I'm building a trellis/clothesline and using 4x4s as the end-poles. The load on these will be pretty small, and mostly bending moments (almost all) and not axial stress. I live in southern California — very little annual rainfall and no frost/ice/snow.

I'm planning for between 4-5' of height above ground and do not want to set the posts in concrete (only backfill, and will install something like this: http://youtube.com/watch?v=hkhz1RY0cY8&t=6m12s). To what depth should I set the posts under ground? I couldn't find rules of thumbs for this scenario (no concrete). I have planned for between 1.5-2' deep holes, which is just about sufficient for posts set in concrete. Should I dig them deeper or will these suffice?

  • Could you update your post with your location and what you plan to use instead of concrete? – Edwin Jan 23 '14 at 20:07
  • @Edwin I've updated my post. – user19445 Jan 23 '14 at 20:09
  • I'll let someone with more experience answer, but I think you'd be disappointed with only 2 feet below the surface. That's more a number for fence posts. A clothes line will in short order make the posts lean towards each other. I've seen this happen many times, but in Florida with sandy soil. I'd use the whole 4x4 if I could and dig down four feet. – Edwin Jan 23 '14 at 20:17
  • An accurate answer would consider the actual lateral bearing capacity of your soil. You would be better off installing a 1 foot deep ring of concrete at the surface even if the rest is set in soil. The bearing capacity of soil is least near the surface, concrete increases the bearing area. Proper compaction of backfill will also be critical for posts set in soil. – bcworkz Jan 24 '14 at 3:57
  • @bcworkz I was thinking of installing something like this: youtube.com/watch?v=hkhz1RY0cY8&t=6m12s – user19445 Jan 24 '14 at 4:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy