Tech
Q1. Can a helical pile segment be simplified as a cylindrical shear zone between helices?
A. This assumption corresponds to the cylindrical shear mode (CSM), but it is not always valid to assume this governs the behavior exclusively. In practice, both CSM and individual bearing mode (IBM) can occur, often at different stages of loading. The governing failure mode cannot be determined a priori and should be verified through analysis or load testing.
Another important consideration is that the “cylindrical zone” formed in reality behaves more like a composite soil–steel system, where soil is confined and interacts with the pile. Accurately representing this would require defining equivalent composite properties, which introduces additional assumptions.
While such simplifications may be useful in simplified methods (e.g., T–Z / Q–Z approaches), they are generally not necessary or recommended in finite element analysis, where soil and pile materials can be modeled explicitly.
In this study, a soil–steel cylindrical zone was not modeled in FEM. Instead, simplified behavior was evaluated using T–Z / Q–Z curves derived from strain gauge data, which can be referenced in Fellenius’ Basics of Foundation Design (Chapter 8).
Tech
Q2. Why use manual interface input instead of the interface wizard?
A. The interface wizard is useful for initial setup, as it automatically assigns interface properties based on the selected soil layer. However, it has limitations in practical modeling.
First, when soil properties are later modified (e.g., stiffness or reduction factor R), the interface properties are not automatically updated. To reflect these changes using the wizard, you would need to delete the existing interface, merge nodes, and recreate it, which is time-consuming.
Second, in models with multiple soil layers, the wizard typically assigns properties based only on the selected (often top) layer. To accurately represent varying soil conditions, the interface must be segmented according to layers, and properties adjusted individually.
For these reasons, manual input is often more efficient and flexible, especially when fine-tuning parameters or working with layered soil conditions.
Tech
Q3. How is the advanced rate modeled?
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.
Tech
Q4. Is the difference large when we model a simplified model of the pile instead of modelling a 3D screwed pile?
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.
Tech
Q5. What is the allowable displacement and safety factor in tension?
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.
Tech
Q6. Is there any specifications or code for the distance between two helical plates? and also the size of helix?
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.
Tech
Q7. Is this considered displacement or non-displacement pile?
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.
Tech
Q8. It's about the 'Interface' feature. When I click on it, I see three options: Line, Shell, and Plane. Which of these is applicable in this particular case?
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.
General
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