How wide should post hole be?

[Hedge]

I am in the process of doing a sleeper retaining wall so I have some 100x200 sleepers I plan to plant vertically. Then lay horizontals between them and bolt together.

In the past when I've done post holes for fencing I've done 12 inch diameter for a 4x4 post.

With sleepers being 100x200 how wide should holes be?

 

[bremen1874]

That's one big hole for a 4x4 post. I'd go for as small as possible while still allowing enough space for some postfix. If necessary get hold of a post shovel or other tool so you can dig smaller holes.

 

[Hedge]

I have a post auger. When doing fencing in the past I had read this link https://www.diydoctor.org.uk/projects/fence-post-and-other-post-hole-sizes.htm

But since the sleepers aren't square I wasn't sure if there was a best way for doing it. I've dug a hole that leaves about 4 inches around which should be more than enough I guess

They are 800mm above ground and 4-500mm below so should be solid

 

[200sols]

The hole should be wider than the post.

/thread

 

[TheFew]

I have just done some sleeper retaining walls. For a wall 1000mm height my vertical sleepers went 800mm underground. I painted the underground portions with bituminous paint. For the post hole I went for as small a hole as possible. Usually with more hole in front of the posts than to the back and sides. Even with fairly tight holes I was using around 2.5 bags of postcrete per hole!

 

[Hedge]

I have just done some sleeper retaining walls. For a wall 1000mm height my vertical sleepers went 800mm underground. I painted the underground portions with bituminous paint. For the post hole I went for as small a hole as possible. Usually with more hole in front of the posts than to the back and sides. Even with fairly tight holes I was using around 2.5 bags of postcrete per hole!

Wow 800mm is deep they sure ain't going anywhere. In the end I did the holes so that they gave around 3 inches or so every side. Ill put some gravel in the bottom or maybe a brick to keep it off the ground.

I wasn't going to paint them because they are oak and I figured they would last but now you've got me thinking I might have to paint em after all.

 

[Abyss]

400mm-500mm is cutting it a bit fine for depth. Considering it's quite a tall retaining wall I think I'd want at least 600mm of the sleeper below ground. If it's clay soil I'd go even deeper due to heaving.

There seems to be differing opinions between half the height up to the same amount above/below, but for a taller wall with significant lateral load (and hazard if it collapses) I'd be playing it safe.

 

[Hedge]

400mm-500mm is cutting it a bit fine for depth. Considering it's quite a tall retaining wall I think I'd want at least 600mm of the sleeper below ground. If it's clay soil I'd go even deeper due to heaving.

There seems to be differing opinions between half the height up to the same amount above/below, but for a taller wall with significant lateral load (and hazard if it collapses) I'd be playing it safe.

I had read / seen in YouTube someone say half plus 100cm so that's pretty much what I aimed for. I didn't think 800mm would be classed as a high wall.

Once the sleepers are locked in place and ran around the corner in an L shape I don't imagine it will be about to move anywhere.

 

[Maccapacca]

Postcrete is a really expensive way of erecting posts, yes it's easy but it's no different to ballast and cement you can buy separately and mix up yourself.

 

[TheFew]

I painted mine as they are softwood sleepers..

 

[pp111]

I have just done some sleeper retaining walls. For a wall 1000mm height my vertical sleepers went 800mm underground. I painted the underground portions with bituminous paint. For the post hole I went for as small a hole as possible. Usually with more hole in front of the posts than to the back and sides. Even with fairly tight holes I was using around 2.5 bags of postcrete per hole!

In passing, for poorly drained soils painting everything with bituminous paint is a really good idea. I actually use car body under-seal. It's really cheap but extremely effective. Just coat absolutely everything the day before you put it in the ground. Make sure its liberal. Continue the paint to at least 100mm about ground level. I have put posts in the ground using this approach for the past 30 years now and all of them are still standing.

Depth of the posts depends very much on the type of soil but with heavy clay you don't need much of a hole, however, you do need them deep. I would not think that 400mm is enough, I would have said 500-600mm but then only you know whats leaning against it!