Disclaimer: I am not a mechanical engineer and the statements made represent my opinions and are based on my observations and deduction. I don’t claim that this is safe for any particular purpose.
This entry is pretty long, so I am breaking it into multiple posts for the individual sections.
Issue: Stakes are used to anchor an item (tarp, canopy, etc.) to the ground. When the stake is driven into the ground and the cord is attached at the head, it generates an unbalanced force on the stake. This produces a pivoting action on the stake when the cord is tensioned. The effect is that a compressive force is applied to the ground in front of the stake above the pivot point and to the ground behind the stake below the pivot point.
The result is that the holding power of the stake is reduced from its full length to the equivalent holding power of two much shorter stakes which are applying their pressure to a smaller areas.
T he more the stake is pulled toward vertical, the more the hole it was driven into is expanded providing less constriction on the stake and the easier it is to pull out of the ground. In addition, as the stake is pulled toward vertical, it is subjected to a much greater vertical force trying to pull it out of the ground.
In short, the worse it gets, the worse it gets.
One alternative to address this shortcoming is to back the stake up with an additional stake. This moves the pivot point up to the cord attachment point on the first stake. This leaves only two options for failure (for my discussion anyway):
1) The entire bottom of the first stake must be ripped through the ground
2) The second stake must be pulled far enough forward to allow the first stake to fail vertically by being pulled from the ground. This is what normally happens.
Additional backing stakes may be used to repeat this method and replicate its advantages to additional stakes and soil.
There are a couple of major drawbacks to this method:
1) To make this method work, you need approximately as much stake sticking out of the ground as is buried to provide the proper leverage for the backing stake(s). This means the stakes are very long.
2) The portion of the stake that is sticking above the ground is unsupported and more likely to bend or break. This requires the stakes to be much heavier.
3) This can be a complex system to setup properly. Since the stakes and cordage must be setup and tensioned to properly distribute the loads from one stake to the next, it can be complex and time-consuming to setup.
This is a very common configuration for BSA pioneering projects, specifically monkey bridges. In this scenario, large wood stakes are commonly used – 2+ inch diameters with lengths of 3-5 feet. Needless to say, a heavy sledge hammer is required to drive these in.
The ideal solution would be to tie the rope directly to the stake at the pivot point under the ground. This would force the stake to be pulled completely through the ground before it would fail.
Unfortunately, by the time you dig out the soil in order to run the cordage to the stake, all the cohesive tension that exists in the soil is broken. In short, the stake would pull through the soil very easily.
There are a number of anchoring systems available that attempt to address this shortcoming - each with their strengths and weaknesses. Many have been discussed in other threads already.
The solution that I have developed is called a stake boom.
The idea is that a rigid boom is attached to the stake so that it runs parallel to the ground. The cordage is attached to the end of the boom such that it matches the angle and direction as if it were run underground and attached directly to the stake.
The leveraging action of the boom keeps the top of the stake from being pulled vertical. The stake is not simply pulled up out of the ground. For the stake to fail, it must to be drug through the ground from the bottom tip.
In my (very unscientific) testing, I have found the stakes to hold many times better than the stake simply driven into the ground (well over 4 times).
This stake boom is made from ½” rigid pipe and a 12”x3/8” steel stake. This happened to be the material I had available to work with, but the principle will be the same for other materials as well. I look forward to additional testing and reporting my results back here.
The biggest issue I have come across up to this point is keeping the cord from sliding off of the end of the boom when the cord’s pull angle is below 45*. I drilled holes and put pins through to stop this. I think a welded bead around the end would probably do the trick as well.
Stake booms have a number of characteristics that meet the specific requirements of another project that I am working on and will be posting soon. As this is a long post already and is really just a component of my other project, I decided to post it separately so I could refer back to this thread.
Very interesting ideas Alamosa. Not UL (yet) but give it time!
There was another thread that talked about a stake with a bend in it that was approximately the same shape as your two-piece version. I like the idea of a tube for the top member because you could store the lower stake part in the upper tube for transportation. I thought about just getting longer stakes and bending them. I wonder if the bent configuration provides more holding power than using a longer stake driven farther into the ground. Lots of variables - soil density and structure, rocks limiting stake depth, angle of guy line, etc. :confused: The hole drilled in the upper part could be a weak point. How about using a stake with a modified point as a twist-bit to drill an angled hole in a trail stick?
Yes WV, the nesting capability is one of the characteristics that really make this design work for me.
The advantage to the offset connection to longer stakes is that it reduces the vertical force that just pulls the stake from the ground.
I have also found that the boom (horizontal bar) must be much stronger than the stake. The stake, being embedded in the ground, has a lot of support on its entire length.
The boom has no support and is much more likely to bend. Believe me, I worked my way up to using the thick walled pipe. It wasn't my first choice.
Alamosa, what a great thread, and a clever line of thinking to take to address an annoying issue that most of us face with some regularity.
Originally Posted by Alamosa
Instead of a thick-walled pipe, have you explored these three ideas that popped right into my head?
a) a thinner-walled pipe, with an "X" brace running down the length to provide additional bending resistance
b) a non-circular boom? Rectangular or square? Again, a cross-brace inside would provide bending resistance, while still allowing the stake to nest inside.
c) Put Dutch to work solving this problem. Should take him about, oh, 45 seconds or so. :lol:
This will be another fun thread to follow.
and they could serve multi-purpose as bars for food grilling or pot supports (!) depending on metallic content I suppose.
KM( who is intrigued by the vector analysis that seems to need to be done..)
I would recommend a shorter boom, such that the "virtual" guyline would be attached above the pivot point on the stake, but still underground. With a longer boom (or one of equal length to the stake) the torque forces on the boom are significantly greater than with a short boom. I think you would find that a boom length of 4 inches or so would be a great improvement from only a stake. Plus less hardware=less weight, and less likely to bend.