Drain Tile Under Retaining wall


Old 06-28-06, 11:49 AM
Thread Starter
Join Date: Jul 2002
Location: Ca
Posts: 746
Drain Tile Under Retaining wall


I'm installing a border for a mixed perenial garden around an existing lawn. Due to extremely poor draining soil I decided to make it a raised bed (raised about 12"). So I attached 2"x12" pressure treated lumber against the fence (reasoning that wet dirt against the fence would quickly rot even redwood) and I'm planning a 12" tall dry stacked retaining wall where the grass meets the bed.

Orginally I was not planning on making the border raised but I had dug some 2-3' deep holes where I was planning larger shrubs etc. and I noticed they quickly filled with rain water and the water did not drain at all. I had to relocate some lawn sprinklers adjacent to the retaining wall (which is not installed yet). Now I notice that when I run the lawn sprinklers some of the holes in the border fill with water. In fact it's so bad I originally thought I had a leak but I think it's just runoff which of course all ends up at the lowest point. Some of it was due to poor sprinkler head selection and aiming etc. Despite the fact that I will have a raised bed I'm concerned that some of the deeper rooted plants may suffer from lawn runoff. Inevitably some of the sprinkler spray will hit the wall and cause a concentration of water under the wall which will go right to the lowest points in the raised bed.

Would it be a good idea to install perforated drain tile under the retaining wall? I actually have an existing drain in one corner of the lawn which I could tie into.

I have read up on drain tile and this is what I have found -

1. It does not need a slope - level is fine.
2. Place a few inches of gravel in the bottom of the trench (is 3/4" drain rock the best type?).
3. Lay perforated 3" drain pipe.
4. Place more gravel to cover pipe and then cover gravel with a soil blocking fabric.
5. Place about 1" of sand over fabric to aid with placement of retaining wall which will be placed over the drain pipe.

Is this about right?
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Old 06-28-06, 03:55 PM
Join Date: Dec 2004
Posts: 89
From responses to my recent thread, it sounds like also underlaying the gravel beneath the pipe with fabric is a good way to keep soil from floating up into your drain system. Maybe even wrapping the pipe itself.

I don't know much about this, but the only thing I would wonder about is whether the weight of the retaining wall could crush the drain pipe .
Old 06-28-06, 04:05 PM
Thread Starter
Join Date: Jul 2002
Location: Ca
Posts: 746

I also saw that they sell some fabric which is made to wrap around a drain pipe but fabric under the gravel is probably even better.

I think my retaining wall stone will be about 8-10" wide so it should be OK as long as the pipe is not too close to the suface.
Old 06-29-06, 05:58 AM
Join Date: Jul 2005
Location: Ottawa, Canada
Posts: 857
Originally Posted by AlexH
1. It does not need a slope - level is fine.
If it is level, where do you expect the water to go? It will just sit inside your drain tile which will not solve your problem. The drain tile must be sloped.

You can purchase drain tile which is already wrapped in a "sock" which prevents dirt from entering the pipe. It is typically used around foundations to gather water and drain it away.
Old 07-01-06, 03:43 AM
Join Date: May 2002
Location: Maryland zone 7
Posts: 1,716
Alex, the drain tile should be either behind the retaining wall or in front of it, not under it. Since the lawn sprinklers will be in front of the retaining wall, and the wall will be less then 3' tall, I would suggest the drain tile go in front of the wall.


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