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Halving the space between posts is not a bad shout. Just means three extra posts (and holes) but I was worried about the 6ft span of the gravel board.
stevo_com-
The posts and concrete boards wouldn't be less than useless, they are a much better bet than the normal fix using sleepers mounted on timber posts. I have seen many horrors used as retaining walls, 1" planks and paving slabs on end are amongst my favourites. There is a special place in my heart for chicken wire on bamboo stakes.
The half length concrete boards with concrete posts are a good idea, the biggest problem is the leverage of the built up ground against the posts. Double the posts would help by, well, it isn't difficult to work out how much. However, the gravel boards are reinforced, so you will need an angle grinder with a stone blade and a metal blade to cut them down.
ColinTheBald
I think if you were to go with the gravel board hack, I'd be tempted to find ones which were half length if possible and double up on the number of posts but basically just do whatever Colin says.
If you have to redo it you'll be cursing yourself.