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I am mid-project in a DIY yard project. I have subbed out a large retaining wall to a civil / wall contractor. It's 220 feet long, between 2-5' in height, and makes 2 90-degree turns. It's built to code with CMU blocks and has an overkill of rebar in it and through the 26" wide and 14" deep footer. i'm in Southern California, so it's a dry climate. I plan to have the face of the wall stuccoed.

The wall just went up this weekend, but hasn't been filled with concrete, yet. That is coming this weekend. Before he comes back on Saturday morning I wanted to get a few things cleared up for my own knowledge and to know what to ask for to make this wall last. The wall has a lot of room behind it now, but will be backfilled with 12" of 3/4 gravel and then compacted and graded soil. No landscape fabric.

My contractor brought in 4″ corrugated pipe with holes on all sides to act as the drain tile at the base of the backside of the wall. I don’t see sock material. I was under the impression that you only wanted the holes on top of a strong PVC pipe so water could easily enter the pipe, but then the continuous bottom of the pipe could run the water out to drainage holes. My local building code does not specify anything other than 4" pipe.

Is it OK for him to use the pipe with holes on all sides or will the water just go in the pipe and run through it and sit below it?

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Counterintuitively, you usually want the holes in drainage pipe to point down, not up. This way, water from below can get forced up into the pipe from underneath, which then has a low-resistance path to flow away. If you install a pipe that only has holes on top, the pipe will want to float on top of whatever water is there, until the water level covers the entire pipe.

Water flowing down from above the pipe will only flow into a pipe with upward facing holes if it happens to flow directly down into the holes, which isn't likely. What actually happens is that the water pools in the lowest spot (which is where you should have your drainage pipe), and as that water level rises to the bottom of the pipe, downward-facing holes allow the pipe to carry the water away. If you have upward-facing holes, the pipe can't really do anything until the water level is above the top of the pipe.

It sounds like your contractor has the right plan - a fully perforated pipe, or one with holes on the bottom, is the way to go. Installing a drainage pipe with only holes on the top is usually incorrect.

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  • I'll consider to place a thin layer of fibric filter over the top of the drain pipe to avoid the accumulation of fines that eventually render the drain ineffective. Also, uniformly spaced weep holes could ensure the wall not to subject to exceeding water pressure during a rain storm event. – r13 Mar 29 at 19:46
  • @r13 Agree, I myself would probably wrap the pipe in filter fabric, although I've seen it suggested that in certain soils it can actually accelerate the pipe becoming ineffective, as the filter fabric pores can clog up before the pipe would have without it. I still use it, but YMMV. – Nuclear Hoagie Mar 29 at 19:52
  • Your worry is not unfounded, the structure of the fabric matters. But anyway, I'll leave the bottom of pipe unwrapped to allow for free flow, at which the flow is less turbulent, thus the infiltration of sandy material. – r13 Mar 29 at 20:03
  • Upward-facing holes also increase the amount of silt that collects in the pipe. They plug much faster (I've seen a lovely research cross section of pipes with holes up, down, and sideways done by an agricultural engineering department - installed, left for some years, and removed with soil cross sections intact.) You could see the higher waterline in the soil sections of the sideways and up pipes, and the downward holes pipe was the only one fully open for flow. – Ecnerwal Mar 30 at 13:29

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