[steveone]

A good family friend of mine who is a contractor recently gave me some advice regarding drainage for a retaining wall that I am building. He said when laying down the drainage pipe along the back of the wall, a pipe with a 0% grade- no high spots, connected at one end to an existing drain pipe leading to the street would do the job. He also advised me to use the perf. solid drain pipe instead of the perf. corrugated pipe. And another thing that didnt make too much sense to me either- he also informed me that when using the perf. solid drain pipe I must lay the pipe so that the holes are at the bottom. I questioned this due to the obvious- how is the water suppose to be redirected with a pipe that has holes on the bottom of the channel. An how would water enter the pipe if the holes are at the bottom?? Can someone please explain this to me? And can someone please explain the 0% grade on teh drain pipe? This is the first time Ive taken on a project like this and I hope someone would be kind enough to point me in the right direction. Thanks.

[PSUscaper]

The drain pipe behind the wall does not perform the same task as say, a drain pipe leading water away from a house tied to a downspout....

The purpose of the drain pipe is to evacuate excess water pressure......not necesarrily excess water. the pipe basically is acting like a sponge, sucking out the excess water and then ejecting it out the opening. The same result can be achieved by having weap holes in the front of the wall, but due the fact that in many cases, such as along driveways, it is not practical to have drains dumping out of the front of the wall onto the drive. Instead, you want it to dump somewhere else.

In all honesty, I have very few walls where I ever see water come out of the drain tile. It really is a worst case scenario, where a large amount of water builds up and needs to be removed.

What happens with a perforated pipe is the water isn't taking away, but more or less 'distributed evenly' across the back side of the wall.....in other words, it is likely that one spot behind the wall may have more water than others. The perforated pipe more or less distributes areas of high saturation to areas of lesser. In a drastic situation, when the entire area behind the wall becomes saturated, it then flows out of the pipe where it is daylighted.

More or less, the pipe is a pressure relief valve....not really a drain pipe that we more commonly think of.

[Stonehenge]

Using a perforated sewer type pipe is fine. Several years ago I did have a retired excavator tell me that they also installed large sewer pipe with 0% slope. "The water will find it's way" - and truthfully, if well-constructed, it will. Even so, I like a little pitch in my pipe.

And for holes at the bottom - you'll likely have clear, crushed stone all around that pipe - if the holes were on the side or top, that would mean the level of the water would have to be as high as those holes just to start draining out.

So in all, I think you got some advice that's worth keeping.

[Bamboo]

dont forget to filter the sediment with a woven mesh (filter fabric)around your nice new rock and drain pipe. I also like to put my pipe in a sock.
The corrigated stuff is crap becoase it will crush under rock pressure, I use triple wall perf pipe. If you want true stability use abs and drill your own and put in a clean out or 2 and drain to daylight.

[Bill Schwab]

Code in San Diego city require a connection to daylight every 50'. So, if you have a wall 70' long, there should either be two T's connected to your sub surface rain water drains heading to the street, or, you can core cut a hole through the wall face and set them flush with the wall right above grade.

There are 3 different types of white rigid pipe in our neck of the woods....Triple wall, which is kind of corrugated on top, then there is Michael Jackson pipe, (black on the inside white on the outside) hey don't laugh, that's what the pipe suppliers call it, and SDR-35. All we ever use is SDR-35 because it is a gluable pipe, and you won't get any root penetration through the connections as you will with the triple wall or Michael Jackson varieties. Christies makes glues and tapes for the lessor qualities of pipe, but typically they will crack and fade after a year or so underground and you have the same root problem as no glue or tape at all. Using black flexible pipe will work but there are two problems. First, it can collapse from the weight of the rock over the top of it. Second, because it is flexible, it can droop in spots, creating a snaky type look and causing water to head for the low roads. Also, this black pipe has to be adapted to any of the white pipes you would most likely have for your sub drains. You usually use pipe tapes for that, but once again you will face the root penetration issues.

The pipe behind the wall in the 3/4 drainage gravel should have a 1% pitch on it, and for runs over 50', we typically will set the middle high, and connect drains at each end. We don't wrap the pipe with a sock. We wrap the gravel drainage rock entirely with geo textile fabric. The reason for that is, a sock on a 4" pipe can clog much easier than a great big wrap over the entire run of gravel.

With respect to holes up down or sideways in the perf pipe. One set of holes has to go down, the other face the back of the cut into the gravel. At least that is what the inspectors called us on at the least wall we had going up. So, we had to make the adjustment. The method to the madness is this. San Diego has 5 basic soil types. If you have the sandy loam found in most of the beach communities (Solana Beach, Del Mar, Encinitas, Carlsbad, etc, usually lying within 2 miles of the ocean, the holes are not as critical. But, if you have Class 4 expansible, silty clays, or even some DG's, the reason they want those hole facing down is for ground perculation. If the holes are down they grab water and "distribute" it as Penn has eluded to. If they are up, and you trap water behind the wall, the water will work the sub soils until it fails causing your wall to blow.

There is yet another trick to the mix here. If you have sub drains, those drains have to be underneath the the bottom of the wall footing in order to make a connection below grade, otherwise the footing itself will trap the water and it will have no where to go.

If you can't make the elevations work to connect below ground, that's when you compact base behind the wall, at the same elevation as the ground on the daylight side of the wall. Then lay the pipe, and core drill at ground level through the wall face and dump your water at the toe of the wall. Install some area drains to get it out and stop puddling in front.

Because of the variable soils we have in the region, every precaution is thrown at us when we install a wall. To many, all this seems way overkill. But it is what it is.

I hope this helps.

[steveone]

Bill...you mentioned...

"With respect to holes up down or sideways in the perf pipe. One set of holes has to go down, the other face the back of the cut into the gravel."

I wanted some clarification- when you say face the back of the cut into the gravel, you mean face toward the soil and not the the same side of the wall right?

Thanks to everyone for all your input. Thanks.

[Bill Schwab]

Yes. Sorry if I was not clear.