Depth of Gate Posts?

[conny]

I want to move 2 gate posts closer to the outside kerb but don't know how deep they should be sunk into the ground. At present the gate is about 16' in from the kerb and actually makes it difficult to park more than 5 cars when we have visitors.

The present posts are 6" x 6" hardwood, (possibly mahogany or maybe oak, not really sure), and they support a 11' wide 5 bar gate. The vertical support rails of the gate are 5" x 3" and the gate height is 48" The current support posts stand 56" above ground and are concreted in but I don't know how deep they are. I have to replace with wood as it is to be kept in context with the house, (a country cottage), so concrete posts are not an option.

Can anyone give any suggestions how deep they need to be below the concrete drive?

 

[r896neo]

You'll need a substantial lump of concrete. A 6x6" post buried 2 ft would be my choice. An 11' gate is quite a weight.

 

[Doggit]

r896 has brought up a good point, but that might make it easer it deal with, but might also make it impossible to do. If the existing slab goes far enough past where you want the new post to go, then you'd need to drill about a 10" hole to let you drop in the new post. Now I don't know the largest core drill that you could hire, but you may be able to do it by drilling 3 pilot holes at the apex of say, an 8" triangle, and then use a 4" core drill down along each pilot hole, and then break out the middle sections. The you could set the post in, fill it with water, and pour Postcrete in to fill it, tamping down as you go to make sure it's completely filled. As to the depth, 2ft could be about right, but I'd ring a company that fits these sort of things, and ask them; this is not something you want to get wrong.

As to the old posts, getting them out is going to be one hell of a job, so I'd be inclined to see how much a new pair costs, then cut the old ones off at ground level, dig them out a few inches, and then fill them over.

 

[conny]

The present posts are on the edges of the lawns but definitely concreted in. I would sink the new ones beside the lawns as well which should give me around the same distance apart as now. (I will of course check the measurements of old and new before setting in.
I like the figure of 2 feet because I was thinking of between 2 and 2 and a half.
With the postcrete is that the way to do it? I have seen builders pour a dry mix in, tamp it down as they go till it was slightly proud, and then just hose the water in last.

 

[Doggit]

I forgot to mention that you'd need to break the cores up as you go down, and an alternative method might be to use a 4.5" angle grinder with a diamond blade to cut down and break up as you go. Obviously, the premise is that the slab goes out far enough to lock the new post in on all sides. How much concrete is there existing around the new post positions.

Postcrete is a standard method for fences, and would allow a lot more flexibility than a dry mix, which I think has ballast in it. It'll depend on how much space you have round the holes to fill, as a large hole would be more expensive with postcrete than a dry mix would.

 

[r896neo]

Don't use postcrete for this. You need to use proper concrete. Postcrete is no use in a large mass, its not strong enough.

Use 3:2:1 gravel:sharp sand:cement.

Unlike a normal post hole you need to dig around and form a good lump of concrete. the hole should be 2ftx2ftx2ft if its in a lawn consider going another 6-8" down as it will be easier digging but looser than a well compacted driveway entrance.

The core drill is a nice idea but clearly you have never tried it doggit.

 

[Doggit]

I used it to check out the sub floor in the kitchen to try and sort out a problem underneath, but I was able to do it in one go.

 

[Ian H]

I'd dig it lawn side and remove the old posts.

Then dig holes so they are buried the same as previously, fix with 25kg bags of ballast with normal cement or extra rapid if you want it curing quicker.