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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!

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  • 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
    Commented Oct 27, 2017 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.

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  • 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
    Commented Oct 27, 2017 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
    Commented Oct 28, 2017 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
    Commented Jan 26, 2018 at 1:00
  • I completely agree.
    – Iggy
    Commented Jan 26, 2018 at 13:59
  • As a counter point, I set fence posts, dumped dry mix in the holes and hosed it down about 25 years ago. I've replaced the fencing 3 times, but the posts (with one exception) are fine. There is some splitting at the top of the posts, but none are loose or rotting out at ground level.
    – FreeMan
    Commented Jul 22, 2020 at 16:59
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I always use exterior clear sealant where concrete meets post at base. This keeps water from getting between concrete and post. I also brush on a waterproofer on entire post that is above the concrete. I then spray the post a rubberized undercoater to about 4 or 5 inches above the base. Very effective in case any water or dirt contacts the post. I use pressure treated posts. Takes no time at all and a very economical way to keep posts lasting many years.

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I am a fourth generation fence company owner in Florida. In my entire career, we have set wood 4x4s in the ground without concrete as does every other fence professional around. The concrete is not smooth in a hole and holds water. Every single post set this way rots quicker right at the top of the concrete in just a few years. However soil alone packed well will over time form a suction-like seal around the post to hold it very well, and the post will last for decades without ever rotting at one spot. ** (this is true unless doing government work. They require a one foot radius of concrete around the post because they obviously like to spend extra dollars and replace the fences more often. In my opinion).

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How about digging down to the clay, inserting a spike, attaching the post then backfilling with crushed rock and tamping?

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