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Hillside terrace support structure

how to shore up a terraced hillside with old timbers and some fence posts


These fence posts are now sunk over 4 feet in the ground.

Will it be enough to hold back the hill from wanting to smooth out?

If it holds for just a few years we might have enough cultivated plants with deep roots by then that will lend a hand in keeping the terrace in place.



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I have a section that is thinly wooded on the south side of my hill. I was thinking of doing this same thing. It is an area where water has made a gully down the side of the hill. I will be watching your terrace reports for the next year to see how it does for you.
Comment by Mona Tue Dec 18 18:57:49 2012
if you're like us you always seem to be having to pound in T-posts somewhere or another and they always seem to be the extra long ones. The job is made much easier, especially if you've got any rotator cuff issues, by welding on "extendos" to your post pounder. It takes no time to cut off the return legs on the bottom of the tube and splice in more material to run to bottom of tube or even past and then back up. Makes a huge difference if you are running long posts.Bob.
Comment by bob Wed Dec 19 03:07:37 2012

Mark, your question is one of (soil) mechanics. You would have to calculate the lateral earth pressure on both the boards and the fence posts to answer it. You can find a nice introduction here.

Basically you need to know the failure plane angle (denoted by the greek small letter phi: φ) and the soil/wall friction angle (denoted by the greek small letter delta: δ). These values will depend e.g. on the type of soil, the amount of water in the soil and the amount of roots holding it together. If you look around you'll probably be able to find manuals or codes to help calculate such supports if you are so inclined.

Realize that all the forces from the soil acting on the boards have to be balanced by forces from the soil acting on the relatively small fence posts. My gut feeling would be that the posts are possibly too far apart.

This construction has at least the following failure modes;

  • If the posts are too short (not far enough into the ground) and too far apart, the lateral pressure on the posts be too great for the soil to hold and will make them shift in the ground, making the whole structure topple. This is the most likely failure mode, IMO.
  • If the distance between the posts is too long relative to the thickness of the boards, the boards might deflect too much and even break.
  • If the distance between the posts is too long but the posts are deep and the boards are thick, the post might just shear off.

Corrosion of the metal posts and rot of the wooden boards will decrease the performance of the support structure over time.

One important point is to make drainage holes in the boards. Otherwise you would get additional water pressure, and water makes it easier for the soil to "flow".

Comment by Roland_Smith Wed Dec 19 04:58:13 2012
Roland --- I'm glad you chimed in! Mark did leave some holes between the boards, which must have been his gut physicist in action --- I wasn't so sure it was a good idea. Rather than doing the math, I guess we'll wait and see what happens and then look back at your rundown to see which failure scenario was the correct one. :-)
Comment by anna Wed Dec 19 08:06:32 2012