soil resistance distribution on an underground "beam"

[cbinder46]

I am working on a concept for design of a helical anchor plate. I have come up with a process that I need to try and make more efficient. My question is how would you apply the soil resistance to what is essentially looked at as a cantilever beam (unit section of the plate)? Is the resistance applied as a uniformly distributed load? Or because of the stiffness at the fixed end (plate/shaft interface) would the resistance be more applied as a linearly increasing distributed load (max at the fixed end). If that isn't clear, see the attachment.

 

[oldestguy]

Lacking some way to measure stresses in the plate, I'd use a triangular distribution, but with the load varying from near zero at the connection to max at the for outside end.

 

[cbinder46]

Thanks for the response!

Is there any way you can show me some resources that back up what you said? I will have to explain to my boss my reasoning for loading it with a triangular distribution with the max at the "free" end.

Thanks

 

[oldestguy]

OG again. Having seen the results of test borings (hey I'm old) where the boring is advanced with continuous flight augers, the main distortion being at the outer edges and considered the likelihood of the anchor being behind a rock or rocks, the load probably should all be placed on the outer edge,not distributed. Running an actual test in soil versus rocks, you will find out. My bet is the rocks by far are more load on the edge mainly.

 

[PEinc]

I suggest that you talk to an engineer from A.B. Chance, a major supplier of helical anchors. Check out Chance's design manuals. See attached page from their manual.

www.PeirceEngineering.com

 

[BridgeSmith]

I don't know of much in the way of resources, but what OG proposes makes sense when you consider that presumably there are multiple 'plates' along the shaft, so one flight is somewhat 'in the shadow' of the flight above or below. If your helical plate is only one turn, then you might have an argument for the uniform distribution. The plate would have to be very flexible (and only one turn) to have a distribution that would be substantially higher on the connected side in most (possibly all) soils.

 

[oldestguy]

I did a search of "soil anchor design" and several references came up.

This one has a lot of scoop.

https://www.fhwa.dot.gov/engineering/geotech/pubs/if99015.pdf

 

[PEinc]

There is nothing in FHWA's GEC No. 4 about helical anchors, except for a very brief mention on page 10 as "other types of anchors." Your best bet is to talk to the tech support at the major helical anchor suppliers.

http://www.PeirceEngineering.com

 

[Okiryu]

I had the impression of that the torque stress during the installation may control the design of the plate...

 

[aeoliantexan]

You can find "Elastic Solutions for Soil and Rock Mechanics" by Poulos and Davis on line for free. Figure 7.3 on Page 166 shows the stress distribution under a rigid circular plate. I have questions about the numbers, but it is obvious that the contact pressure near the edge of the plate is much higher than near the center.

My mind understands it like this: A flexible, uniformly-loaded circular area deflects more at the center than at the edge because the pressure dissipates rapidly under the edge, due to the absence of load beyond the edge. Witness the dishing of flexible tank floors. To force the circle to settle uniformly, much higher pressure has to be applied near the edge.