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edit...I ended up digging a 11"w x 11"l x 25"h hole with a 11"w x 20"l x 11"h shelf in the direction of the slackline. I welded some steel wire frame pieces, that I cut up from a box spring mattress, to the post and also laid some in the shelf. Took 6 bags of concrete to fill. I set the slackline up around 22" and it barely moves when I'm on it. I doesn't seem like I'll need any additional support which is exactly what I was going for. It's been a couple weeks since I've finished and it's been working great. Thanks again for all the input!

enter image description here

I'm trying to figure how much concrete I would need to prevent my slackline post from tilting and how deep I would need to bury the posts in dirt, but I'm unsure how to calculate this.
I have a 3-1/2" x 3-1/2" steel square tubing that is 6' long. The slack line will be spanning 40'.
My hope is to not have a wire or frame for support, but maybe I will need too much concrete for this to happen.

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    depends on the supporting strength of the dirt.
    – Solar Mike
    May 28, 2023 at 9:19
  • First you need to calculate the lateral force, more information is needed. Height of the attachment to the post? How slack is the line (how low your feet will be below the attachment point)? Weight of the person? Are there any dynamic forces (are you walking quietly or jumping)?
    – Mattman944
    May 28, 2023 at 11:57
  • If, say, you had a welder attach plates to the bottom and ~3" below soil line when buried of sufficent size, no concrete at all would be needed. But the size of the plates needed would depend on the soil and loading. Or you can use screw-in anchors and guy wires beyond the posts.
    – Ecnerwal
    May 28, 2023 at 12:22
  • - the dirt is medium firm? not sure how much strength that will provide. it wasn't too difficult to dig up. - lateral force will be whatever a 200lbs person will exert while standing on the slackline. not sure exactly what the lateral forces will be. - I like your plate idea. i'll keep that in mind!
    – Ty H
    May 28, 2023 at 18:19

2 Answers 2

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Lateral Static Force = (Weight_Of_Person * Length) / (Sag * 4). This assumes a trig approximation for small angles, a reasonable assumption since this is a rough calculation anyway. https://slackline.us/wp-content/uploads/2015/08/grundsatzflyer_A5_EN_final2015.pdf

For a 200 lb person, 40 ft long line, 2 ft sag -> Lateral force = 1000 lbf

I would multiply by 2 to account for dynamic loading.

This is in the range of the force on a 6 ft fence post during a strong tropical storm. In my experience, for soft florida soil, a 9 inch wide hole, 2 ft deep is marginal for these conditions. I would use a 12 inch wide hole, 3 ft deep for your slackline posts.

If you have hard clay soil. Like so hard that it is difficult to dig a hole with a hand post-hole digger, 2 ft deep may be sufficient.

In any case, be sure to test it well under safe conditions before using it for more dangerous conditions. e.g. with the line near the ground, have 2 large people sit on it.

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  • this gives me a good idea of what i need. thanks for the info and the link!
    – Ty H
    May 28, 2023 at 18:44
  • @TyH, the proper way of saying "thanks" around here is to click the up vote arrow next to any answer that helps you and to click the check mark next to the one that helps the most.
    – FreeMan
    Jun 27, 2023 at 17:27
  • @FreeMan Thanks for the reminder. I wasn't able to and still not able to upvote because I'm too new. Apparently I need at least 15 reputation points to upvote. I didn't know about the check mark though. Done!
    – Ty H
    Jun 28, 2023 at 18:45
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It depends on high you want the slack line. When I built a playground for the kids some 20 years ago, I gave them a swinging log, fastened by chains to a post concreted in the ground. The ground was very wet and clay, the fastening point was 2ft above ground and I had dug a hole 18 inches deep, filled with rapid-setting concrete. The post was a left-over round timber pile (6 inches in diameter) They could walk on the long as it swayed, the posts did not move.

When I google, it I can only find this product, supposedly designed for the purpose

enter image description here

It says to put it in 390mm, which sounds deep enough to me. The fastening point is 360mm high. From Slackline posts

I am not associated with this site in any fashion.

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  • that's good to know. not exactly sure how high i'll need the slackline. i'm hoping no more than 3'. i think i'm going to have to set it up temporarily at 40' somehow to find out how much it bends when standing on the slackline. thanks for the input!
    – Ty H
    May 28, 2023 at 18:20

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