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Retaining Wall Drainage

Discussion in 'Hardscaping' started by dw8, Nov 8, 2007.

  1. dw8

    dw8 LawnSite Member
    Posts: 5

    I'm located in Tucson Arizona.

    PROJECT (In process)

    I'm building a mortared rock retaining wall that is 5 feet high from grade.

    The wall also sits on the high side of a mostly dry creek bed and is contoured along the creek. Consequently, grade slopes downward and the wall is open at one end (doesn't wrap back to the bank) at the low point.

    A portion of the the back part of the retaining wall faces the house where it will receive roof runoff.


    How to handle the volume of water that runs off the roof during our "monsoon" season that will end up behind the wall and/or flow over the wall.


    Weep holes (1 inch PVC pipe) spaced approx. ever 4 to 5 feet. I plan to cover the fill-side end of the weep holes with filter fabric to keep sediment from leaching out.

    I plan to use (in the following order) going backward from the wall:

    -- 6 inches of crushed stone
    -- 1 inch layer of pea gravel
    -- 3 1/2 ounce filter fabric
    -- ABC (screen fill) compacted

    The object of the pea gravel is to cushion the filter fabric from the crushed stone during installation to keep if from tearing or punching holes in the fabric I also planned on laying the fabric on top of the crushed stone at the top and covering with top soil to help control filling up the drain field with sediment.


    1) Does the above planned drainage appear OK?

    2) In some cases the weep holes will not be at the lowest point (due to rock variances). Should crushed stone or compacted fill be used below where the weep pipes lies?

    3) Would putting a drain pipe either at the top of the wall or located near the bottom of the wall be of benefit?

    4) If pipe is recommended is perforated flexible or solid preferred -- wall curves?

    5) Is the use of filter fabric both over the weep pipe and as a vertical layer between the crushed stone a backfill recommended or will the fabric clog (how long?) and not allow water to weep into the drain field?

    Any suggestions greatly appreciated.

  2. Captains Landscape

    Captains Landscape LawnSite Senior Member
    Posts: 333

    Sounds like you are on the right track. I look forward to some of the responses you get from the seasoned guys. Is it possible to put a swale above/behind the wall?
  3. RockSet N' Grade

    RockSet N' Grade LawnSite Silver Member
    Posts: 2,454

    check out "elgen drainage systems". we have gone to this wherever possible vs. gravel.
  4. dw8

    dw8 LawnSite Member
    Posts: 5

    Thanks for both responses.

    Re: "elgen drainage systems", did some checking.
    One con indicated clogging occured after 4 years.
    My primary concern is unlike leeching field, would be almost impossible to re-do wall drainage if clogging occurs.

    Any specifics appreciated regarding options and facts to support options presented.
  5. Captains Landscape

    Captains Landscape LawnSite Senior Member
    Posts: 333

    When you mortar in your "junk" or fillers behind the wall, leave gaps and space. Pile up rubble and mortar over the top in layers. There should be sufficient room for hydraulic pressure relief with that. I wouldn't use the filter fabric on that application, might get clogged. Use the drain stone like you said And make a swale at the top of the wall.

    free bump to the top- I'd like to see more responses on this one myself....
  6. dw8

    dw8 LawnSite Member
    Posts: 5

    Captains Landscape.

    Had hoped for a few more comments. Plan to go as you suggested in last post after Thanksgiving.

    Would you recommend 3/4 crushed stone or #5 or a 57 mix (#5 and #7).
    IMHO crushed stone would leave larger void for water but also might fill up faster with organics/sediments.

  7. crazymike

    crazymike LawnSite Senior Member
    from Ontario
    Posts: 454

    for walls that recieve a lot of run off I've gone with filter cloth and gabion stone against the wall.

    This is also what the engineer has recomended when I've had him do wall drawings.

    I've yet to have a problem.
  8. Captains Landscape

    Captains Landscape LawnSite Senior Member
    Posts: 333

    Any size 3/4" or bigger, crushed ANGULAR stone will be ok. Try to get some compaction out of the drain stone as well, should reduce the silt deposits. I don't think I would use the filter fabric, but I had hoped for more comments on this as well?!?!?
  9. dw8

    dw8 LawnSite Member
    Posts: 5

    Thanks for responses.


    Have looked at "Mirafi" carried by one of our local suppliers. This is a plastic waffle with nipples about 1/2 inch high and glued to the nipples a polyethlene filter fabric. For installation on vertical walls (poured concrete) this would be an easy install, but for stone with a rough back area IMHO there is no way to get this around the rock and attached to weep pipe.

    That aside, I have yet to get a definitive answer regarding the longivity of the filter fabric. Seems to range between 8 - 20 years. Clogging of the fabric appears to be a function of soil content.
  10. dw8

    dw8 LawnSite Member
    Posts: 5

    Note of possible interest: Did some reseach on Net searching for "ancient drainage methods". Came up with an interesting article on Inca drainage.

    Seems they built retaining walls to terrace for farming in a tropical climate with lots of H20. Supposedly drainage is still working to this day. Article indicated they layered about 1 meter of stone chips (assume from rock wall construction -- size not indicated), then a layer of gravel (again size not noted), and lastly a layer of sand before adding top soil. They also had several outlets on top of the wall to direct water to flow to next terrace and this was routed in what seemed to me to be a swall of some sort. NO FILTER FABRIC was used.

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