Below you will find answers to commonly asked questions that you may have on PLAXIS.

The focused topic for this article will be on: Interface elements

 

Q1: What are interface elements and what are they used for?

A1: Interface elements act as zones of interaction between two distinct components, essentially functioning as connection elements that facilitate their interaction. They are used to simulate the interaction between structures and soil/rock – which allows for relative displacement (slipping or gapping), which is crucial for soil-structure interaction, e.g., interaction between tunnel lining and rock, a pile and its surrounding soil.

 

Q2: Can interfaces be used to control groundwater flow?

A2: Yes, interfaces have the capability to affect groundwater flow (including consolidation) by having the following settings: impermeable (flow is restricted), semi-impermeable (flow is controlled through hydraulic resistance which controls the flow in the normal direction of the interface and drainage conductivity which controls the in-plane flow), and fully permeable (flow is unrestricted). Ensure that the ‘Active in flow’ option is enabled for the interface so that the aforementioned settings are applied during the analysis as it is disabled by default.

Here are some practical applications of groundwater flow control using interfaces.

 

Q3: How do I know whether I need to use interfaces or not in my project?

A3: The decision to use interface elements depends primarily on whether relative movement (such as slipping or separation) is expected between two connected materials, typically between soil and a structural element. For example, in the case of soil-geogrid interaction, the connection is generally rigid due to the interlocking nature of the materials, so an interface may not be necessary. However, for soil-geotextile interaction, some degree of slippage is often anticipated. In such cases, interface elements are essential to accurately simulate the mechanical behaviour at the contact surface.

 

Q4: What value of Rinter should I use when defining my interface parameters?

A4: The choice of Rinter (interface strength reduction factor) depends on the expected interaction behaviour between the two contacting materials. You can think about the smoothness or roughness of this interaction, and it reflects how much of the adjacent soil’s strength is mobilized along the interface. Lower values indicate a smoother interaction, For example:

• For steel-soil interfaces, lower Rinter values (typically between 0.5 – 0.7) are used due to the smooth surface and limited friction.
• For concrete-soil interfaces, higher values (typically between 0.7 – 1.0) are common, as concrete provides more frictional resistance.
• For bored piles Rinter is usually lower than for cast in-situ piles because the boring process disturbs the surrounding soil, reducing skin friction.

It is generally not recommended to use Rinter values below 0.5, as this can lead to convergence issues in the analysis.

 

Q5: Do I need interfaces as well for embedded beams or rock bolts?

A5: No, although both embedded beams and rock bolts require interface elements to simulate the soil-structure interaction, these structural elements themselves have special (internal) interfaces integrated. Therefore, you do not need to explicitly model interfaces for them.

 

Q6: Will the model behave differently when I have a positive or negative interface? And how to decide whether I should use a positive or negative one?

A6: The terms positive and negative interface refer to the side of the structural element where the interface is applied – not to any physical property of the interface itself. The model will behave differently only if the interface is applied on the wrong side, especially in cases where slipping or separation is expected on a specific face of the structure. For example, in a retaining wall, the interface should be applied on the side in contact with the soil. 

 

Q7: How can I be confident with the values used for my interface parameters?

A7: Confidence in interface parameter values should be based on a combination of engineering judgement, relevant design codes, published literature, and most importantly, calibration against laboratory or in-situ test data. Interface parameters should not be the most critical factor in your model. If small changes in these values significantly alter your results, it may indicate deeper issues in the model setup or assumptions. In such cases, you should revisit the overall modelling strategy.

 

Q8: Why do I need interface elements at the boundaries of my model?

A8: Interface elements are essential when applying free field and compliant base dynamic boundary conditions in dynamic analyses. These interfaces generate node pairs that allows for relative movement between the model and the boundary enabling the application of input motion (e.g., seismic loading) and the absorption of incoming waves. Without these interfaces, the free field and compliant base options cannot function correctly, and the model may produce unrealistic results due to wave reflections at the boundaries.

 

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