I would like cue on a issue I have with my FE model. I do model (or try ot) the ultime bearing capacity of a footing resting on a purely frictionnal soils (I do take account the weight of the soil). I use quad element to mesh to soil. The stress-strain rel. is a Drucker-Prager. I did use result from 3-axials tests I did in the lab to calibrate the model.
I wish to compute the displacement - contact pressure of the footing during the loading of the foundation.
I do impose displacement at foundation's nodes (so if I have 3 node for a footing I do impose 3 vertical displacement, if I have 11 nodes I do impose 11 displacements).
The foundation dimension (B) is 0.5 m right now.
When I use large mesh (0.5x0.5m), I get very large displacement on the soil before I obtain numerical instability. As I reduce the dimension of the mesh, I get less displacement of the soil before numerical instability (I do reduce the increment of displacement when I decrease the mesh dimension).
The curves of displacement - pressure does looks good with small mesh (I get a plastic plateau like I would expect from a elastic-perfectly-plastic) whilst with the large mesh I do obtain a plastic region but it nevers gets to a plateau, it still looks like a unfnished curve. Also, the value with the large mesh are 3-4x larger than those with the smaller mesh (even considering the edge effect of the pressure distribution under the foundation).
Any1 got a clue here? Thanks!