A. I see exactly what you mean. In theory, we can assume either cylindrical shear mode (CSM) or individual bearing mode (IBM). Here, you are assuming a fully cylindrical shear mode. There are two main points I should point out in your question before answering: 1- The assumption that only cylindrical shear governs the failure (which is assuming that the helical zone is a perfect cylinder) and no IBM will occure is not accurate; becase we do not know for sure which mode can occur until we test the model itself with regular helices or through very detailed instrumented field testing. In my case, both modes occurred at different stages of the loading, so we cant assume cylindrical mode form the start. 2- The second important point is that the cylinder that is forming in reality could be considered as a composite section of soil and steel, since it is formed by wrapping soil around the pile (that is what happens in the real soil). Then the new section should take the composite properties and they should be calculated it accurately to represent the soil-steel section. This could be a simplification for simpler modeling (i.e Tz-Qz method) for modeling piles, but not in finite element, where we can actually model soil and steel as soil and steel. So to answer your question, I did not try to model a steel-soil cylinder in FEM for this particular pile; however, I did that using another approach simpler than FEM using Tz-Qz curves taken from strain gage data analysis, the best refernce for Tz-Qz approach is Fellenius' Red book "Basics of Foundation Design" chapter 8 it is available for free online.

A. Yes, it does when you start the wizard and create an interface for one soil layer. but if you start changing the soil properties to fine-tune your model, or if you want to change the reduction factor of the interface (R) the interface properties will not be automatically updated, you have to either: Update them manually (using the equations) OR use the wizard which mean you have to >> delete the interface and merge the nodes (interface creation will split nodes shared by pile and soil mesh and you have to merge them if you want to delete the interface) and create a new interface using the wizard to update the properties. This is a very long process, and the easiest is just to update the properties manually. The second reason, I found it more efficient to use the manual update of properties, is that I have multiple soil layers, and to create the interface using the wizard I can only select the pile and create only one interface using the wizard which will take the properties of the top soil layer and ignore the other layers (that was what I noticed). I have to split the interface mesh to match the soil layer, and I ended up adjusting the other properties manually.

A. I understand that you men advancement rate during installation. Unfortinatlly FEM does not model installation process. It can only handle the digridation of soil due to installation process. you need a more advanced approach to model installaiton effects.

A. For a single helix, there is no big difference, but if you are modeling multiple helcies I recommend modeling a full helix. Because if your helices are closer than 3D they might interfere with each oterh and the pitch will play a big role in that interferance.

A. The allawable displacenemt and safty factor are not generial for all piles and they are case specific, depending mainly on the superstrucre carried by the piles. For example, Generally the displacement should not be more than 25mm, and safty factor of 1.5 is acceptable. Some designers will not allow more than 10% of helix diameter multiplied by a factor of safty to insure the pile stays in the elastic zone.

A. There is no code spicifieng exaclty how far the helices should be or how big they should be; however, it is common in practice to space them 3 times the helix diameter to avoid helical interaction and increase eficincy. There are acceptance criteria published by the International Code Council (https://helicalpileworld.com/ac358%20-%20revised%20June%202012.pdf), it talks about the necessary procedure to accept the helical pile based on performance.

A. They are considered displacement piles; however, the amount of soil displaced is not significant, and their capacity is not based on the increase of lateral stresses caused by soil displacement. Their capacity is based on the bearing resistance of the helical plate and/or the cylindrical shear plane formed between the helical plates and the soil (two main theories related to helical pile capacity: CSM and IBM). Therefore, we don’t usually model the lateral displacement of the soil; the soil lateral displacement (if there is a significant lateral displacement) is modeled using the cavity expansion inside FEM software, which is the approach for displacement piles.

A. In modeling a pile using a volume element, use the Plane interface, which is the case I presented. If you model a pile using a beam element, use line interface; if you model using a shell element ( I dont think engineers use shell element for piles, but maybe) use shell interface.

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