Screw Pile Selections

[Sparweb]

I am installing a 40' tall self-supporting tower and considering the use of screw piles for its foundation. The previous plan for a concrete foundation was abandoned when the soil was excavated. Fine silty clay "alluvium" for the first 2 meters, and a gradual progression to sandy clay below that.

The contractor I have been talking to insists on custom-making the screw-piles they use, rather than purchasing them off the shelf. On reviewing datasheets from AB-Chance, I can't see a significant difference, except, of course, the cost. This Chance anchor: SA150-0019 would do just fine, in my estimation. Of course, I haven't selected a screw-pile anchor before, but I can read a drawing and do a stress analysis. It will be just fine, even with a reduced Kt factor.

Is there any experience working with Chance anchors that would justify the opinion of my contractor?
Are there other suppliers that should considered or recommended?

Those who read the (aerospace) line beside my member name will surely notice that this isn't my discipline of engineering. I admit that this is a personal project, not a commercial one. I think it's about time I got high-speed internet at my house, for less than 200 USD$per month. I live in a fairly remote area. I need the tower to get a line of sight with a lower-cost provider.

STF

 

[GeoPaveTraffic]

I've used Chance anchors several times. They are well made and work very well. I would not use a homemade anchor. Just too many variables that your contractor hasn't had to work out yet and that Chance already has worked out.

Mike Lambert

 

[humanengr]

Is the "off the shelf" screw pile design for the site-specific soil conditions?
The structural capacity / strength of the pile itself is one thing. The ability of
the soil to resist that load is another - "normally" determined by a geotech. evaluation
and site-specific design of the pile. I'm not sure how its handled in your case.

 

[PEinc]

The contractor may be trying to "re-invent the wheel." There are a number of different helical pier/anchor manufacturers with both similar and different products. Quality control during the contractor's manufacturing process could be an issue. I think it is a bad idea for the contractor to make its own helicals.

http://www.PeirceEngineering.com

 

[Brad805]

Home project, like to save money, I have been there. What is the uplift force you are dealing with from the tower? If your contractor can find someone to buy the pile from it is easy to find a piece of equipment to generate the torque one needs to install a screw pile. We have seen guys use bobcats and excavators in lieu of the larger machines. The problem with those methods is knowing the torque during installation. Rarely do those have a gauge to measure the oil pressure while drilling nor do they have anything that prints out what the final torque is. That is a problem because you need to know that to know if the pile will work. Now if your forces are very small and you have some knowledge of what others have done in the area you might be fine. I would judge this decision on what I can potentially save. If you are talking percentage points, I would not risk it.

As for the contractor building the pile yourself, that I would not suggest. The process of heating/stretching a flat plate to form a helix is not straight forward. The typical helix plate thickness we see is 1/2" or greater. If you look at the image below keeping in mind the shape must be built from flat plate steel (with square edges) I am sure you will see that.

 

[Sparweb]

Thanks to everyone for their comments.
As I write, I am waiting for the contractor to arrive to put them in, provided his morning job finishes up on time. This is a different contractor, not the first that I asked about earlier. Your replies confirmed my intuition and added valuable reasoning. I made more phone calls, and found an installer ready to answer questions and respond quickly.

The guy that's coming has equipment that will measure installation torque, and is bringing extensions in case the first 10-feet of pile isn't sturdy enough. When I called the office, their engineer there quickly checked my calculations and made recommendations for me, no fuss.

I'll let you all know how the install process goes.


STF

 

[Sparweb]

All done (after a little delay matching up with the installer's schedule).
Really went smoothly and plenty of installation torque at depth. Three places: 4500, 6000, 5000 ft-lb, respectively.
Interesting to feel the "crunch" through the soles of my boots as the helix scraped past some rocks, roughly 7 feet down.


STF