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  1. #1
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    Default OT: Filling in a concrete pit

    Inside my building is an old car alignment pit ,about 10x10x 30" deep

    Expanding the shops and its time to fill it in and top off with concrete.

    Figured someone here has "been there done this" so..........

    what's the best material to fill the hole that will pack down the best so the concrete doesn't sink later??

    Figured I'd rent a tamper..........

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    Local shop uses "flowable fill" get it from the ready mix supplier.

    I'm told it's like very weak concrete, is very wet, so will "Self Level" and is able to be removed with an excavator (no hammer needed)

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    We call it controlled density fill(CDF) and that what I would recommend too. It's like 1 sack concrete. Often used to fill in soft spots under foundations.

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    CDF would be a be a very simple solution to the problem. It may not be the most economical though. If I were filling this shallow pit, I would certainly look at using crushed gravel similar in size to that commonly used on country roads. A ten yard dump truck load would do it with some left over. Some time with a small gas-driven compactor and a shovel and you’d have an excellent base that would stand up to very high loading.

    The time-honored cost difference vs convenience would make the decision.

    Denis

    Added: I did a quick search to get relative costs and see that CFI (a compounded and mixed material) may cost about 80 to 100 dollars per yard and gravel (a raw material) more like 25 to 35. We are looking at roughly 9 yards for this job. Cost of material and labor vs convenience vs regional variations in the above=which way to go.

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    Using that flowable fill I would think you could drop in big hunks of clean broken concrete or natural boulders as filler. Not really a good idea with dry gravel that will not flow up and around them to make it all one piece when it cures.
    Bill D
    Last edited by Bill D; 10-14-2021 at 03:22 PM.

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    Thanks guys so far......checking out stuff here .......

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    Yer going about this all wrong - that there hole is a gift! It's just what you need to pour a proper foundation for a new medium V or HMC. Chose what'cha want, get the anchor points settled, and Bob's your aunt after the realignment surgery.

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    Not the same situation but close. I had an in ground oil tank (about 1000 gallons) that my homeowners insurance company said had to go or they would not sell me insurance. I cut a hole in the top of the tank and had a dump truck deliver a load right into the hole. This was much cheaper then pulling the tank out of the hole and satisfied the insurance company. They used #57 gravel which looks like the stuff put on driveways. It was cheap.

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    Milland

    well...........how did you know??

    I do have a vmc on the way........but when the guy built this building the floors are a true 10" min of concrete.
    Yea total overkill for what he was doing here but thats what I'm be told by his family.
    I have done some drilling and vouch for that.
    LOL

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    Quote Originally Posted by MwTech Inc View Post
    Milland

    well...........how did you know??

    I do have a vmc on the way........but when the guy built this building the floors are a true 10" min of concrete.
    Yea total overkill for what he was doing here but thats what I'm be told by his family.
    I have done some drilling and vouch for that.
    LOL
    I knowz all, seez all (and haz a Sawzall).

    Sure, it's overkill, but I'm actually not kidding. Maybe another $200-400 out of pocket for a proper reinforced concrete block rather than fill and cover, but there's no other downside to having a really massive support foundation for a good VMC.

    It's just a matter of whether location of the hole will work.

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    Well since you haz a Sawzall

    Hole is not really in the right place....but
    most likely do the fill ,tamp..real concrete thing.

    Wondering if a sand/stone mix would tamp down a bit harder??

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    Think of all the bodies you could get rid of......

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    I would talk to a contractor in your area--someone who uses fill for building driveways or small building
    foundations. They should be able to tell you what "locally available" material is best for you needs.

    Where we are the Fraser River runs through here and provides a ready source of river sand which would
    be ideal for what you want to do. It's a finer material than bank sand and has a bit of clay mixed in so it
    packs very densely. A good grade of pit run gravel would probably work but I'd be wary of any kind of
    crushed rock. Without anything to fill the voids it's much more likely to settle over time.

    Whatever you use for fill you're going to lay a concrete slab on top so you want to make sure you "key"
    the slab to the walls of the hole. Drill a series of holes around the perimeter of the hole--at the correct
    height to line up with the centre of your slab--and glue in some rebar J-hooks using a good construction
    epoxy. This will tie the new slab to the rest of the structure...

