On Apr 21, 7:07�pm, John wrote:
My backyard is higher than my neighbor in behind me. �To keep the
ground level, there is a three foot retaining wall at the back of the
yard, which is only a few inches from the fence. �The wall is built
from 6x6's, and is not normally visible (even the top of the wall is
hidden by bushes and flowers in front of it.

The problem is that the 6x6's appear to have started to rot. �I
imagine they were put in place when the house was built 25 years ago.
This year, the neighbor's want to rebuild the fence, so I was thinking
of replacing the retaining wall at the same time to give easier
access. �It doesn't have to be pretty, just functional, and as long
lasting as possible (and preferably fast to build without breaking the
bank). �The wall is about 25m (around 90') long, and I live in Canada,
so there's lots of freezing and thawing going on.

I'm wondering if I should consider concrete, brick, or wood again (and
what the advantages are), and if anyone has any advice that I might
not have thought of.

Thanks

John
Wood is cheap and non-permanent as you have discovered. You might be
able to get some old railway ties.
The cheapest permanent solution is poured concrete if there is access
to have it delivered. You would need to rig up shuttering. There are
several technologies. If no access you would need an expensive
concrete pump.
Or build out of concrete blocks. You would still need a substantial
foundation. This would be a gravity wall, the usual thing is to make
the thickness 1/4 of the hieght.