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I'm prepping to build a fence in the back yard and am going back and forth between using crushed gravel and concrete. I've read lots of differing opinions on the pros and cons of each.

I'm leaning towards using crushed gravel as it's easier to repair posts if they rot. I'm just worried that with clay soil it will just fill with water and stay there for days. I'm not 100% sure the concrete would help that though as I assume water would still get in to the post and sit there.

On the other hand I'm also thinking concrete would be easier to install, as tamping 30ish holes 48" deep with crushed gravel doesn't sound very fun. I will be using pressure treated lumber if that helps

Anyone else build fence in heavy clay with any luck? Any recommendations greatly appreciated, thanks!

  • Side note: the best tamping rod I've found is an 8-foot length of 3/4" steel water pipe with a cap threaded on one end. You want the tamping rod long so you don't have to bend over to tamp the fill at the bottom of the hole, and the steel pipe has a good mass. – Jonathan J Oct 27 '17 at 18:58
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In clay soil, the water just sits in the hole, especially where the water table is close to the surface. Gravel lets the water get right up against the post, which will rot it out fast. Using concrete will help the post to last longer, if you create a dome of concrete above soil level and make sure it is smooth and tight against the post. This will ensure that no water gets down into the post. Also, make sure the bottom of the post is not below the concrete level or the post will soak water up from the bottom (pour a little concrete in the hole, then set the post, then fill the rest of the way)

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Your reservations are very warranted. I've never had a problem in using just the soil I took out to put the post in. Even posts that warped severely after installation moved the fence, not the ground.

The good thing about clay is that water runs off and doesn't penetrate deeply at all. So, stay away from concrete, it retains moisture to the full depth and rots the post. Same goes for gravel, it just creates a pocket to receive water to then drain into the soil. Crown around your posts to force water away and keep fence panels/pickets 1/2" off the ground, you'll be set for decades.

  • Crushed gravel, especially if it has a lot of "fines" (everything from silt & sand up to the largest gravel in the mix) packs VERY tightly, as tightly as concrete. As a bonus, you don't need to brace the post like you would waiting for concrete to set. Remember, if water can get in you need to provide a way for it to get out. Crushed gravel, being porous, allows water movement both ways. Concrete can trap water that get in between the post and the concrete, and the wood can get compressed over time rendering the post wobbly in the concrete. – Jonathan J Oct 27 '17 at 18:55
  • Absolutely correct! I've run into a number of posts that were set into concrete and they literally pivot, can hear them hammering in the wind. Swirling winds rock them back and forth to quickly break them right in half. I really don't know where the crack-pot "engineers" came from to sell concrete for all circumstances, just silly for 90% of any continent. And gravel, I'd only use in loose or very sandy conditions. Just going deeper fixes most any setup. – Iggy Oct 28 '17 at 2:59
  • The soil removed from the hole only works well as backfill if it is not saturated and not loose sand. You can't tamp muck. Well, you can try, but you won't get good results. – Jonathan J Jan 26 '18 at 1:00
  • I completely agree. – Iggy Jan 26 '18 at 13:59

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