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Old 11-24-2008, 02:18 PM   #1
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Drainage ? for an 8' tall Retaining Wall


We've got an 8' tall retaining wall which is holding back a large hill. We live in a wet climate and alot of water comes off that hill, which then gets behind the wall. But there are weeps all along the base so that the water evacuates freely. (pics below)

However, I am going to build a planting bed along the wall. By putting dirt in front of those weeps, I would be impeding the water flow. I am concerned that impeding the water will allow it to back up behind the wall, which will eventually bring it down.

My plan is dig a trench along the base of the wall and install drain pipe, bedded in gravel, which is covered with drain cloth. Then above this gravel trench I would mound the gravel up to cover the weep holes. And cover it with drain cloth. Then I'd build a 2' planting bed wall a few feet off the retaining wall, and back fill it with planting soil.

Do you think this would work?





What grade of gravel would be best?

And do you have any advice as to which type of drain pipe and cloth I should use?






Water constantly drains out these weeps.



Last edited by White Elk; 11-24-2008 at 02:26 PM.
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Old 11-24-2008, 03:02 PM   #2
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Drainage ? for an 8' tall Retaining Wall


That's a tall retaining wall to dig near the base. I'd be concerned of an avalanche and consult with an engineer.

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Old 11-24-2008, 03:38 PM   #3
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Drainage ? for an 8' tall Retaining Wall


Since that is an older style tall rigid reaining wall instaed of a modern segmental retaining wall you will have a spread footing since things will not be what you show on the sketch.

You can measure the height of the wall face, but it is also holding back a large portion of the sloped soil behind and above the wall.

If it was a typical DIY wall, you have a small footing and the likelyhood of problems is greater than if it was engineered.

Dig a little in a couple of spots to determine what you have to support the wall.

Dick
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Old 11-24-2008, 03:42 PM   #4
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Drainage ? for an 8' tall Retaining Wall


I've been wondering that same thing! My thought was to work in 6' sections and backfill 4' of it immediatly. And also to use some 2x6's for support (braced against the wall and nailed to posts driven in the ground). We are tight on money but the 2x6's are leftover from another project and will later be used for a chicken coop. But perhaps I can rent some better braces cheaply? Has anyone ever done anything like this before?

I'd prefer to dig the full length of the trench, then lay the pipe etc. But risking wall damage or my early burial makes me think of playing it safer. Once its done I think the compacted gravel will be more stable than the dirt. But then my expereiance is all in labor and not structural considerations like this. So I need advice on this part of it as well.
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Old 11-24-2008, 03:52 PM   #5
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Drainage ? for an 8' tall Retaining Wall


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Originally Posted by concretemasonry View Post
Dig a little in a couple of spots to determine what you have to support the wall.

Dick
Will Do! And then post the pics.

I think the wall is at least 50 yrs old. I know it was built by my neighbor when he was young (he built this house and raised his family in it till his divorce, his ex wife got the house.. he parceled his land and built a home nextdoor). He was a commercial builder but not very experianced in his youth. So I guess it could be considered a DIY wall. It's been so long that he doesn't remember some of the details of things.
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Old 11-24-2008, 04:14 PM   #6
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Drainage ? for an 8' tall Retaining Wall


I wouldn't dig out anything for fear of compromising the stability of the footing that is there now - whatever it is. Leave all of that in its virgin state.

You can still do as planned and drawn. I think that is a sound concept.

Build the new wall, and port the bottom joints basically the same as what was done with the taller wall. Install crushed stone size:3-4 up to the weeps in the old wall. Lay in a perforated drain pipe lateral same as used in leech fields. Allow the drain pipe to penetrate the ends of the new walls on either end. What you do with that run-off is up to you. (Should there be any.) Then add some additional crushed stone size: 3-4 about four inches above the drain.

At that point the crushed stone can be covered with two layers of the Phillips landscape drain cloth then filled with soil above that.

During wet conditions there is a tremendous amout of pressure on that wall. I wouldn't do anything that would have an effect on the existing footings or foundation.

Last edited by Bud Cline; 11-24-2008 at 04:16 PM.
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Old 11-24-2008, 04:22 PM   #7
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Drainage ? for an 8' tall Retaining Wall


Since the water and pressure is coming from the up hill side is there any way to put a drain such as you are thinking of on the high side? I would have it run to the right side of the picture that you showed. Then just up hill of that I would berm it to run off some more of the surface water in the same direction. Is the wall leaning some already or is it just the picture?

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Old 11-24-2008, 04:53 PM   #8
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Drainage ? for an 8' tall Retaining Wall


I suggested to dig in a few local spots to find out what you have. If it 50 years old and still appears sound, it probably has a footing. You should find out how deep the footing is and far the spread portion sticks out into your yard.

Do not dig below an imaginary line estending out from the bottom of the footing at a 45 degree aniamal since you could be reducing the strength of the footing bearing and the resistance to siding.

The idea of a drain with cloth and rock is a classic method if you do not upset the original stable construction. A failure can be rapid and dramatic if the soil is saturated. The tree growth and the vegetation make sure the water is absorbed in the soil and does not just run downhill and over the top of the wall.

Dick
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Old 11-24-2008, 05:33 PM   #9
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Drainage ? for an 8' tall Retaining Wall


I don't think I would dig at all. Just clean up the bottom of the wall to a level surface and lay your pipe and gravel as in the diagram.
I don't see any advantage to digging down in front of the wall.
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Old 11-24-2008, 05:56 PM   #10
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Drainage ? for an 8' tall Retaining Wall


I would suggest that you would build the new bed. lay the drainage tile in the bottom, near the old wall and then cover it with gravel.
Filling it up, the rest of the way with top soil.

