questionswhat is the best way to dig a deep hole for a 4x4?

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just checking: you are aware that having wood underground will rot, aren't you? for underground if needing structural you are prob better off to bury concrete then put the wooden part above ground with a seporator. if not structural i'd consider pvc/vinyl

you might find that a old fashion hand operated post hole digger is your best option to get that deep.

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@moosezilla: Yes, I am aware wood rots. I am using pressure-treated wood to help resist rotting and the posts will only be used for 5 years max, and wood doesn't rot that bad that fast. It is not for axial structural purposes either.

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Why do you want to go 5 feet deep? 2 feet max is a general standard. Just curious.

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@jsimsace: The posts will be used for a slackline. Which involves quite a bit of horizontal/bending load, and I'm looking for minimal deflection.

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for a slackline post you might want to look at angled posts with guy wires

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A stick of dynamite should do the trick! ;->

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Your rental a post hole digger with a 6" Dia auger at $30/hour should have extension available to go to even 30 feet sometime.

A properly pressure treated post will not rot in the ground for 30 to 50 years.

I dig down 4 feet using a soil auger. Refill the hole with 2 feet of gap sized pea gravel for drainage the put 2 feet of the post in the ground with concrete. I started doing 6 foot high fences since most places will not make you get a building permit.

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Make sure you bell-out the bottom of the hole once you reach the 5 foot depth. When I say bell-out, just like it sounds.... picture a bell. This will help resist lift out from freeze/thaw and any reaction in the horizontal direction. Definitely fill the bottom foot of the hole with either dry concrete mix or concrete/gravel mix. The only way the post will rot out is if the post is sitting directly on earth. Encased in concrete will bring a long life.

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Sorry, when I read your question I thought you were looking to bury a 4WD truck. Figured there'd be an interesting story there. Around here the soil is too hard and rocky for digging like that, you have to dig by shovel as some of the rocks are quite large. You will generally pull out rocks creating a 3-5'wide x 3+'deep hole to plant a tree with a 1' wide x 2' deep pot. On the plus side, you and up with enough rocks to build a nice little decorative wall around your new tree. Sorry I don't have anything constructive to offer.