Grid vs geotex

Discussion in 'Hardscaping' started by Edgewater, Jan 28, 2006.

  1. Edgewater

    Edgewater LawnSite Senior Member
    Posts: 457

    can someone clarify a few thngs for me.
    I will not attempt a project before I am convinced that I know how to do it inside and out.

    For a wall, i have 3 questions.

    1) The drawings in the tech manual for the stone we use says to layers of geotextile to tie the wall back into the soil. What is the difference between goetex and grid. Also, how is it ancored to the block.

    2) When backfilling the wall. Lets say it calls for 48 inch lengths of geotex as ties, is that whole area done with 3/4 clean and wrapped in geotex, or are the horizontal layers of geotex sandwiched with soil.

    3) In the drawings, it show the 4 inch perf drain in the compacted 0-3/4 base below the footing, with the compacted base extending to the back of the backfill zone at a 2% slop. Do you guy put your drains in the 0-3/4 or in the 3/4 clean??

    Just want to get this clear befor I build any wall. I have done several patios, but no walls.
     
  2. cgland

    cgland LawnSite Bronze Member
    Posts: 1,929

    Edge - THe difference between geotex and geogrid is huge! Geogrid is material used to reinforce SRW's. Geotextile is a fabric used as a soil separator.
    1) It is anchored to the block simply by laying it on the block and utilizing the block pins and next course to hold it in place. If your SRW has a lip system the weight of the next courses holds it in place.

    2) The backfill material used in the grid zone would be up to the engineer. This depends on many factors i.e soil type, toe slope, slope behind the wall..etc. If you were just doing a 3-4' wall grid may not be a necessity depending on the block. In those situations (where I am approaching the manufacturers max height w/o grid I may throw a pull in depending on the slope and what I will be retaining.
    Most cases you will be able to use topsoil for your backfill zone (behind your 1 foot of drainage stone. If this is the case and you have multiple pulls of grid you will have to wrap each segment with geotex. You don't want to wrap it totally, but just enough so that the soil will not leach into your clean stone. Then just place grid as usual.
    3) We place our drain tile on the 3/4 mod. We slope the compacted stone to an adequate degree to achieve runoff. (If you use black corrugated, make sure you don't have any dead zones (dips that could trap water and impede flow)) Once we have our pitch, we place the drain tile on the mod stone and continue the backfill process w/ clean 3/4".

    I hope this helps. I am not the best at explaing things while I am typing. I will try to find a diagram to explain #2

    Chris
     
  3. cgland

    cgland LawnSite Bronze Member
    Posts: 1,929

    Edge - go to Anchorwall.com

    There, click on contractors and then cad drawings. There you will find a wealth on info and diagrams on all facets of SRW installation.

    Chris
     
  4. UNISCAPER

    UNISCAPER LawnSite Bronze Member
    Posts: 1,426

    If in fact that drawing tells you to use geo textile fabric to back tie your wall, call the design firm and question what they wrote. Ask for a revision on the specifications. If they refuse, fire the engineer and get someone who knows what the hell they are talking about.

    Chris explained the differences very well. Even though homeonwers style manuals are the most basic and manipulated things ever written, I can't imagine a block maker writing that you use Geo Textile for back ties.
     
  5. Edgewater

    Edgewater LawnSite Senior Member
    Posts: 457

    I have posted a drawing of what the maual shows. I noticed that in the site Chris sent me to, the drain is right behind the footing and there is a pocket of 3/4 clean that brings water to the drain from the excavation line. Does it vary from wall type to wall type, or do you guys think that one particular drain placement is better?

    Also, if you were to do 1 foot of backfill with stone, and the rest was soil, then I assume the drain must be right behind the wall. In that case, you have to wrap each pocket of drainage gravel with geo right?

    I would like to see this done first hand, but most of the guys around here (In my area) are very paranoid and think that helping someone out is bad for themselves.

    Thanks for the help.
    Adam
     

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  6. cgland

    cgland LawnSite Bronze Member
    Posts: 1,929

     
  7. UNISCAPER

    UNISCAPER LawnSite Bronze Member
    Posts: 1,426

    Actually, depending on the site situations, you could have a pipe behind the wall, connected to daylight, and also a pipe at the cut line by the end of the grid. In this diagram, they show the cut line as being one straight line tapering up to the top of the grade. And, in reality, this is what we usually find when we arrive at sites where a grading contractors job was to make those earth cuts to build the wall along with his other site work.

    What the ideal situation is, would be to step cut the back of the earth, so one cut would be at the back of the grid, the other about half way up would be another 2' deeper (farther away from the wall) than the bottom cut. This stops, or slows drastically, water from running down that cut line to the bottom of the footing.

    The pipe you see drawn at the back of the cut in the drawing is used to extract water that would follow that cut down between the backfill in the reinforcement zone (footing) and the virgin soil where the footing cut stopped.

    In the event of heavy rain, or a wet year, if water follwed that cut and found its way to the bottom of the grid, the soils could get wet and develope a condition the engineers call "plastcitiy". When that occurs, they turn to pudding, and that 95% compaction you worked so hard to get comes apart because there is no holding capacity. In the event water, (hydrostatic pressure) builds behind the wall, the grid can actually slide through the soil as the wall pushes away from the footing. The wall in this scenario can and will fail.

    So, if the engineer calls for a pipe at that cut, you need make it perforated and place 3/4 gravel around it and daylight the pipe. You also need a pipe behind the wall. Connect them to a standardized drain pipe, and make sure water going through that pipe can be kept separate from the perforated pipe behind the wall. Don't use that black snake pipe. That stuff is not rigid and will eventually fail. I prefer SDR-35, rigid PVC plastic pipe.

    There is one other thing I am curious about in this drawing. I see how they set a reversse pitch in the grade so water will drain towards the pipe.

    What will not work is the place they show the bottom block. You show compacted base under the wall, but there is no embedment course, i.e. a course of wall in a trench that sits below grade. Not doing that will cause a global stability issue, because the wall toe will kick out. The standrd requirement for SRW walls is for every 8" of elevation, you need 1" of wall course buried (embeded) with a minimum of one full course below grade regardless of height.

    I hope this helps!
     
  8. Edgewater

    Edgewater LawnSite Senior Member
    Posts: 457

    Not having and of the wall buried is my fault in the drawing.

    What I wanted to know was weather the pipe should go on the same level as the footing, or below it. Also, as you said, there is a pitch away from the wall in the base agg., our local manufacturers spec it that way, and you guys (Chris) have show images of it pitched towards the wall with the drain right behing it.

    I understand what you have said, and also that the homeowner type manuals are probably to generalized.

    The only project that is on the bid horizon right now is a 3 foot tall wall about 50' long with flat lawn and no extra load at the top. I think that would be a good starter.


    Thanks for all the clarifications

    Adam
     
  9. UNISCAPER

    UNISCAPER LawnSite Bronze Member
    Posts: 1,426

    It depends. In the ideal scenario, you would place the pipe on the leveling pad, with connections to drain the water that ran under the wall. Unless you can do that, you need to use the next best scenario. Place the pipe at the same elevation as the outside grade at the toe of the wall, then dump it out the wall face. If you place the pipe below grade with no place for the water to go, all it will do is sit there and cause trouble in the footing.

    You should also have grid on the bottom course.
     
  10. Edgewater

    Edgewater LawnSite Senior Member
    Posts: 457

    Is there a particular manufacturer of Grid that is better than others?

    I have never even seen it at any of the garden centers I buy from. But then again, we do weird things here like set pavers on stone dust.

    Thanks again for all your help.

    Adam
     

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