Putting a drain pipe through a retaining wall

[plowhog]

I'm building a retaining wall using stacking stones (not the kind with a lip in back.) It's kind of a lazy quarter-circle shape, slowly descending, about 100 ft long, with a gentle slope behind it.

I have excavated a trench about 2 ft wide and roughly a foot deep-- for 6" of compacted type 2 road base, then the first course of stones below grade. The top of the first course of stones should be roughly at the current grade. As my wall goes up I am planning to backfill with 3/4" clean gravel. I am undecided yet on whether to put a pipe on the back side of the wall, or whether to use fabric if I do add such a pipe. I am wondering if the gravel and natural slope will provide the drainage I need.

My concern is trapping water behind the wall. I need to get from behind the wall to daylight. I want to somehow pipe from behind the wall, through the wall, under a gravel driveway, then to daylight. I am not sure the best way to do that?

Should the bottom of my drain pipe (behind the wall) be at the same grade as the bottom of the first stone? Both would be sitting atop compacted road base. If so, this means the pipe needs to go through the wall and I am not sure how best to do that, such as cutting stone blocks. Or should I have my drain pipe even farther below grade, with the pipe embedded in the compacted base (or perhaps in embedded adjacent gravel instead) so the top of the drain pipe is roughly equivalent to the bottom of the first course of stones?

Regarding piping, I am guessing SDR 35 4" pipe is a good choice once I am outside the wall and to take it (underground) across the gravel driveway. But what about a pipe behind the wall to connect to the SDR 35? I have heard horror stories about the 4" black corrugated drain pipe collapsing or getting plugged.

Any suggestions?

 

[deezler]

Most of the time when I see it, the first course of block is put in atop compacted base, then you toss a drain tile with sock on it behind the block, cover with gravel and call it good. I dont think you would want the drain pipe within the fully hard compacted base, as this will inhibit drainage flow into the pipe itself.

Hard to tell from your pics, but kind looks like you could get away with no drain pipe/tile at all. As long as there is a drainage path on the surface, you don't have heavy clay soils, and your wall won't be too tall (say, under 2-3' high), you'll be fine. Is this in rainforest northern CA, or dry northern NV?

 

[plowhog]

Dry rocky crappy Northern Nevada including some clay and freeze / thaw cycles.

I phoned my excavation contractor this morning. He suggested SDR 35 (green) pipe from the back side of the wall, under the wall, then out to daylight. He said staying under the wall is less risky as if there becomes any future movement in the wall it could break the pipe. Under the wall would be safer.

He suggested bringing the SDR under the wall to a 90 degree fitting, then use perforated pipe (in gravel on the back side of the wall for as many feet as I can go considering the curve.

The tricky part is the stone blocks need to be on compacted type 2 road base but the pipe embedded in gravel. I think I can level/compact the entire area, then make a slight trench as necessary for the pipe and gravel.

btw I have heard pros/cons on the sock or landscape fabric to keep debris out of the pipe. It seems there is some evidence that over time the sock/fabric fills with silt and debris and no longer passes water .....

Still looking for thoughts or ideas ... that was his. I'm very unsure of how to get the water from behind the wall to in front. In our climate there are downpours and it would overwhelm small outlet holes in the wall. And, I'd rather pipe it to daylight as otherwise the water is crossing a gravel dirveway.

 

[2manyrocks]

If it is essential that the pipe actually drain, my understanding is to use solid wall pipe, perforations down, placed in a sock that is laid in a bed of clean gravel that is also enclosed within a layer of sediment filter fabric. The fabric keeps the sediment out of the rock drainage gravel so the water can pass unobstructed into the drain pipe.

Solid pipe can be installed so there is a constant pitch. Corrogated pipe is harder to get a fully consistent pitch and the corrogations don't allow water to flow as well.

My opinion is pipe is cheap to install during building of a wall. If the situation warrants pipe drainage, then just do it.

 

[rScotty]

Dry rocky crappy Northern Nevada including some clay and freeze / thaw cycles.

