| Application | PLAXIS 2D PLAXIS 3D |
| Version | PLAXIS 2D 2023.1 and later PLAXIS 3D 2023.1 and later |
| Date created | 21 November 2023 |
| Date modified | 21 November 2023 |
| Original author | Faseel Khan - Bentley Technical Support Group |
Interface elements are formed by node pairs, i.e., two nodes at each node position: one at the 'soil' side and one at the 'structure' side or the adjoining 'soil' side if an interface is present between two adjoining soil layers. In the calculation kernel the two nodes have the same coordinates (see Modelling soil-structure interaction: interfaces).
An interface is usually assigned between the soil and the adjoining structure to capture the soil-structure interaction, such as shear stresses which cause shearing and relative movement between the soil and the structure. Shear stress in interfaces can be seen visually in an interface plot and its associated data table.
A clear understanding of the tabulated results, especially the sign convention associated with the shear stresses, makes it easier to understand the relative movements in complex 2D and 3D models.
The interface node movements in the local directions relative to the adjoining ‘structure’ or adjoining ‘soil’ nodes define the shear stress (τ) sign (+ or -). Fig. 1 and Fig. 2 show the nodal pairs at a plate along with the interface local axis in PLAXIS 2D and PLAXIS 3D respectively. The sign of the interface (+ or –) does not play any role in the behaviour of the interface and simply depends on the way the interface is created (left to right or right to left for example).
From the movements of these nodes, the sliding movements or gap formation can then be interpreted. To assess relative sliding movements between adjoining soil and the plate, the shear stress sign plays an important role. The shear stress sign depends on the relative movement between soil node and structure node and also on the shear stress already present in the previous phase.
Fig. 1 Node pairs on plate and interfaces and interface local axis in PLAXIS 2D
Fig. 2 Node pairs on plate and interfaces and interface local axis in PLAXIS 3D
The interface shear stress sign is determined based on the following information:
- Interface local axis direction
- Soil node movement in the interface local axis direction
- Plate node movement in the interface local axis direction
- Interface shear stress in the previous phase
The interface shear stress and its sign can be viewed by double-clicking on an interface and viewing the shear stress τ1 in PLAXIS 2D. The interface local axis directions will also be displayed in this plot in both PLAXIS 2D and PLAXIS 3D.
The shear stress sign for τ1 and τ2 in PLAXIS 3D can be viewed by first double-clicking on an interface and then by clicking on the Interface stresses menu and then clicking on the Table of stress point results. In PLAXIS 3D, the interface nodal results for τ1 will have an inverted sign and the plot for τ1 will also have an inverted sign because plots are generated from nodal results whereas the sign for shear stress τ2 is not inverted.
The soil node movement in interface local axis 1 in PLAXIS 2D and in interface local axis 1 and 2 in PLAXIS 3D can be found by double-clicking on the interface and then by viewing the movements from the Deformations menu.
The plate node movement in interface local axis 1 in PLAXIS 2D and in interface local axis 1 and 2 in PLAXIS 3D can be found by double-clicking on the plate and then by viewing the movements from the Deformations menu.
The magnitude of interface shear stress in the previous phase will determine if the magnitude of current phase shear stress is sufficient enough to change the shear stress sign from say positive to negative or vice versa. This is because if in a previous phase, the interface shear stress (τ1) was -100 kpa and if in the current phase the relative movements produce a positive shear stress of only +50 kPa, then the interface shear stress (τ1) will still be (-100 + 50) = -50 kPa and displayed as negative even though the relative movements produced a positive value of shear stress in the current phase.
Fig 3a to Fig. 3f show all cases that will result in a positive shear stress and Fig. 3g to Fig. 3l show the cases when shear stress will have a negative sign. In this figure, one node is part of the adjoining ‘soil’ and the other node is part of the adjoining plate. The blue arrow indicates the interface local axis 1 in PLAXIS 2D and local axis 1 or 2 in PLAXIS 3D.
Fig. 3 Sign convention for interface shear stress in PLAXIS 2D (τ1) and PLAXIS 3D (τ1 or τ2)
In Fig. 3a, the soil node moves in the interface local axis direction whereas the plate node is stationary resulting in the positive sign (+τ). Similarly, in Fig. 3g, the soil node moved opposite to the interface local axis direction resulting in the negative sign (-τ).
In Fig. 3b, the soil and plate nodes both move in the interface local axis direction but the soil node moved more resulting in +τ. Similarly, in Fig. 3h, the soil node moved more compared to the plate node in the opposite direction of the interface local axis direction resulting in -τ.
In Fig. 3c, the plate node moves opposite to the interface local axis direction whereas the soil node moved more in the interface local axis direction resulting in +τ. Similarly, in Fig. 3i, the plate node moves in the interface local axis direction whereas the soil node moved more in the interface local axis direction resulting in -τ.
In Fig. 3d and Fig 3j, the soil and plate nodes both move in equal amounts but in opposite directions. In these cases, the interface shear stress sign will take its sign from the previous phase.
In Fig. 3e the soil node moves in the direction of the interface local axis whereas the plate node moved more opposite to the interface local axis direction resulting in +τ. Similarly, in Fig 3k, the soil node moves opposite to the interface local axis direction whereas the plate node moved more in the interface local axis direction resulting in -τ.
In Fig. 3f both the soil and plate nodes move opposite to the interface local axis direction but the plate node moved more resulting in +τ. Similarly, in Fig 3l, both the soil and plate nodes move in the interface local axis direction but the plate node moved more in the interface local axis direction resulting in -τ.
In summary, interface shear stress will be positive when the soil node coordinate is more towards the interface local axis direction whereas interface shear stress will be negative when the plate node is more towards the interface local axis direction.
When interface is present between two adjoining soil layers, one of the node from the interface node pair becomes the soil node and other is named as structure and so the same sign convention applies as mentioned for the case of soil and plate.