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Old 09-26-2015, 02:54 PM   #1
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surface drainage above a retaining wall


Hi everyone, I am new here but this looks like a great place to ask.

Short version, I want to terminate a short 15' run of mini french drain (with 2" perf pipe) into a shallow gravel hole right behind a retaining wall. There is a legit french drain with pipe at the base of the retaining wall, as is proper. I want my new pipe/drain then drain down into the wall and eventually into the real french drain. Bad idea, or okay?

Here is the longer version with details, pictures attached.


I have a retaining wall that is near my front patio. There is one small section of land between the retaining wall and patio, about 28" wide and 6 feet long, that tends to take on water coming off the patio, which has created some negative backflow toward the house foundation (see photos). I have filled the spot with paver base to create positive drainage away from the house there, but I am considering installing a small mini-french drain to better conduct water coming off the patio there further away from the house and to the back side face of the retaining wall, about 15 feet away.

I plan to use 2" PVC perf pipe (since the area I am trying to drain is actually small), but my main concern is how it terminates. My current plan-- which I am unsure about and therefore am seeking advice-- is to bring that pipe to the back side of the retaining wall, and have it terminate in a shallow gravel-filled hole right behind the wall, with the hope that it drains into the hole and down into the wall itself. There is a real french drain at the base/backside of the retaining wall so any water going down into the wall and behind the wall eventually go to that drain, where it exits the base of the wall from a "daylight" pipe.

Am I crazy to think that just ending this short 15' run of 2" perf pipe (that will be in a shallow/small french drain trench w/ gravel) in a gravel hole behind the retaining wall is a workable plan?

Planning to do this today and tomorrow so any advice is very much appreciated. THANK YOU!
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Old 09-26-2015, 03:11 PM   #2
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What is the point of this piping?
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Old 09-26-2015, 03:18 PM   #3
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SeeMs like the water is going to end up behind the retaining wall either way, just farther from the house with your plan. Ideally you could have a grade across the top of the wall away from the house, but that looks unlikely. I'd use a small catch basin and abs cation of solid pipe before transitioning to perf pipe and a French drain fArther from the house
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Old 09-26-2015, 03:34 PM   #4
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Quote:
Originally Posted by Ron6519 View Post
What is the point of this piping?
Hi Ron,

The piping in the photos is just showing the approximate path I'd be taking with the drain. This pipe would of course be in the french drain trench, in gravel and wrapped, below the surface etc etc. It is just there now to show the path.

The purpose in general of this project is to collect water that tends to run off the patio onto that narrow 28" by 6' strip before the retaining wall, and move it farther from the house before it hits the retaining wall, to avoid constant water collection there from creating a sink that draws the water back toward the house.

I can tell that water was collecting there and backflowing toward the house in the past. That's why I started by beefing up the positive drainage from the foundation by putting in a good 2 feet surface area of paver base in that section nearest the house. Maybe that's all I really need to do, I often make things more complex than needed in an attempt to be thorough. I just want to move the water away from the house, out further.

My main concern is, once I run that pipe and french drain to the back (high) side of the retaining wall about 20 feet from the house, and have it terminate into a gravel hole there, will I be creating erosion problems there, or will it drain nicely to the back side of the wall, and down to the drain at the base of the retaining wall? It's a small deal and there shouldn't be a ton of water going through.

Last edited by Benjamin5; 09-26-2015 at 03:56 PM.
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Old 09-26-2015, 03:44 PM   #5
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Quote:
Originally Posted by red92s View Post
SeeMs like the water is going to end up behind the retaining wall either way, just farther from the house with your plan. Ideally you could have a grade across the top of the wall away from the house, but that looks unlikely. I'd use a small catch basin and abs cation of solid pipe before transitioning to perf pipe and a French drain fArther from the house
Hi Red, I agree a little more slope of the wall away from the house would have been helpful, but its not there. It is pretty level. SO the main directional flow of surface water right there in that corner is down off the patio, toward the retaining wall. Since it is pretty level right there moving away from the foundation, a little more movement-inducing stuff to direct it away could help.

Right now my plan is to put plastic sheeting, about 2-3' worth in the location between patio and retaining wall, over the new paver base, helping direct water away. The paver base I put down was in response to a sort of hole forming in the corner close to the foundation, but now it slopes strongly awat from the house there. So, that plastic sheeting will have to act as my collection basin...

The actual trench and pipe will start, then, where the paver base ends approximately (water on the paver base area should drain into the trench area.)

It's true water ends at at the wall regardless, I just want it further out. I am a little concerned that since the trench will pick up water all along its route, maybe it would create erosion at the wall when it hits the gravel hole, and where does it go from there? Will it sok down into the wall and hit the foot drain of the wall okay? These are my concerns.

Thank you for your input, I really appreciate it.

Ben
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