I phoned my excavation contractor this morning. He suggested SDR 35 (green) pipe from the back side of the wall, under the wall, then out to daylight. He said staying under the wall is less risky as if there becomes any future movement in the wall it could break the pipe. Under the wall would be safer.

He suggested bringing the SDR under the wall to a 90 degree fitting, then use perforated pipe (in gravel on the back side of the wall for as many feet as I can go considering the curve.

The tricky part is the stone blocks need to be on compacted type 2 road base but the pipe embedded in gravel. I think I can level/compact the entire area, then make a slight trench as necessary for the pipe and gravel.

btw I have heard pros/cons on the sock or landscape fabric to keep debris out of the pipe. It seems there is some evidence that over time the sock/fabric fills with silt and debris and no longer passes water .....

Still looking for thoughts or ideas ... that was his. I'm very unsure of how to get the water from behind the wall to in front. In our climate there are downpours and it would overwhelm small outlet holes in the wall. And, I'd rather pipe it to daylight as otherwise the water is crossing a gravel dirveway.

I like the contractor's thoughts but would upgrade the pipes under the wall to pipe material that can withstand some weight and movement. Nobody can forecast what the settlement and movement is going to be when clay is involved, but the pipe needs to take all the weight if necessary. 4" iron is a common for municipal water & sewer pipe - which keeps the price reasonable. Get ductile iron if you can and cast iron second choice. For as little amount as you are looking for I would see who local stocks overage from projects. Ask at the water utility. There are also decent heavy wall PVC for that matter - but not at Lowes/HD.

About the sock, we had a flood here & rebuit some perf drainage with some left bare, some socked, and some with double sock and PVC peanuts. Some silted up and some hasn't yet. My latest effort is to lne the bottom of the ditch with strips of geotextile 3 or 4 foot wide, put down an inch of washed gravel, then the perf pipe, then cover the pipe completely with gravel and fold the sides of the geotextile fabric over it all and backfil.
That system worked until the willow roots from the windbreak found their way into the end of it.... Now I have to roto-root my drain pipe every year.

So my last piece of advice is whatever you do, make it "roto-rootable". If 300 ft long consider putting in a pair of back to back risers for access to the pipe every 100 feet
luck,
rScotty

 

[plowhog]

Get ductile iron if you can

Good idea thanks. When I installed my rear driveway I used 10" ductile iron to move irrigation ditch water under that driveway. I chose that as I could only embed 6 inches and I drive my motorhome across it (50,000 lbs). The 20' piece of pipe was $900 (then) but I think it was well worth it.

 

[rScotty]

Good idea thanks. When I installed my rear driveway I used 10" ductile iron to move irrigation ditch water under that driveway. I chose that as I could only embed 6 inches and I drive my motorhome across it (50,000 lbs). The 20' piece of pipe was $900 (then) but I think it was well worth it.

Ductile iron isn't well known - it's a fairly recent process and a real problem solver. Apparently municipal drinking water mains and laterals are using it more and more. I only ran into it because of taking oa job with a municipal water works.
I wonder if ductile has hit the used pipe market yet? For drainage, a good-used 300 foot run would work as good as new. And nothing better for short pieces under a heavy wall on clay....

 

[dodge man]

I would put the pipe in. They also make a geo grid you can lay down between some of the layers of blocks that ties back into your gravel backfill that hold the blocks from tipping over.

 

[Streetcar]

If they use ductile water main in your arena you should be able to get short piece of pipe from water main contractor and sleeve the sdrthrough it. They make perforated sdr pipe for behind wall

 

[dstig1]

The fabric should go against the dirt behind the wall, not around the pipe. And then you don't really need the pipe. If you lay the fabric down against the back of the trench, then fill between the wall and fabric with 3/4" washed (or bigger), you really won't even need a pipe. The goal is room for water to freely move and the 3/4" provides that plus plenty of space for it to freeze without pushing on anything. If your wall is natural stone, water will go through the joints easy peasy as there will be gaps everywhere. Also a good reason to use larger stone fill...so it doesn't come through those gaps.