In PLAXIS 2D or 3D, a thick slab can be modelled in two ways:

• Create a polygon (2D) or a volume (3D) and assign concrete material to it. A model that simulates concrete behaviour as a volume element can be more accurate as you can use the Linear Elastic model or the Concrete material model to capture real concrete behaviour.
  In this case, the geometry definition is straightforward, and you can set the exact dimensions of your slab.
• Create a plate element and determine the equivalent thickness of your slab (d) based on the ratio of EI/EA in PLAXIS 2D or directly define the relevant parameter (d) in PLAXIS 3D.
  A plate can be created as a line element in PLAXIS 2D or a geometric surface in PLAXIS 3D.

When using plates to model a thick slab, note that plates are structural elements used to typically model thin structures in the ground with a significant flexural rigidity (bending stiffness).

There are, however, cases when a plate needs to represent a thick concrete slab embedded in the ground. In such cases, the thickness of the plate, which is essentially a surface in PLAXIS 3D and line in PLAXIS 2D, overlaps partially with the soil, depending on the plate’s thickness.

When the plate is representing a slab embedded in the ground, the true volume of the plate is already occupied by soil elements, and the actual weight of the plate, w, is, therefore, the difference between the weight of the plate and the soil occupying its true volume.

Remarks

• When you are modelling a plate element as a concrete slab on top of a soil material, then the line representing the plate should be in contact with the soil.
  In the same logic, anything that will be constructed above your concrete slab should also have a bottom elevation matching the elevation of the plate.
• In this case, the weight of the plate element is not the full weight of the structure, but it is the weight of the structure minus the weight of the soil removed (see figures below)
  
  
  
  
• Although, using plate elements versus soil volumes depends on the modelling approach a geotechnical engineer follows, nevertheless, there is no clear guideline as to when to use plate elements versus soil volumes.
  Nevertheless, when the ratio of slab thickness to length (d/L) increases, the use of soil volumes would give a better representation, as the real thickness of the concrete may play an important role in the structural behaviour of the model.
  Note: when using soil volumes structural forces can be extracted using either the dummy beam technique or by using, where applicable, the Structural forces in volume plates (2D) or Structural forces in volume piles (3D) functionality in Output.