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What type of soil or other material should be used in a shed foundation?

I'm building a gravel foundation for a timber shed on a sloped site (about 8%). Due to the slope, I need to fill to level. At the lowest point the foundation will be about 420mm above natural ground level. I will construct retaining walls with sleepers and fill (with what?) to within 100mm of the top, and then add crushed stone for the last 100mm. So the soil fill will be a wedge which is about 320mm at the thick end. I will use a plate compactor to ensure both layers are well compacted.

I'm just a bit unsure what I should buy for the fill. Is basic soil fine? This, for example? I assume the main quality I'm looking for is that once I compact it, it won't settle further under the weight of the shed. Are there other factors to consider?

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The linked soil that you provided appears to contain little cohesive blobs of soil. This suggests to me that the soil has non-trivial clay content. Under the Universal Soil Classification System (USCS), it's probably either "poorly graded sand" or "clayey sand" (you'd have to get it wet and play with it to know for sure). Poorly graded sand won't compact, so that would be no good for your purposes. The website mentions that it's used for "backfill," which sounds consistent with the poorly graded sand theory.

If it's clayey sand, then it will compact (if you get the moisture content within a range of acceptable values). It also has medium frost susceptibility, however, so the ground below your timber shed might heave a little during the winter. For habitable spaces that could be a deal breaker, but for a shed it doesn't sound terribly offensive.

Crushed rock with the fines ("3/4 inch minus" in the US) is idiot proof on the moisture content and doesn't have frost susceptibility, so that's what I would use. Crushed rock gets categorized as "well graded gravel" under the USCS. That and "well graded sand" are the two categories with low frost heaving, low expansiveness, and good compactibility, making them the best choices for subgrade material, especially above the frost line.

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  • There is a clay based product available here too (confusingly to me, it's known as granite). Here and here. Apparently it has excellent compaction. Would that be a sensible foundation? Frost isn't really an issue in my area.
    – Igby Largeman
    Commented Oct 20, 2023 at 2:40
  • The notion of using crushed rock for the entire foundation, rather than just the top 100mm, did cross my mind, but I thought it seems a bit extravagant due to the cost. But if something like the "granite" I mentioned above is appropriate, the cost is actually around the same as crushed rock.
    – Igby Largeman
    Commented Oct 20, 2023 at 2:47
  • @Igby Largeman, I've got a local quarry that charges 16 USD per cubic yard of "5/8 inch minus" and "1-1/4 inch minus" (that's 34 Aussie dollars per cubic meter), so we just throw the stuff all over the place. The issue with clay even if there's no frost is "expansiveness," where it can swell with water. Your "red granite" lists a plastic index of 10, where less than 20 implies low expansiveness. 10 is the exact transition point between "silty" versus "clayey," so I'm a little suspicious that the 10 was chosen to dodge the label "clay."
    – popham
    Commented Oct 20, 2023 at 3:40
  • @Igby Largeman, they also say "no swell rate," which is reassuring. It sounds fine. I'm a little uneasy about its compactibility. If it's used locally as subbase material, though, then it should be fine. Like I said, crushed rock is a sure thing.
    – popham
    Commented Oct 20, 2023 at 3:41
  • Thanks. Maybe I'll just go for crushed rock all the way then. It certainly simplifies things having only one product, and I also feel more comfortable knowing the entire foundation has great drainage.
    – Igby Largeman
    Commented Oct 21, 2023 at 23:31
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In the US, I frequently see signs posted for "clean fill dirt" (either wanted or available). What you've linked to seems to be "clean fill dirt", so it would probably be quite appropriate.

I'd say you're on the right track, but that you should give the company a call, just to be sure.

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  • Good point about calling them, thanks.
    – Igby Largeman
    Commented Oct 20, 2023 at 2:42

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