To simulate the interaction between a structure and soil you can apply interface elements. Without an interface the structure and the soil are tied together: no relative displacement (slipping/gapping) is possible between structure and soil.

By using an interface, node pairs are created at the interface of structure and soil. From a node pair, one node belongs to the structure and the other node belongs to the soil. The interaction between these two nodes consists of two elastic-perfectly plastic springs. One elastic-perfectly plastic spring to model the gap displacement and one elastic-perfectly plastic spring to model slip displacement. Also see the connectivity plot of a soil-structure connection with and without interface.

Note: for a clear display in the connectivity plot the node pairs are drawn with a certain spacing in between, where actually in the calculation kernel the two nodes have the same coordinates. The spacing between the node pairs can be influenced by the scale factor in the connectivity plot (this scale factor is then remembered in other Output plots).

The level at which (plastic) slipping occurs is directly controlled by the strength properties and the R_inter value of the relevant material set. These parameters are by default taken from the material set of the adjacent soil cluster. Directly appointing a material set to the interface is however also possible and allows for direct control of the strength properties (and thus the interface strength) without changing the properties of the soil cluster.

The elastic shear and normal stiffness of the interface springs are internally calculated from the stiffness properties of the relevant material set. The stiffness is chosen in such a way that the program is numerically stable and elastic deformations are negligible. Note that the relevant properties by default are also taken from the adjacent soil cluster. However, when you want to influence the stiffness of the interface, in general you should directly apply a material set to the interface. By adjusting the stiffness properties of this material set you can directly influence the stiffness of the interface without changing properties of soil clusters.

Note the following:

• Changing the R_inter influences both the stiffness and the strength properties of the interface;
• When using very low values for R_inter the interface stiffness may become so low (it has a quadratic dependency on R_inter) that it results in (possibly unrealistic) gapping or overlapping between soil and structure;
• Gapping is also possible between the structure and the soil when the tension cut-off of the relevant material data set is activated (meaning that no tension is allowed in the interface).

Please note that a connectivity plot may be helpful in gaining insight in the way soil and structures are connected. When looking at the connectivity plot you can also select “Table” to explicitly look at the different connectivities of all interface nodes (which node is connected to which node). Also see the PLAXIS 2D/3D Reference manual for more information on interfaces.