Compaction questions...

Discussion in 'Hardscaping' started by GreenLight, Jan 29, 2010.

  1. GreenLight

    GreenLight LawnSite Senior Member
    Messages: 508

    We are presently working on a paver patio approximately 20 L x 12 W and find ourselves hitting a wall a bit. I certainly do not claim to be an expert mason, but I do have a few years experience with masonry and the base material incorporated in the install. I am running into a bit of a unique situation on this job.

    1) Site was excavated and dug down to virgin soil generally 8 inches below our grade everywhere the patio would sit. I do admit that even after 8 inches of excavation the ground was still a bit damp, but I figured this to be the result of starting excavation 2 days after some heavy rains and the machine was simply spreading surface water into the new ground floor as it traveled.

    2) Put out 3 inch layer of what we call 57 stone which is generally similar to french drain style gravel surrounding a corrugated perf pipe. Tamped this is. Overall it doesn't get ultra firm, but certainly pretty tight.

    3) Next we installed the 4 inch 8910 layer (crushed gravel base layer). Using a vibratory plate tamp we compacted this numerous times every 1-2 inches.

    4) The crushed gravel layer was not getting firm and rock solid like it usually does, in fact it started getting almost spongy with every pass we made with the machine. So we left it overnight and almost magically it got a lot stronger so we attempted to use the plate compactor again and once again the crushed gravel started sponging and actually was beginning to sweat out moisture a bit but not enough to pump out.

    It appears we have some sub surface percolation occuring that is drawing up into the crushed layer with every pass we make. Oddly it firms back up pretty tight (not perfect, but certainly not bad) after removing the machine from it. Has anyone else ever found themselves in this situation where it seems the more actively you compact the worse off it becomes. Obviously I firmly believe in compaction, but at this point it almost seems to be hindering progress.
  2. DVS Hardscaper

    DVS Hardscaper LawnSite Fanatic
    Messages: 6,553

    well, #1 - paver work is a whole different world than "masonry". I'd venture to guess 98% of the participants are a far cry from masons :usflag:

  3. GreenLight

    GreenLight LawnSite Senior Member
    Messages: 508

    Haha, well I just wanted to clarify for the record that im not generally a "paver layer". Most of my experience is with flagstone patios w/mortar joints etc. That being said, the pavers and flagstone work are similar in that they both require a firm base and subbase so I figured they were somewhat relevant in that capacity.
  4. Bru75

    Bru75 LawnSite Senior Member
    Messages: 582

    It's been my experience that once the ground starts to pump you have to dig it out, no amount of compaction will fix it once it has started to move, it will only make it worse. I have had to dig as much as two feet to remove undesirable soil.
    The solidification you are seeing overnight is the top layer drying out and bonding. Once the compactor vibrates it it breaks the bond and caused the ground underneath it to move again.
    On a wet subgrade you can spread lime or portland cement to firm things up, it also helps a lot to put down a good woven geotextile fabric under your base.
  5. coolluv

    coolluv LawnSite Platinum Member
    from Atlanta
    Messages: 4,281

    Sounds like you are going to have to remove what you put in and start over. If you have ground water weather it be a temp situation or permanent situation, like a spring or drainage issue you will have to fix that first. Get a probe rod and test the soil once you removed the stone you put in. If you can push it down easily or more than a few inches then you have to keep going down until you hit something solid. At some point you should hit solid ground. If you don't then you may be able to bridge over it, meaning you could take out an extra 3ft, put down some fabric and then 3ft of 57 stone, followed by extra sub grade material. Crusher Run or GAB what ever your using. 57 stone is 100% compaction so there is no need to put it in lifts or tamp it. When you tamp the 57 stone you are pulling the water to the top and it is contaminating the sub grade.

    You could static roll it (no vibration) but thats it. When you put the sub base stone in then you could vibrate it like normal. Another thing is you could have unsuitable soil underneath (topsoil, organics ) whatever. Builders are notorious for doing that to save money. You could also hire a Geo tech to come out and do a bearing test and they will give you a path to follow.

    Just my 2 cents.

  6. downrigger

    downrigger LawnSite Member
    Messages: 3

    We had the same situation arise on a significant size patio which is identified as "ground" or "spring" water table. The way we solved the problem was excavate another 6", put a layer of woven textile down, added 6" of 3/4" clear gravel to create a drainage bed (which goes down at 95% standard proctor density without any equipment). The woven textile stops all fines from clogging upwards into your drainage bed as the water rises. Put down another layer of textile then start adding your base. Turned out great.
  7. GreenLight

    GreenLight LawnSite Senior Member
    Messages: 508

    I appreciate all the responses and it seems my nightmare is confirmed. Remove and excavate more and seperate with geotextile. Thanks everybody.

Share This Page