I need to Install 4″wood posts into 1 3/4″ drain rock 6ft above grade. I installed one post in an 8″ sono tube 30″ deep as a test. Auger and digging did not work so well.. The walls of the hole colapsed in as I dug so I shored up the walls of the hole with plywood scrap. Way to time consuming and not as solid as I would like. Frost is not an issue but I need to put 80 linear ft of cedar fence 6 ft tall. I feel like I am missing something. There are better ways.
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Replies
Used smaller rock if the "wiggle factor" is to great. 3/4" and smaller will hold the post tighter than the 1 3/4" stuff and you still have plenty of drainage. The smaller rock will also allow you to use a tamping rod to keep things tight as you fill the hole.
The sono tube test isn't very valid because the sone can't get a "bite" in the sorrounding soil.
As far as the hole collapsing, can't help you there. My soil is such dense clay you colud use a core bit in it :)
Yeah, and it occurs to me that if the soil collapses that easily it's likely sand, and it's going to be hard to set the posts securely with any technique.
You might have to use schedule 80 2-1/2" pipe if you can't get a hole dug. You can rent a pneumatic post driver that you just hook up to your compressor, and just drives'em in. Works great for "T" posts. Never used one for larger dia pipe.
Bolt your rails to the pipe. It'll never blow over...
Yeah, don't use sonotube --
Yeah, don't use sonotube -- the rock needs to be able to press into the soil. And 1-3/4 is too large for setting posts -- you maybe want something that large in the bottom of the hole for bearing/drainage, but you should use about 3/4" for the rest.
If caving in is a problem, you could use a piece of sono temporarily as you add rock. Pull the sono up a few inches at a time as the rock is added. (But remember that you should stop every six inches or so and compress the rock by using a piece of 2x4.)
You're saying that the drain
You're saying that the drain rock is already in place everywhere you need to dig? This doesn't sound good... that rock is going to be impossible to dig a hole in, and it's not going to hold up posts if you DO manage to get one in there. I'd get a small backhoe to remove the gravel in each post area, then install the posts into 3/4" crushed rock with plenty of fines in it. That will tamp and lock up well. Of course I have no idea WHY the drain rock is there and if it's needed....
The 1 3/4" drain rock is already in the ground as part of the stormwater management system. I need to run a fence parallel to it.. I should have installed some footings when it was all exposed.. Coulda woulda..