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  1. #1

    Join Date
    Apr 2001
    Posts
    388
    Location
    Southern Maryland
    Tractor
    L3010DT

    Default How much gravel under a slab?

    I am currently in the midst of a 30x40 garage construction. I am pouring a 5" slab over 4" of compacted gravel. The problem is, I don't know how much 4" of compacted gravel is when it comes to buying the stuff. How much does it compact, how many ton (the usual purchasing measurement) do I need? Ordering concrete is easy. It's sold by the yard. Ordering gravel, well? I am pretty much clueless. Any help out there?

    The volume roughly equates to 13.8 yards when compacted. What's that weigh?

    Thanks in advance.

    Nick

  2. #2
    Super Member
    Join Date
    May 2002
    Posts
    5,809
    Location
    Wylie, Texas
    Tractor
    JCB165HF

    Default Re: How much gravel under a slab?

    Figure gravel and sand at or about twenty five hundred pounds a yard.

    I have a bud who has a fleet of trucks hauling sand and gravel. He's told me to figure a ten yard load will cover four inches five hundred square feet.

    I've heard some of the cities now are getting away from wanting cushion sand under a slab while others still demand it. The high school where I'm doing a ton of fence has a lot of hard pack sandy soil. Stuff is harder than a bad girl's heart freshly broken. What blew my mind is they had to excavate down two feet into the sandstone and put in topsoil before they could do the tennis courts. Something about how the sandstone when it got wet would swell like clay and since it's so tightly packed the pad for the tennis court wouldn't last a year if placed directly over it.

    Here I was thinking the sandstone would be the most stable soil available for a pad. Go figure.

  3. #3
    Super Star Member
    Join Date
    Aug 2001
    Posts
    12,099
    Location
    Upper Midwest USA
    Tractor
    JD 4300, JD X485 JD 4x2 Gator, JD 425, JD455

    Default Re: How much gravel under a slab?

    Around here we order by the yard, same as concrete. However, when it is billed it is by the ton. I expect the trucks are such a size that they know how many yards of material they hold. Then, selling by the ton is the easiest way to 'measure' how much is actually on the truck. AND they then get paid for the water that is in the gravel too, which is okay. I would expect the pit will be able to make the conversion to yards or to weight for you.

  4. #4

    Join Date
    Jul 2002
    Posts
    104
    Location
    Michigan
    Tractor
    JD 4200, JD B, JD 50

    Default Re: How much gravel under a slab?

    Gravel is usually sold by the cubic yard. If you want 4 inches of gravel after compaction you will need about 20% more volume than the 40x30x4inches. The 20% is how much the gravel will compact to its maximum density. You will need to add the right amount of moisture the gravel to get it to compact its fullest. Better a little dry than too wet. So in your case you would need to figure enough gravel to cover 30x40x about 5inches deep of loose gravel. Remeber you are going to compact to about 4inches. Thats works out to be
    18.5 yds. Order 20 yds you will find a use for the extra. Around here that would cost $200. That is generally 2 truckloads.
    I use a vibratory plate compactor to squash the gravel. They can be rented at most rental places for about $50 for an afternoon. Loose gravel weighs about 110#/cubic foot.

    If you are building up the gravel above the existing grade you will need all of that 20 yds because you will have to taper the gravel back to the existing grade. Compact the &^%&^$ out the sloped gravel too. even if you are not above the existing grade its a good idea to extend your gravel drainage outside the perimeter of your garage about a foot. slightly more if above grade.

    btw you will need 18.5 yds of concrete too. it weighs 150#/cu ft. and btw its 14.8 yds after compaction

  5. #5

    Join Date
    Apr 2001
    Posts
    388
    Location
    Southern Maryland
    Tractor
    L3010DT

    Default Re: How much gravel under a slab?

    Thanks all,

    That makes it simple. Looking for minor clarification. On the concrete, are you saying that ordering concrete by the yard on the truck will compact down 20% as well. I did not realize that or really think that. So I need to order 20% more concrete than I think I need? Never bought concrete and here I thought it was easy, or am I misinterpreting you. And am I SOL when the 7.5 yds of concrete for the footer shows up tomorrow and it settles in and only fills 6 yds of space?

  6. #6
    Silver Member
    Join Date
    Mar 2002
    Posts
    213
    Location
    Northern Michigan
    Tractor
    Ford/NH 1715

    Default Re: How much gravel under a slab?

    I don't think I'd use compacted gravel under a slab. Maybe for a base, but then I'd put 4" of compacted sand on that. Sand compacts tighter than gravel.

  7. #7

    Join Date
    Jul 2002
    Posts
    104
    Location
    Michigan
    Tractor
    JD 4200, JD B, JD 50

    Default Re: How much gravel under a slab?

    Sorry Argee, gravel will compact far more than sand will because gravel has better gradation. Sand has more particles of the same relative size than gravel does. The same size particles prevents the sand from filling the all the voids. Gravel has a wide variety of size particles so when it is compacted the smaller particles will fill in between the larger ones. This gives you much higher density than sand. The only time I bother to use sand above gravel is when the layer beneath your final course must be flat. For brick pavers or stone etc. Having said all that it does not hurt to use sand above compacted gravel to bring your base course up to grade. It is easier to spread.

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