In some conditions while using complex plate connections (like ⊢ or + shaped connections) in combination with interfaces the node tying may give unexpected and possibly even unrealistic results. This especially may be the case if the plates of the complex connection are not all activated at the same moment but in a staged construction manner.
A typical example of such a situation is the excavation of a building pit with the installation of a base plate at a certain excavation level. The situation may look like this:
Here we have a T-shaped connection of plates (between wall and base plate) with interfaces. Now in the staged construction process first the wall with interfaces is activated and only after reaching the desired excavation level the base plate with interfaces is activated. Now the unexpected behaviour appears in the phases prior to installing the base plate. In this case the shared node of the (inactive) horizontal interface beneath the base plate and the vertical interface along the wall becomes rigidly connected to the wall preventing local heave of the soil. This behaviour is unrealistic since in reality the soil can heave along the wall.
There is no straight forward solution or work around available since it depends on the situation at hand if and what kind of solution/work around needs to be used.
The user should decide if the results are significantly affected for his/her case. If this is the case the user might for example decide to run a separate model in which the complex connection has been simplified such that at least some of the phases without the undesired node tying can be analysed and compared with the initial calculation. Where relevant the user might also send his/her model to the Plaxis support team (contact page) to discuss what other approaches are possible.
Plaxis is further investigating the possibilities to overcome these issues in the future.