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I need to replace an old 2ft retaining wall(railroad ties) along my property line. There is not currently a fence there so I was going to take this project to add one. I am located in the PNW so there is plenty of wet.

I was planning on using concrete blocks for the wall and treated wood for the fence posts. My issue is I don't understand how to best apply good fence post-drainage to the situation. I read up on fence posts for my area and it looked like the gravel for the bottom of the whole and concrete to just above the ground was the way to go.

The wall would be 2ft tall, the fence 6ft tall from the bottom of the wall, so about 4ft tall. If I did a 2.5ft hole relative to the bottom of the wall. Then in the hole gravel for 6" then concrete for 2ft to the bottom of the wall, I would have 2ft of post that is in the gravel backfill of the retaining wall. Is that OK assuming my wall is well drained? Or should I have 4ft of concrete?

The CAD below roughly shows my idea minus gravel and not to scale(the wall is 1ft in this CAD).

Thanks for the help!

enter image description here

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    I am unsure as to exactly what drainage issue you are asking about but the post must be set into the ground at the depth require by code regardless of the different wall/fence combo so it will not be heaved up . Is frost line code in PNW 2.5' ? If the wall/fence is sitting on the ground but the post is set below frost line will the fluctuation of the ground up and down put stress on the fence/wall that is connected to the non fluctuating posts ? If the wall was independent of the fence it could move without moving the fence. – Alaska Man Jun 26 at 18:54
  • Thanks for your response. 2' is the requirement here. – Kris Jun 26 at 19:02
  • Drainage is in concern to the post rotting out prematurely. – Kris Jun 26 at 19:03

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