is called railroad ballast around here. The advantage to using a clean crushed is that it does compact some (not as well as a crushed with minus - the minus or smaller pieces make it compact really well) so when you are done it is more solid than a river rock or drain rock.
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Old 05-18-2012, 09:48 AM   #9
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Retaining Wall / French Drain Question


Resurrecting this old post with a new question...

How close together do the "daylight" drains have to come out of the wall?

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