Go BottomRetaining wall drainage
 | kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon 
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2008-08-13 156067
When a retaining wall is built I've always ASSumed that the drain tile had to drain to daylight, but the manufacturers installation guide never actually says that.
Is draining to daylight necessary? Or is the drain tile and gravel supposed to function as sort of a dry well that can hold water until it can seep down?
Our last house had a wall in the front, not built by us, and it didn't seem to have any visible drain (but it could have been tied into the underground gutter drains).
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2008-08-13 156068
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Necessary? No.
Advisable, yes.
As long as the water is carried away from the wall, especially out from BEHIND the wall, it will be fine.
The problem is if the drain does not come to daylight, it's VERY hard to tell when or if it's working or clogged up. Also, if the drain is installed properly, no sharp bends, etc., and does clog (say with dirt) you can push a hose up the opening and flush it clean. If the drain does not come to daylight and it clogs up, get out the shovel!
Having said that, I've seen lots of them built where they drain to a 'dry well', basically a hole in the ground filled with clear gravel and then covered with landscape fabric and topsoil. The idea being that the gravel-filled hole acts as a reservoir to absorb drainage and hold it safely away from the structure until the earth can absorb it.
Best of luck. ....
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| kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2008-08-13 156074
Murf, thanks as always. I'll figure out some way to get it drained. It's funny, a few years ago I knew we'd have to put in some retaining walls and drew scale drawings of what was needed, and even made a bill of materials to order. Now that it's time to build them everything has changed due to site details that haven't changed at all. :( ....
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 | earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2008-08-13 156078
Just for clarity for others who may or will be following along...
Dry wells are great IF:
- You have low water tables lower than the bottom of the dry well,
- Have soil that drains easily and quickly,
- Have more than enough water storage capacity under ground for worst-case scenarios e.g., bad storms, runoff, snow melt.
And ideally the dry well should be free of infill (stone, gravel, broken concrete) for maximum storage. If infill is used, keep in mind that by filling the dry well DECREASES the capacity to just a fraction of the original volume---all the capacity you have left is the air spaces between the infill.
The so-called dry wells I've done at the insistence of the client had a perforated 6" dia. drop tube to the bottom of the hole and capped like a water well. That way the water level can be monitored and pumped out if necessary.
Also the dry well should be lined with a geotextile before infilling to keep the air spaces free of surrounding dirt--otherwise over time as everything settles you'll have just a sunken depression in the ground with no capacity underground. ....
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