When modelling pile raft in plaxis 3D such that the piles will carry around 70% of the load. Do we need to use interface elements to capture this effect or
by modelling pile + raft + soil materials, this effect will already be captured?
The purpose of the interfaces is to allow for a proper modelling of soil-structure interaction.
By default when no interfaces are added, the nodes between the soil and structure (e.g. plate) will be rigidly connected. This means that any deformation that happens to the soil will happen to the structure and vice-versa.
The decision to add interfaces or not falls back to the geotechnical engineer who needs to clarify what needs to be covered in the analysis.
Typically interfaces are added so that the behaviour of zone at the contact between the structure and surrounding soil is properly modelled and included in the analysis.