In a short term undrained calculation for a 20m excavation in soft marine clay, I have chosen to use undrained poisson ratio of 0.45 in the view that this is a middle value for this kind of soil. The Plaxis manual recommended value is 0.495(non-compressible). By doing so, d-wall displacement, moment and shear are all smaller than those using v=0.495 and I am comfortable with the out come.
Can anyone familiar with FEM method using /not using 0.495 in total stress analysis advise on this?
Note: soil cluster-drained, water level: at bottom. I set the model this way to avoid excess pore pressure being generate in the calculation and only C=Su is used, phi is set to zero, Eu/Su=250
I assume you have your reasons to do what your last paragraph note says. If that is in fact the case, your poisson ratio should be an effective poisson ratio!! Your mentioning of the undrained poisson ratio and its sensitivity are not elevant.
Concerning treatment of poisson ratio in Plaxis for modelliing "undrained" behaviour
(1) If you are are using “undrained” for your material, the poisson ratio of 0.45 is in fact too high and should be reduced to about 0.3 to 0.35. This is to ensure that the equivalent undrained total stress poisson ratio is not too close to 0.5 that may lead to numerical instability.
(2) If you are using non-porous option in Plaxis in attempt to model undrained behaviour using undrained parameters, then your poisson ratio (0.45) is to the low side and i suggest you limit to numbers not less than 0.495. It was proven for certain engineering applications, e.g. strip footing, the use of value of 0.49 can even lead to noticeable error.