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    Plain tamped crushed limestone with an average stone size of 1/2” tamps very well and locks together well because of its sharp edges. Quality fencing companies use it locally preferentially over concrete, using a small tamper to place it around wooden posts. On advice of a experienced landscaper 14 years ago we built a 3 foot high retaining wall using 4” treated poles “set” in tamped crushed gravel. It is still standing nice and vertical. Even untamped, crushed gravel would hardly settle at all. But tamping it locks it in place. Typical river gravel or pea gravel having round edges does not interlock well. Sand may tamp, in general pretty, well and some may lock up very well if it is angular and contains clay. Generally, the smaller the granule size and the more angular the granules, the better gravel/sand locks up. Again, local materials and knowledge should help.

    Denis

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    MW, I think I would use crusher run in 6 to 8” lifts, tamping after each. Ideally pug mix, which is crusher run that has been run through a pug mill. When it sets up it is hard as concrete. It’s what is used in interstate highways, in Tennessee at least.
    When you get to concrete depth, you’re ready to pour.
    Bob

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    I would use 2" minus/crusher run/road base and pack with a jumping jack until solid.

    I have a plate packer and a jumping jack. Jumping jack is more work, but it will make shit solid. 10x10 area is not a big deal for a jumping jack.

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    Quote Originally Posted by digger doug View Post
    Think of all the bodies you could get rid of......
    -Nah, eventually they rot and leave a void, which can lead to cracking of the floor if you then put a load on it...

    ... Or so I've heard!



    Doc.

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    Fwiw I've spent enough time doing work like this to have some
    recommendations.If you do back fill, a crushed rock and sand mix will always compact the best. But it's not quite as simple as it seems. You can't just fill the hole and then use the compactor.It has to be done in layers. With one of the small vibratory compactors you walk behind, maybe a 2"-3" thick layer at most. And pay extra attention to the perimeter that's against the concrete walls already there as it's the hardest to get adequate compaction. Depending on the ground type, in construction your maxed out at around a 7"-8" layer, and I've used 824 Cats or larger and what's called a hydraulic vibratory sheep's foot compactor. So they put a lot more pressure and a higher vibration rate per square inch than the small walk behind can. With work like this you have one chance of doing it right, do it half assed and it then costs big bucks to tear it all out and do the job properly. I'd also use a bit of water sprayed on the loose material on each layer before compacting it. About 5%-7% moisture content is what you want for the best rate of compaction. That water acts a bit like a lube to help the material slide and fill in any small gaps. Most don't really understand that there's almost constant minute ground vibrations and that's what does the ground settling over time. So what your doing is trying to duplicate what would happen naturally. If you don't do the best job possible it's inevitable you'll get further settling later. In construction or road building they actually have test equipment that measures the density of the compacted material so they know for sure it won't settle later.

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    Thks guys.... more good info....
    BTW the pit does have a solid concrete floor.

    Understand the layer thing...

    Understand the pin thing, ......contractor did that when we had a pad poured next to the building.

    Hey, if i dont tamp well then the pins can hold the concrete up

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    Quote Originally Posted by MwTech Inc View Post
    Thks guys.... more good info....
    BTW the pit does have a solid concrete floor.
    THIS ^^^ .... makes a difference.

    Presuming the "alignment pit" housed a stout underfloor machinery frame & such, good chance the PO built the sides stoutly and poured the bottom of it over properly compacted subgrade and made it as thick, if not MORE so, as the adjacent slab?

    Had it been DIRT, Neanderthal called it on the "thin" lifts for best compaction, and I'd have suggested bog-standard V-DOT 21A, 'coz everybody has it handy and priced predictably.

    IF it is built as stout as the rest, OTOH?

    I'd be VERY tempted to preserve the pit, have a removable, sectional steel deck fabbed!



    But Lybarger's Corollary to Sod's Law sez it will be in the wrong place for a VMC, so the CDF fill, "mostly".. then decent-strength re-barred slab atop is probably saner.

    You CAN saw keying slots to tie the bar to the rest of the slab, but ISTR it needs 20 diameters of run for grip and doesn't do the adjacent slab any sort of kindness.

    I'd actually not even fight thermals. I'd just take it as given stuff may move, but not by much, given same-same temps, year-round, and would build-in expansion joints.

    2CW


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