Interface_Material Mode and Virtual Thickness

Question

Hi all, 
 
 Material mode 
 Can anyone please tell about what is the meaning of this with example?

Virtual thickness 
 I have modeled this system (as shown below) to test the virtual thickness effect of the interface. I changed both interface virtual thickness values at the same time for 1 and 0.1 and the result outcome is shown below in the table.

Parameters

@ Virtual thickness = 1.0

@ Virtual thickness = 0.1

Effective normal stress (kN/m2) @ negative interface side

18.72

16.40

Total normal stress (kN/m2) @ negative interface side

64.50

66.30

Shear stress (kN/m2) @ negative interface side

5.679

6.343

Total Displacement @ negative interface (mm)

17.39

14.66

What is the reason for the change in the stress in the interface due to virtual thickness changes? 
 Thanks.

Dennis Waterman · Answer

Dear Nitha, 
 Interfaces are to model the interaction zone between structure and soil. This zone can be disturbed (for instance due to installation of the structural elements) in which case we may want to give the interface lower stiffness and/or strength than the undisturbed soil. It doesn't say that elastic deformations are ignored, it only says that the stiffness of the interface is chosen such that the elastic deformation is small. In the interaction zone the majority of the deformation will be plastic due to reduced strength, not elastic. Hence, the spring stiffnesses in the interface are chosen such that the elastic deformation of the interface is not suddenly very large compared to the elastic deformation of the soil. 01) Indeed, virtual thickness is not used when the user directly inputs the kn and ks values. How do you define correct? We can't .... we have typically no measurements of displacement of wall and soil so you can't validate if you don't have data. Hence, it's a question that can't be answered. If you do have data, it's a matter of calibrating.... 
 02) vi and Gi area chosen based on experience to give the best numerical stability for a wide range of soil parameters. 
 03) In PLAXIS not the friction angle is reduced, but the tangent of the friction angle is reduced. So indeed tan(phi_i) = Rinter*tan(phi_soil). Or, if you prefer : phi_i = arctan(Rinter(tan(phi_soil)) 04) The stresses in the interface are really calculated in the interface. The stresses in the soil come from a cross section for which soil stresses are extrapolated from soil stress points to the soil element nodes and then from the nodes they are interpolated to the cross section. As you can imagine, the extrapolation and interpolation can cause inaccuracy, and if the stress gradient is quite high (which is usually the case close to a structural element) the inaccuracy is quite high. This is the most probable cause for the differences. The stresses from the interface are the most accurate here. With kind regards, Dennis Waterman

Stefanos Papavasileiou · Answer

Dear Nitha, 
 The custom option is for you to be able to directly control the stiffness and strength of the interface behaviour. You can therefore select a dedicated material to better control the interface behaviour. This gives you the option to avoid using the Rinter property which affects both stiffness and strength (as explained in the Reference manual). 
 Regarding the virtual thickness, this purely affects the numerical part of the analysis. As explained, the virtual thickness does not exist in reality but is the way to handle the presence of the interface in a finite element method world. The difference in results can be explained as mentioned in the Reference manual as a consequence of using a larger elastic zone to be considered, which should not be the case. 
 I would recommend though to try it in a simpler example and not involving water, diaphragm wall and change in soil loading to see in a simpler way how it behaves. 
 If you have further questions about it please submit a service request. Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport

Dennis Waterman · Answer

Hi Martin, 
 I'm going to add to the confusion I'm afraid .... 
 The parameter you're now referring to is the "real interface thickness", this is available when using HS or HSsmall and the dilatancy cut-off option activated. Dilatancy cut-off is based on void ratio and void ratio is calculated from volume strains. So in order to have a dilatancy cut-off in an interface we need to calculate the volume strain in the interface. Since interfaces don't have a thickness by default, the user should specify this "real interface thickness". Usually one chooses a few centimeters... The "virtual thickness factor" that Nitha refers to is not a material parameter, it's a property of the interface itself. This virtual thickness factor is used to determine ks and kn when the standard stiffness determination is chosen. 
 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Ant&oacute;nio, 
 Yes, that is possible. Just use a cylinder and create a plate on the outer surface of the cylinder. Through the plate properties one can set the thickness of the steel case. 
 By the way, since this question is not related to the original question, it's better (and common practice) to start a new forum question for it. 
 With kind regards, 
 Dennis Waterman

Interface_Material Mode and Virtual Thickness

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