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I'm putting in a stepping stone path. The stones will be on top of 4" of packed, 3/4"- (three quarter minus) stone - which packs to a nice, solid base.

My question is, if I know that the volume of the stone in the path is going to be (for example) one cubic yard (3'x3'x3'), what amount of stone should I order?

In other words, if I ordered one cubic yard, spread it out, and tamped it down, how close would the resulting volume be to one cubic yard? Is it close enough to not worry? Or would it compact down to only be 75% of the cubic yard?

Edited to add: For the sake of this question, assume that I've already added a buffer in my order for voids I've not accounted for, that I've already adjusted for loss while wheel-barrowing the stone to the area I'm going to put it, that I've accounted for deviations in how much the gravel company delivers, and that I've already accounted for extra slough off the shoulder of the base layer.

The question is NOT about "how to build a path?" or "it's just a path, why do you care?" - the question is simply, how much volume is lost when compacting 3/4"-? It's obviously non-zero because it actually does compact (as opposed to river rock which does not compact).

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    Less than what all those other additions call for in extra stone. So double that overage; the loss is going to negligible. It will be close enough to not worry. The quality of the job without using a sand topping will suffer. Considering the smallest order is probably half a yard (if that), how accurate do we really need to be? This is a stretch for DIY... I can't wait for Material Science SE.
    – Mazura
    Nov 20, 2014 at 2:48
  • @Mazura -- Seattle's major rock and topsoil vendor sells crushed rock in increments from 5 gallons to the truckload. The bulk price (for picking up material at the vendor's yard) is available in quarter of a cubic yard increments. A quarter of a cubic yard is enough to fill 10 5-gallon buckets.
    – Jasper
    Jan 14, 2018 at 6:50

7 Answers 7

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When something calls for x" of compacted y, you typically calculate the amount of y prior to compacting.

So if something calls for a 4" compacted base, order enough to cover your area with 4". Then compact.

(Actually, order to cover 4", but the only lay down half, compact that, then lay down the other half, then compact that.)

In other words, you don't typically order the stone to allow for compaction. You order the stone based on the volume needed prior to compaction.

UPDATE:

If you're just asking about the rate of compaction 3/4 minus has, the answer is...actually, that's a tough one. There seems to be a lot of opinion, but no specific engineering spec that I can find. General googling seems to imply you'd be in the 20% loss due to compaction range, but I can't find any hard data to back that up.

Keep in mind that there's also issues of your soil base compacting as well.

So based on that limited info, I'd suggest ordering 20% over. Keep in mind that this may be a trial-and-error process that may not be worth the headache. If you can't get 20% compaction, then you're left with extra crushed stone that may or may not be easy to get rid of.

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  • -1 Doesn't answer the question. The question is how much will it compact?
    – Trey Jackson
    Nov 19, 2014 at 20:56
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    @TreyJackson no, you are asking how much to order. If you are asking what the compaction rate is, then I suggest changing the title of your question to reflect that.
    – DA01
    Nov 19, 2014 at 20:59
  • Correct, my comment is 1/2 wrong. You didn't answer the question, but I wasn't asking how much it would compact. I was asking how much to order allowing for compaction. It appears you're deliberately trying to misinterpret the question - or perhaps you only read half the title, and not the 2nd half, or the body of the question. All of which talk about compaction - which has been explicitly mentioned 3 times in the question by the time you gave this answer.
    – Trey Jackson
    Nov 19, 2014 at 23:36
  • @TreyJackson read my updated answer.
    – DA01
    Nov 19, 2014 at 23:47
  • If I may, I'd suggest you edit your question down to the bare essential: "What % of volume is lost when compacting 3/4 minus?"
    – DA01
    Nov 19, 2014 at 23:49
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From my experience it depends a lot on how is the compaction done. If you are compacting road crush with a vibratory plate you will get about 10 to 15% compaction. (This is based on you having a stable compacted base first. If you are putting the road crush on top of top soil you will lose more to compaction.) If you use a jumping jack, about 20% would be accurate. I personally like a jumping jack because it is very evident when it is fully compacted (The jumping jack will 'jump' when it is fully compacted. It will also keep compacting into soft areas till they are fully compacted. If you get to clay base you can expect about 20%, but if you are in top soil, it can take significantly more volume to fill and compact).

