Here's a picture of some ground contact pt 6x6's used as a retaining
wall. It was 18 years old. I had to replace about half of it. The
rest was in "ok" condition. A lot of the rot started where it had to
be cut because the wall curved. The fresh cuts were done to make it
easier to dispose of.

http://home.earthlink.net/~jamesgangnc/ebay/ptwood.jpg