Retaining fence

[brianham]

I would like to use 9ft concrete posts with 3ft of concrete gravel boards to retain soil and a 3ft fence on top of the gravel boards. Is a 9ft post adequate and should I use any special method to put the posts in to take the weight of the soil?

[masona]

Difficult one to answer, if the soil is level then you might get away with it then depend how much rain in the short peroid of time. I have a 9' concrete post and now have a hairline crack at the soil level due to the wind pushing onto the fence panels over the years.

It would be best to build a 9" wall thickness (double skin) with the concrete post built inside the walls to retain the soil pressure.

[david and julie]

I have seen fence posts in windy places that were 6x6 instead of the usual 4x4?. Don't know where they come from though.

Many of the smaller garden shed and fence suppliers cast their own posts, and bases. I would try and find and talk to one of those places if possible, perhaps half an hour in yellow pages under fence manufacturers?

An industrial fencing suppliers may also be worth a try.

[mr_bigben1]

The technique of using gravel boards spanning between post to retain soil is call a "Berlin Wall" in the construction industry. The principle is that the active earth pressure on the boards and the wind loading on the fence above is resisted by the passive earth pressure in front of the buried section of the posts.
The posts must be surrounded in sufficient concrete (at least 60cm diameter) in order to transmit the loads into the soil in front. The length of the post in the ground depends on the type of soil but the penetration must be AT LEAST equal to the retained height of soil and if the soil is poor, then a penetration of twice the retained height will be needed.
It will be probably be easier and more certain to build an independent 25cm gravity wall on a concrete footing between the posts as a previous member has suggested.