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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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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.

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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? – Paul Price Jul 13 '18 at 16:11
  • 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 – The Evil Greebo Jul 13 '18 at 16:15
  • 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"? – Paul Price Jul 13 '18 at 17:24
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    So 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. – The Evil Greebo Jul 13 '18 at 17:27
  • Of course, that's what we're here for – The Evil Greebo Jul 13 '18 at 21:43

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