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As I was planting a few fruit trees last weekend, I discovered an odd and very annoying phenomenon. About 6" down from ground level, I ran across a 2" layer of really crumbly concrete/gravel mixture. Beneath that was clay for as far as I dug. The dirt above the concrete layer was slightly better quality topsoil.

I found this in every one of the holes I dug for the trees, which spanned a length over 60' about 10' away from my privacy fence. (The trees were dwarf fruit trees, hence the closer spacing. The recommended separation between trees was 15'.)

At first I thought I discovered some sort of massive French drain covering the entire backyard. But when I looked closer, I realized it wasn't just gravel - it was actually crumbly concrete. Does anyone have any idea why this was in my backyard? And will it significantly affect the trees I just planted?

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I think @winwaed's answer (fill) is probably the reason for the concrete. Did you dig out the concrete for each planting hole and replace with good soil? – bstpierre Dec 9 '11 at 1:50
@bstpierre: Yes, I dug out each hole about twice the size of the root ball, both vertically and horizontally. I backfilled with a mixture of about 10% mulch for drainage, 10% original topsoil, and 80% good topsoil. – Doresoom Dec 9 '11 at 14:43
up vote 10 down vote accepted

Who knows? There could be lots of reasons. It might have been to help drainage (although proper rounded gravel probably does this better), but it might be just builder's fill - and a bulldozer leveled it out before the final top soil was added.

A large number (majority?) of urban backyards in the US are made of varying degrees of builder's fill. All they are doing is leveling it out with whatever is lying around, what was dug out, demolished, etc. Older houses tend to fare better (but have the most 'history' from previous owners instead) - newer houses tend to be worse.

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Probably from the builders, as winwaed and LarsTech already point out, and previous structures that were built there.

Though our backyard sounds much smaller than yours, it sounds like you've been just as unlucky. Our house is on land near a city centre that has undergone an "urban renewal" project, transforming from really shabby houses to slightly larger houses on slightly smaller blocks.

Our backyard's soil was so bad, and so crammed full of the builder's fill that winwaed describes, that we decided to get the top 15cm (6") soil removed and replaced with a sandy loam/compost mix. During the process we not only found crumbly concrete from previous house foundations, but many well-rusted copper pipes from what must have once been a bathroom and bits of tile and crockery.

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It sounds like the original builder may have dumped any remaining concrete from the mixing truck into your yard and top soil was just thrown over that later. Although 60' is a lot of concrete. Maybe it was once a drive way.

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Yes, that's a huge amount of concrete -- way too much to be the washout from the truck. More likely that it's fill, as @winwaed says. – bstpierre Dec 9 '11 at 1:45

We caught our builder burying crumbled concrete below our garage floor when building. They were bringing it from other job sites to save on dump costs. Since our house was the next to have concrete poured they thought they could hide it all below our garage floor. My husband, a concrete/materials inspector for over 16 years, just happened to show up while they were doing it. He made them move it all because it can make any structure built on it unstable. 2 months later while laying sod we found it under our dirt in the back yard. We once again made them remove it.

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