Iv'e put fence posts but originally they were used to support new trees, and my husband acquired them (legally) now he wants to use them to support the soil at the back of a border, laying them horizontally two posts high, any ideas how we can support them to stop them falling over, wooden supports would rot well before the posts, so something long lasting, they wont be seen so need not be anything attractive. Thanks
I would Knock in some vertical stakes in the ground at intervals along the horizontal posts and either nail or screw them to the uprights to support them.
I've just re-read your question (more thoroughly) and thought that a more durable method, rather than wooden stakes maybe to use some angle-iron, drilled through and bolted, probably one each end and one in the middle.