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What is a reasonable depth and diameter for postholes dug in clay soil where the frost line is no more than 12” and that will support an 8 foot tall solid wood fence that uses PostMaster metal posts?

I hope to have someone else install an 8 foot high PT wood fence. There will be no light or air passing through or under the fence and it will be heavier than average, given the additional height and board weight. One side will run for 135’. The attached diagram provides more detail.

The PostMaster Installation Instructions describe a 30” deep, sloped, 10” diameter hole, filled with 6” of gravel plus 24” of concrete for a 6 foot high fence. They caveat that the “exact diameter and depth will be determined by local weather and soil conditions.” Got it. Huntsville, AL; very clay soil. Maximum wind gusts of 65 mph. (This is not in the normal tornado path and is not intended to withstand a tornado.)

In reviewing the StackExchange posts on fencing and post holes I came across the concept that the diameter of the pier may be increased to compensate for a shallower post hole. enter image description here I would like to know the recommended depth and diameter without relying on the 2 buttress posts, which are my small overbuild idea. We will be able to walk under them.

26 APR 18 update: I just found a Master Halco youtube video specifying 6-8" diameter holes dug 36" deep for an 8' board-on-board fence, such as I would like to have. https://www.youtube.com/watch?v=mgjryMn7jI4 - I also spoke to a fence builder experienced using PostMaster posts who opined that the 10" diameter and 30 deep holes were more than adequate, but that the gravel was of no value in our climate. So, 30" x 10" dia inches of concrete vs. 36" x 8" dia of concrete.

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  • Bit late, but the standard metric is you want about 1/4 to 1/3 of the post's entire length to be underground... more if it is a high-wind shear area. So for 8ft above ground, you want 3-4ft below ground. Of course, that's the rule of thumb for wooden fence posts, and I see below that you opted for metal fence posts.
    – TylerH
    Commented Aug 28, 2020 at 15:01

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I say the 30" depth of post hole is adequate for said fence with given construction method; no need for additional support.

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Posts were installed this week in 9" diameter holes, with a minimum depth of 30". They were pounded a bit into the clay before 30" of concrete was 'poured' into each one, with a few inches of clay on top. They flex, so should handle any wind loads with ease.

enter image description here

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