Unplugged pile behavior - 3D Model

Question

Dear all, 
 I am trying to model a monopile, hence an open ended pile with large diameter (more than 8m), in PLAXIS 3D (21.01.00.479). At this point, I am more interested in the axial capacity of the monopile. The pile is installed as "wished in place" in a sandy profile (HS small) and the interior of the pile is filled with soil. Monopile is simulated with shell elements, while two interfaces have been modelled, the interior one and the exterior one (both with an R=0,67). After the pile installation, I activate vertical prescribed displacements in the pile top. 
 Since the monopile has such a large diameter, I expect to see an unplugged behavior, as also mentioned generally in the literature. However, the pile acts as plugged, with a very large resistance from the soil volume in the tip of the pile and virtually almost no internal shaft resistance. Soil displacements also indicate a fully plugged behavior. I was wondering if there is a solution/tip that would potentially allow me to see an unplugged behavior by essentially mobilizing actual internal shaft resistance. Maybe someone else had a similar issue/experience? 
 Kind regards, 
 George Chortis

Dennis Waterman · Answer

Dear George, 
 There are several things that you could take into account: 
 1) In reality if the monopile is load axially, the wall of the monopile would "cut" through the soil separating soil either going into the pile or along the outside. In a FEM model you can't "cut" the soil.....and element below the pile will remain below the pile and the wall would get some artificial (numerical) resistance. This can be solved by extending the interface along the monopile a bit further down below the monopile foot. That extension should just have Rinter = 1, but it will force the mesh generator to generate elements either outside or inside the radius of the monopile, which will improve the behaviour. 2) The mesh inside the pile should be sufficiently fine. So let's say the element size should be less than 10% of the diameter of hte pile. 
 Hopefully this helps, 
 Dennis Waterman