Gal Fence Posts in Rock: How Deep?

[strangerep]

I need to get a short fence replaced. It's about 7.5m long, over sandstone.
I'm want it at least 2.1m high. 3-rail lap-and-cap in 15mm treated slash
pine, with peak-and-groove capping.

People I've talked with suggest 3mm steel all-gal posts 50x50.
The rock must be core-drilled (obviously) and the posts
concreted in.

In this case, how deep do the core holes need to be? Does the
usual 600mm minimum depth still apply? Should it be deeper?
Can it be shallower without losing rigidity? (The position gets a
reasonable amount of wind, so the fence needs to be good and strong.)

Thanks in advance for any advice/suggestions.

[Skew ChiDAMN!!]

In this case, how deep do the core holes need to be? Does the usual 600mm minimum depth still apply? Should it be deeper?
Can it be shallower without losing rigidity? (The position gets a
reasonable amount of wind, so the fence needs to be good and strong.)

I'm no residential fencer, so the following is only my opinion and far from being "professional" advice, but I've put in one or two residential fences that haven't fallen over yet... and all too many miles of bloody paddocks! Besides, any misinformation I may inadvertently post here'n'now is sure to provoke prompt responses to correct me.

I've always worked on the rule of thumb that at least 1/4 the total post length should be buried. Or looking at it another way, 1/3 of the final fence height. Going into sandstone you should be able to get away with slightly shallower holes, unless it's overly friable, but for 2.1m (7') I think I'd still go down to at least 600mm (which, after all is only 2') even if solely because of the wind factor.

It's easier to over-engineer the first time around than have to start all over again.

[DavidG]

What about drilling the rock with a 1" drill and welding 1" reo rod on the bottom of the posts and epoxying them in?
Depth of say 450mm approx.

[ian]

I need to get a short fence replaced. It's about 7.5m long, over sandstone.
I'm want it at least 2.1m high. 3-rail lap-and-cap in 15mm treated slash
pine, with peak-and-groove capping.

People I've talked with suggest 3mm steel all-gal posts 50x50.
The rock must be core-drilled (obviously) and the posts
concreted in.

In this case, how deep do the core holes need to be? Does the
usual 600mm minimum depth still apply? Should it be deeper?
Can it be shallower without losing rigidity? (The position gets a
reasonable amount of wind, so the fence needs to be good and strong.)

looked at another way, if the posts were attached to a flat plate and going onto a concrete slab, four 6" loxins would probably do.

the standard 600mm is for earth, on the basis that when neighbour's Great Dane decides to rub up against it (after the ground's been saturated by two weeks solid rain) the fence wont completely fall over.
you have sandstone but how weathered is it?

Without seeing your rock or the wind exposure of the site or knowing your council's requirements (often you need a BA for higher than 1.8m) or doing any calcs on the post bending strength, I'm guessing 200mm deep and epoxy grout is more than enough.


ian