Last year and this year, there has been a fair bit of discussion about "pole structure smunch". This is where you make a wall that is ten feet tall on the uphill side that is a perfect fit. And a year later the poles have moved down two inches. So you have a ten foot tall wall in a space that is two inches shy of ten feet. And the next year you might lose another inch or two. At about three years you might lose just half an inch, and then a quarter of an inch after that.
I think there are two things to be done:
1) mitigate. When the posts are put into the ground, use the excavator to push down on the posts as hard as the excavator can. This might reduce the smunch about 90% to 95%. And it will probably create another problem where the posts will have a tough time draining their
water. So I now think the wise thing to do is to put a little gravel in the bottom of the hole to mitigat THAT problem.
2) engineer. After #1, there might still be an inch of smunch that will be spread out over five years. Design everything to embrace that happening. One idea is to have some dimensional lumber along the top edge of the wall. Maybe two 2x6's on edge attached to the wall, and one attached to the log at the top. At the time of the build, the log is two inches taller than the 2x6's on the wall. In other words, the 2x6 on the log has 2 inches of
wood exposed. Five years later this might be reduced to an inch.
Just felt the need to write this down as I am thinking about it.
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