You also need to asses what you are placing over the top of the road crush. If it is a sidewalk it will not get the same psi as a driveway. If you are parking a motorhome then you want to ensure a fully compacted base. Think of it this way. Spending a weekend with jumping jack is worth not having cracked driveways.

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A number of websites have recommended adding 4% to the order to account for compaction.

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  • This doesn't actually answer your question. Your cited sources refer to driveways--not base layers for pavers. You need to compact hardscape base layers much more than surface gravel for a driveway.
    – DA01
    May 30, 2017 at 2:25
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I was actually looking for an accurate range for a specific stone and wound up here... The answer is complicated but boils down to about 10 to 25% over order based on a bunch of stuff.

The loose drop weight of a relatively uniform (all the rocks are similar size and shape) 3/4" stone will be about 80-90% depending on the stone (angularity and phi). Presuming that you are compacting to 100% of some form of "proctor density", because you're awesome at compacting and have a monster machine (and thin lifts), that's 100/80=125%. You will not achieve 100%. You will not achieve close to 100%. 100% is what some sledge hammer thing does to a couple of buckets of soil over the course of 15 to 30 minutes with some guy standing there adding water until it's perfect. You will be lucky to get 93% with standard effort. That gets you about 10% loss in volume.

However, as others mentioned, you will get more loss on a finer substrata. This is mostly because the finer strata (sand silt clay) will ooze around the stones as you literally drive the stone into the subgrade. If the subgrade is rocky, the stone will "hang up" on the underlying rocks and you won't have such a loss... at first... upwelling water will probably manage to settle your gravel a little by moving surrounding finer soils over time.

With a more properly graded 3/4" washed stone (such as a good filter stone ranging between the 3/8" and 1" diameter), you will have a loose drop of more like 75%, but you will also actually be able to get closer to 100% without talking about your subgrade. If the subgrade is sandy, it will not invade such a well graded gravel as much. This is a superior situation.

If you're a glacier, you will take a loose dropped stone weighing roughly 105 pcf, and make it into metamorphic stone weighing 167 pcf, meaning that you should have ordered 159% of your required thickness... for your glacier walkway or whatever.

~Geotechnical P.E.

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75% sounds like the upper end, I don't really know. I will suggest using sand, 1/4" minus or fines, shown as the 1" of sand in the picture. The 3/4s is for drainage, 1/4 or sand is so you can set them properly allowing for deviation of materials. Delivery is expensive, buy a little extra. Or build back up what you've lost by adding the sand topping. Sand you can be pretty sure about.

enter image description here

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  • Agreed 75% is not a realistic - that's why I'm asking the question. ;)
    – Trey Jackson
    Sep 30, 2014 at 15:28
  • I think your measurements will be off by more than what will actually be compressed. At least %10 is going to disappear into voids you hadn't accounted for. @TreyJackson
    – Mazura
    Sep 30, 2014 at 16:18
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Without having done any experiments, my scientific wild-ass guess (SWAG) is 4 to 8 percent. Here are two reasons for thinking this is reasonable:

  • The densest packing of spheres is about 74%, whereas a common less compact packing is about 68%. (74% - 68%) / 74% = 8% potential volume loss.

  • A standard specification for careful tamping of backfill is "in 6-inch lifts to 95% of maximum density". In other words, after tamping, there is still a potential for 0% - 5% potential volume loss.

The 74% packing density is "face-centered cubic" or "hexagonal close pack" or "tetragonal body-centered cubic". The 68% packing density is "body-centered cubic".

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Shame this question is so old as I can offer a far more comprehensive answer. The truth is it's impossible to create an exact formula. Too many variables. What proctor density are you compacting to? What is the density of the substrate? What is the moisture content of the substrate and the material to be used? How much weight will be applied to the finished product? Some of our roads have a weight limit, compaction is a very complicated thing.. when ordering stone it depends on availability. Under-order easily available stone and top up if needed. Over-order difficult to access stone and dispose of if necessary...

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