Retaining Wall Drainage

Discussion in 'Landscape Architecture and Design' started by GreenMonster, Aug 30, 2004.

  1. GreenMonster

    GreenMonster LawnSite Silver Member
    from NH
    Posts: 2,702

    I'm quoting a wall rebuild:

    175 x 4. Middle section (50') has failed. Surcharge behind this Allan Block wall. No grid, and backfilled with sand/gravel. No drain field or tile.

    So, on a wall of this length, how would you guys vent the drain? Do you think it would be beneficial to vent the drain at more locations than just the ends? Only probably being, if I were to vent it in the middle somewhere, base course/drain is below grade?

    Just kind of thinking out loud. Any thoughts?
     
  2. D Felix

    D Felix LawnSite Bronze Member
    Posts: 1,898

    Is it planted in front, or bare?

    Could you install weep holes?

    If you can take it out both ends, I'd probably go that route, otherwise I'd probably bump up to a 6" perforated pipe behind the wall. Once the water is inside the pipe, it'll find it's way out. You just need somewhere for it to go.....

    Are you rebuilding the entire wall, or just the section that failed?


    Dan
     
  3. GreenMonster

    GreenMonster LawnSite Silver Member
    from NH
    Posts: 2,702

    Wall runs along sidewalk.

    Weep holes by drilling into bottom course with hammer drill?

    Original bid called for repair of failed section only. I talked to them (City bid) and told them that it's only a matter of time before the rest of the wall comes down too.

    So, of the 175', they want a bid to correct the worst 120', then a separate $$$ amount to rebuild the entire wall.

    The wall has to be totally disassembled, excavated for crushed stone base and drain, geogrid install (due to surcharge). Rebuild and backfill.

    Wall is 5 courses high, with cap. I'm working out manhours in my mind right now.

    What say you, Dan?
     
  4. D Felix

    D Felix LawnSite Bronze Member
    Posts: 1,898

    For the weep holes, yes, that's what I had in mind if you couldn't do any thing else...

    Ideally, as you know, re-building the entire wall would be the way to go, and do it right when you do.....

    As far as man hours go.......... Hmmmmm............ Roughly 7 hours for excavation....

    Around 130 or so for re-build? That's quick and dirty time, but close to our production numbers...... Doesn't include any material procurement, etc, etc. And that's two of us. Add a third, and it may drop to 110 or so, I dunno.

    If you want the way I figured excavation times, let me know, I'll email you the spreadsheet I've got set up.....

    HTH.


    Dan
     
  5. GreenMonster

    GreenMonster LawnSite Silver Member
    from NH
    Posts: 2,702

    Spreadsheet? Yeah, cool. send it over. I hope to work on a bunch of stuff like that over the winter:rolleyes:


    so 137 manhours? Did you figure that at 120' of rebuild, or 175'?
     
  6. D Felix

    D Felix LawnSite Bronze Member
    Posts: 1,898

    120 feet.

    Based on a wall we did last fall....

    I'll try to send the spreadsheet and an explaination sometime this week.


    Dan
     
  7. Randy Scott

    Randy Scott LawnSite Bronze Member
    Posts: 1,915

    Sorry to sway from your question, but I've discussed this with a few people and don't think I have a good answer yet with drainage behind walls. As far as perforated pipe goes. My question is, what good does a perforated pipe behind a wall do? I understand it collects the water as it seeps through the fractured stone behind the block, and goes into the pipe. The problem I have is I think it's a joke. The pipe is perforated all the way around. The water that as easily goes in the top, will just as easily run out the bottom. In my mind, the only way water would drain through the pipe, would be a rush of water. There's no way you will get a rush of water 3 4 5 or whatever feet the wall is tall. I personally think it's useless.

    If someone could explain that it truly does something, I'm all ears. One guy told me at least it's a piece of mind. No it's not. It doesn't make sense that this could even drain anything. It's a fricken' perforated pipe that lets water in as easily as out, right to your base buried course. Please indulge me with a good answer. Thanks!
     
  8. GreenMonster

    GreenMonster LawnSite Silver Member
    from NH
    Posts: 2,702

    :)

    Well, that makes me feel good. I came up with 144 manhours!
     
  9. D Felix

    D Felix LawnSite Bronze Member
    Posts: 1,898

    Plain and simple, the perf pipe is a hollow area for water to collect.

    As the water travels down through the clean crushed backfill, it will find it's way into the pipe. From there, as the water level inside the pipe rises, it will find its way out. That's the reason to "outlet to daylight".

    The perf pipe does not need to have slope; it's the trench that it's in that does. Make sense? Basically if the trench is level, and the pipe has slope, the water will have to rise in the voids of the gravel until it hits the pipe in order to drain very fast.

    No, you won't have a "gush" of water in most cases. And it's more than just "peace of mind". It's a necessity behind almost every SRW.

    The reason I mentioned bumping up to a 6" pipe for the length of the 175' of this particular wall, is that it is possible that a 4" pipe could be filled to capacity fairly easily. There's less than .15 gallons of capacity (I think I did that math right) per foot of 4" pipe. It doesn't take a lot of rain once everything is saturated to reach that capacity.....

    Not bustin' ya, Randy, just 'splaining.:D


    Dan
     
  10. Randy Scott

    Randy Scott LawnSite Bronze Member
    Posts: 1,915

    I understand the pipe will be sloped or pitched by the crushed stone backfill. My question again is, how can a perforated pipe drain water to the low end when the pipe is perforated? It takes water in through the perforations on top, it can just as easily have it run out the bottom perforations. How, with all the perforations, does it flow out the end. It magically comes through the top perforation's, but doesn't exit the bottom perforations, rather instead it flows out the end? Do you understand what I mean?
     

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