I'm putting up a 6ft vinyl privacy fence. The holes reach 42 inches (city mandates 36 inches). I was planning to put a couple inches of gravel in the bottom of the hole before filling with cement (per fence kit instructions), but I'm getting a bit confused now I'm thinking about it. The thing is, I've seen my holes in the clay soil hold water for days, so gravel at the bottom of the hole would just trap water with nowhere for it to go. Now, it's below the frost line so it shouldn't heave the post up, and it's a vinyl post so it shouldn't rot, but it's not doing any good either, so what's the point?
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No, he says it is per the instructions.– The Evil GreeboJul 13, 2018 at 14:18
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@isherwood The Instructions say: "If frost line exceeds 30 inches, dig hole to the appropriate depth and then add 6 inches of gravel for post drainage." (Step 2c in pdf.lowes.com/installationguides/040933139507_install.pdf) The people renting me the post hole digger also mentioned gravel.– Paul PriceJul 13, 2018 at 14:21
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1 Answer
The clay, as you state, will trap water with nowhere to go as well. By digging the whole deeper and lining the bottom with gravel, what you do is provide a place for extra water to drain to until it can dissipate/drain.
Without that extra space for water to drain, it can fill up the hole above the frost line and cause lateral displacement.
answered Jul 13, 2018 at 14:23
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The hole is, of course, full of concrete. When you say the water "can fill up the hole", do you mean it fills up the minuscule gap between the concrete and the (clay) soil? Jul 13, 2018 at 16:11
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The hole wont be full of concrete. It will be full of gravel at the bottom which has spaces in it, and concrete above. Some concrete will penetrate the gravel to some depth but it won't go all the way. Water will seep around the concrete down into the gravel. Image attached for clarity Jul 13, 2018 at 16:15
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Please forgive me for being slow to understand, but if the gravel isn't there then the concrete is right up against the clay soil and there isn't any hole for the water to accumulate in, right? Then it can't "fill up the hole above the frost line"? Jul 13, 2018 at 17:24
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1So I think part of your issue is you're thinking of clay as being impermeable. It isn't. It's very SLOW to drain, but it does drain. Second, the concrete will be flush with the clay when it first cures, sure, but over time there will be gaps that develop, and water will seep from the clay into the space between the clay and the concrete. Without the drainage pan (aka: gravel) that water will sit against the concrete and gradually fill up the edges of the hole till it gets above the frost line - then you get heaving issues. Jul 13, 2018 at 17:27
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