Fluidsolidporous material vs fully coupled elements

[oscar]

Whats the difference between using a quad element with fluidsolidporous material and using a fournodequadup element? We can get one pore pressure value (as for one element) by using quad element with fluidsolidporous material. For the fournodequadup element, we can get 4 pore pressure values at each node. Thus should the average of the pore pressure values from the 2nd case are the same as the one from the first case ? Which one should be used for which kind of applications?

And final question..

How can we use buoyant unit weight for fluidporous material for the undrained analyses ? Shouldnt undrained analyses should use total unit weights ?

Thanks