3

I want to place a permanent anchor for a slackline in my parents' backyard. I looked around at the different options, but I don't have access to some of them (e.g. no duckbill anchors), so I think I will go for the deadman anchor option as in this video.

I also found a diagram for what it should look like, but I am taken aback at the suggested trench depth of 3-4 foot.

diagram of anchor setup

I will be digging with a hoe and my own two hands. The soil where we live gets quickly into compacted clay, after maybe only 2 feet of something more humuslike - my father tells me he once rented an excavator to make a posthole and it needed over half an hour for that hole (operated by somebody experienced, not by my father).

So, I wonder if the diagram I found is not overly sturdy, and if I could get away with a shallower trench. Especially since this diagram also foresees getting concrete poured into the ground, and the person in that video gets away with just a buried length of pipe.

I found a slackline calculator and it says that the normal load will be in the 7-8 kN range, and possibly up to 11 kN under some circumstances (e.g. if it gets tightened a lot and somebody heavy tries to walk on it). So let's say I want the whole line to hold up to 15-17 kN just in case (that's breaking load, not nominal load). There is a tree to hold one side up. For the second side, if I bury a 3 feet long pipe with maybe 2 inches diameter, what depth should I choose?

  • If it's what I've typically seen (i.e. low to the ground and you expect to fall off it regularly while learning anyway) you can figure that the deadman ripping out of the ground will not likely injure anyone (you'll either find the line on the ground or fall off as it lurches) and you can try what you like and re-do if it fails. Otherwise I'd suggest borrowing, renting, or purchasing a pick & a shovel to aid the digging in compacted clay, which will be ludicrously difficult with only a hoe. – Ecnerwal Apr 28 '19 at 18:42
  • In Chicago that's after the first six inches. A pick won't help you. Use a spade shovel (and weigh enough to be able to sink it while standing on it... which I do not; I have to wiggle it ;( - Also, screw freakin post hole diggers... just lever-out small chunks at a time. Buy the expensive shovel or you'll break it. Sometimes it's so bad all you can do is remove little chips while stabbing at it with a 6' railroad breaker bar. – Mazura Apr 28 '19 at 23:45
  • I will actually have more tools than a hoe at my disposal, I am not exactly sure which ones, nor what are the proper names in English. What I meant is that I will be fully reliant on my own muscle power, which is rather low. But yes, I could look for a pick specifically. I said hoe, because on this page, the hoe method looked easier than the others: easydigging.com/guide/trenching-guide.html – rumtscho Apr 29 '19 at 11:11
  • +1 on the diagram – Sickest Apr 29 '19 at 19:53
5

It’s based on the 1) angle of repose, and 2) weight of soil, and 3) length of deadman.

7-8kn equals about 1500 - 1,800 lbs.

If the angle of repose is about 45 degrees and your soil weighs about 80 lbs. per cubic foot, then you’ll need about 2,000 lbs. (safety factor) resisting.

Therefore, you need to bury your deadman about 3’ deep if the deadman is 3’ long. (I’d compact all material in the ground.)

I’d worry about a 2” diameter deadman getting pulled through the soil and compacting the soil immediately adjacent to the deadman. You’ll need a wider diameter deadman, such as a 12” wide x 3’ long steel plate welded onto your 2” pipe.

Adjust accordingly for your 15-17kn.

  • To clarify, I rhink that it is 15-17 kN in total, and it is half per anchor. Also, doesn't the calculation have to involve some kind of cone? I saw this discussed, but since I am not an engineer, I couldn't imagine that cone properly. – rumtscho Apr 28 '19 at 20:03
  • @rumtscho Yes, the “cone” is the angle of repose based on the type of soil. An easy explanation of “angle of repose “ is: Take a shovel of your dirt and slowly pour it on the ground. The slope on the sides of your pile of dirt is the angle of repose. Now, turn that pile on its side and drag a deadman through it and it will create a “cone” of resistance. – Lee Sam Apr 28 '19 at 20:12

Your Answer

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

Not the answer you're looking for? Browse other questions tagged or ask your own question.