It appears that your neighbor did a good job of the wall, as it has retained the hill for this long!
Excavating probably wouldn't cause a problem, but why tempt fate, if its unnecessary!
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Old 11-24-2008, 06:52 PM   #11
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Drainage ? for an 8' tall Retaining Wall


Quote:
Originally Posted by concretemasonry View Post
I suggested to dig in a few local spots to find out what you have. If it 50 years old and still appears sound, it probably has a footing. You should find out how deep the footing is and far the spread portion sticks out into your yard.
There is a 4" slab (at the thinnest point I found) of what I'm certain is poured concrete. The top of this footing runs level with the ground. There is no spread beyond the face. I also trenched an inch deep for 13' and saw no seams. Under the footing there is an inch or so of dirt and then a sandy clay. It is saturated (but all our ground is). Which actually might be a good thing since theres so much sand in the soil?






Above is a better shot showing the elevation and hill slope. Beyond that crest is a gentler slope and then a greater slope for near 60 yards to the tree line. Alot of water comes down that. The Cedar tree and the decidious are gonna need to come down. I found cedar roots growing through some weeps. There is a bunch of Ivy still on that hill (it used to cover the wall). And I'm still fighting the blackberry there, but I've nearly won that battle. I will wait to remove the ivy until spring so as not to make the slope barren. LoL I have plenty other blackberry patches and nettle root systems to battle this winter anyway.

Last edited by White Elk; 11-24-2008 at 06:56 PM.
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Old 11-24-2008, 07:16 PM   #12
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Drainage ? for an 8' tall Retaining Wall


Quote:
Originally Posted by Bud Cline View Post
I wouldn't dig out anything for fear of compromising the stability of the footing that is there now - whatever it is. Leave all of that in its virgin state.

You can still do as planned and drawn. I think that is a sound concept.

Build the new wall, and port the bottom joints basically the same as what was done with the taller wall. Install crushed stone size:3-4 up to the weeps in the old wall. Lay in a perforated drain pipe lateral same as used in leech fields. Allow the drain pipe to penetrate the ends of the new walls on either end. What you do with that run-off is up to you. (Should there be any.) Then add some additional crushed stone size: 3-4 about four inches above the drain.

At that point the crushed stone can be covered with two layers of the Phillips landscape drain cloth then filled with soil above that.

During wet conditions there is a tremendous amout of pressure on that wall. I wouldn't do anything that would have an effect on the existing footings or foundation.
Thank you for the detailed info and the specific material suggestions!

What do you mean by "port the bottom joints"? Below is a pic of a small retaining wall I built adjacent to that big retaining wall. I'll be using the same material. I set the bottom row 2" below ground level at its lowest point and leveled as I went. At the highest point, the bottom row is just below the surface. Under the wall is a 4-6" deep gravel bed (with no drain pipe) which rests on clay. It is sloped away from the big retaining wall to encourage excess water to flow out of the backyard (mighta been unecessary?).







Last edited by White Elk; 11-24-2008 at 07:19 PM.
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Old 11-24-2008, 07:31 PM   #13
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Drainage ? for an 8' tall Retaining Wall


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Originally Posted by Gary_602z View Post
Since the water and pressure is coming from the up hill side is there any way to put a drain such as you are thinking of on the high side? I would have it run to the right side of the picture that you showed. Then just up hill of that I would berm it to run off some more of the surface water in the same direction. Is the wall leaning some already or is it just the picture?

Gary
This is on my project list for late summer 09. I think it will help, but will not allow me to forgoe the base drain. I want to build a gravel path there so I can move a heavy wheel barrow across as needed. This gravel path will also reduce some of the landscape maintenance along the top of the wall. My plan is to drain it to the left side (to the right is a garage and above it is a rising slope). Then tie that drain into the base drain and run it all down the side yard and down into a little bog below the front of the house. LoL I have wondered about having it flow into the waterfall that I have planned for that left hand corner. Then construct the pond to overflow into the drain as needed. But I think it may be better to run the drains seperate, and then tie in below the pond.

That wall is leaning top side. Good eyes.
In 4' there is up to an inch off level.
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Old 11-24-2008, 08:17 PM   #14
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Drainage ? for an 8' tall Retaining Wall


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I don't think I would dig at all. Just clean up the bottom of the wall to a level surface and lay your pipe and gravel as in the diagram.
I don't see any advantage to digging down in front of the wall.
Quote:
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I would suggest that you would build the new bed. lay the drainage tile in the bottom, near the old wall and then cover it with gravel.
Filling it up, the rest of the way with top soil.

It appears that your neighbor did a good job of the wall, as it has retained the hill for this long!
Excavating probably wouldn't cause a problem, but why tempt fate, if its unnecessary!
Do you guys think that will pick up enough of the water? The water drains out the bottom of the weeps which are 8" tall. Wouldn't some water still back up behind the wall if I put the pipe at ground level?

Perhaps it would be better to install the pipe with half its diameter underground?
And place it a couple inches out from the wall face, with the trench sloping 45 degrees away from the wall face?

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Old 11-24-2008, 10:48 PM   #15
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Drainage ? for an 8' tall Retaining Wall


Quote:
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Do you guys think that will pick up enough of the water? The water drains out the bottom of the weeps which are 8" tall. Wouldn't some water still back up behind the wall if I put the pipe at ground level?

Perhaps it would be better to install the pipe with half its diameter underground?
And place it a couple inches out from the wall face, with the trench sloping 45 degrees away from the wall face?

You could clear the topsoil of the top of that 4" footing! Then lay weeping tile on top of the footing, then backfill it.

You don't say, but frost doesn't seem to be a factor in your area.
With such a shallow footing, I would expect the wall would have failed by now, if you have freezing!

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