concrete fence post problem

I've got a fence that's collapsing and want to replace the short concrete posts (half the height of the fence) that hold it up. I decided to dig the posts out (one had actually rotted and the concrete was crumbling!) to replace them and found that they were not concreted in. Is that normal? I always thought a hole was dug, the concrete post put in and then a load of concrete put aound it. Anyone any advice for how to correctly put the new concrete posts in? What is the depth they should go into and how much concrete should I put around them? The soil around the posts is soft and not clumpy or clay. Are there any tips for how to get the posts to match up with the metal bolts that are sticking out of the wooden fence post that the concrete post is attached to?

thanks

As a rule of thumb, bury 25% of the height of the pence posts. So, 6ft fence, 8ft post, buried 2ft.

Dig as small a hole as you can because all dug earth will be loosened and have to be replaced with concrete. If you have dug too big a hole pack in old bricks or rubble if you have it to economise on hand-mix.

Make the concrete wider across the axis of the fence than along it, as fences never blow over that way.

Jack up the old fence so it is level and upright, thread the concrete spur onto the bolts* and pack the concrete round to hold it there.

Reduce the concrete fill near ground level so you do not have a large unsightly patch of concrete showing through the ground.

*use new bolts, or threaded galvanised studding which you can cut to length.

thanks for the advice.

a few points I need to know:
1) the original posts were half the height of the fence. There are not really visible because they are behind shrubs etc. Would it be better to go for the tall ones that reach the top of the fence or are half sized ones as good? One of them was rotted, but the other one looks fine. Is it ok to reuse them? Also, the bolts can of easily with a bit of wd40. Would the be ok if reused?

2) There was no concrete holding the posts in (probably why they fell!), but the hole did have a little puddle of water in it, underneath a brick that was in there. Will water damage the concrete?

3) roughly how much concrete should be used (ie. volume)?

1) these are called repair spurs, you use them to put a rot-proof RC end on a wooden post. They are mostly added when an original wooden post has rotted but you can use them from new, if for example you like the appearance of wooden posts and want to avoid them going rotten (it is essential to raise the bottom of the wooden posts away from the ground in this case) If you like concrete you can use full-height RC posts (and if you want to, you can use masonry paint to make them blend in with the fence. This also makes them last longer as it reduces water penetration in the top. Concrete does not rot, but if it is not dense and has big air bubbles it will take water in, which will freeze and break up the surface, eventually reaching the reinforcing steel and making it rust).

I expect your fence is about 6 ft high, so 2ft of concrete spur underground, and 2ft above, bolted to the wooden post, will be right.

2) it is usual to backfill the hole with concrete and rubble, because where you dig it out you will have loosened the soil so the post will move relatively easily. Unless you have an unusually sheltered spot the wind will then rock the post about and it will get looser and looser. It might be possible to ram some kinds of soil down hard enough, but I live by the coast and haven't tried it.

3) Enough to fill up the hole (!) so make the hole as small as you can; try not to have a cone-shape with a big top, as more of the strength comes from lower down. One day you will have to dig out that post, and you will find it very difficult to get out a barrowful of concrete with hand tools. It will also obstruct your flower bed/lawn or whatever. If you are not doing it for a living, and can spend a bit of time on it, you get a neat result if you put a bit of scrap wood shuttering around the post at ground level, and just have a inch or two of concrete showing as "shoulders" round the post at ground level. You can economise on mix by packing in clean bricks or lumps of old concrete rubble, or big stones a "plums" in the mix. No mud or soil on them though.

thanks for answering JohnD. Now for me to get digging!