The lack of understanding can
result in serious consequences and software like Plaxis is not to blame.
Except in rare occasions where the software is actually wrong the answers given
will be a direct result of the input/instruction given by the user. This problem
is not helped by the simplicity of Plaxis where someone without understanding
quite easily could; create, run and obtain results from a model and therefore
believe that everything is OK and correct.
It is vary important to check
input, assumptions, model and output and more importantly have someone
else check. The problematic thing is that even thou everything is checked
multiple times by different people, things can get overlooked.
Here are a few things where
people tend to make mistakes in Plaxis:
Generating
initial conditions with k0-procedure
Engineers often run the k0 without thinking of what the k0 procedure does to generate the initial effective stresses. The k0-procedure is only correct when all the geometry of the ground surface, the ground layers, and the ground water table are horizontal. If this is not the case the k0-procedure will lead to the existence of unbalance forces or non-equilibrium of initial forces.
Engineers often run the k0 without thinking of what the k0 procedure does to generate the initial effective stresses. The k0-procedure is only correct when all the geometry of the ground surface, the ground layers, and the ground water table are horizontal. If this is not the case the k0-procedure will lead to the existence of unbalance forces or non-equilibrium of initial forces.
Changing
(SUM)M_stage in a faulty manner
In some cases the user can choose to change the Sum M_Stage to a value smaller than 1.0. If the user does this it is important that the user knows what happens. Say that the goal is to run 50% then 25% and then the final 25% the way to do this is to have the M_Stage set to 0.5, 0.5 and 1. 50% in the first stage, 50% of the remaining 50% in the second stage (i.e. 25%) and 1, 100% of the remaining 25%. (This is explained in more detail in the Plaxis Tunnel Webinar. LINK)
In some cases the user can choose to change the Sum M_Stage to a value smaller than 1.0. If the user does this it is important that the user knows what happens. Say that the goal is to run 50% then 25% and then the final 25% the way to do this is to have the M_Stage set to 0.5, 0.5 and 1. 50% in the first stage, 50% of the remaining 50% in the second stage (i.e. 25%) and 1, 100% of the remaining 25%. (This is explained in more detail in the Plaxis Tunnel Webinar. LINK)
Switching back and fourth
between Mohr-Coulomb and Linear Elastic material
If the switch is made for a material from Mohr-Coulomb material model to Linear Elastic the user needs to pay attention to the input-parameters. These can change between every flip that is made and should be checked so that they correspond to the values that the user wants to use.
If the switch is made for a material from Mohr-Coulomb material model to Linear Elastic the user needs to pay attention to the input-parameters. These can change between every flip that is made and should be checked so that they correspond to the values that the user wants to use.
Putting Interface
Elements where they don't belong
The user should check if the interfaces used actually obtain the results that is sought. For example interfaces on node-to-node anchors and embedded beam rows are sometimes inserted.
The user should check if the interfaces used actually obtain the results that is sought. For example interfaces on node-to-node anchors and embedded beam rows are sometimes inserted.
Not Checking
the undrained shear strength when using Undrained (A)
The shear stress values have to be checked so that they do not violate the undrained shear strength from soil or laboratory tests.
The shear stress values have to be checked so that they do not violate the undrained shear strength from soil or laboratory tests.
Dilatency angle when
modelling with Undrained (A)
Dilatency angle for Undrained (A) should be used with care. Plaxis recommends that the dilatency angle is set to 0 in their manuals. Positive angles may lead to unrealistic values in tensile pore stresses i.e. unrealistic values of shear strength. Negative angles may lead to unrealistic values in pore pressures and therefore behaviour.
Dilatency angle for Undrained (A) should be used with care. Plaxis recommends that the dilatency angle is set to 0 in their manuals. Positive angles may lead to unrealistic values in tensile pore stresses i.e. unrealistic values of shear strength. Negative angles may lead to unrealistic values in pore pressures and therefore behaviour.
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