Modeling Prandtl 1920 undrained bearing capacity condition in Plaxis 3D and 2D

Hello,

I was trying to model Prandtl 1920 standard case for the bearing capacity untill failure of strip footing stands on the surface of undrianed clay soil with Eu = 200 Su , Su = 100 kn/m2, and vu = 0.499. The expected value is 5.14 x Su.

The value i got in Plaxis 2D was 5.19xSu which is very close value. But when i tried to model the same case for a square footing using plaxis 3D, i get a doubled value of 11.1 x Su which is unrealistic.

I would like to ask what is the best way to model the bearing capacity untill failure of square or rectangular footings?