4 node QuadUP excess pore water pressure ratio exceeding 1

[unnikartha]

The problem which I am analysing is that of sand layer with stone column embedded in it and subjected to sinusoidal loading.
The model used is plain strain, 4-node quadUP element with soil properties suggested in the documentation. The permeability values are also used to simulate partial drainage. To identify liquefaction, the excess pore water pressure ratio (ru) was computed from the pore water pressure results (pwp) available at the nodes from analysis. To calculated ru, the pore water pressure obtained from gravity analysis is deducted from the pore water pressure during dynamic analysis and then divided by the stress in y direction at the node from gravity analysis i.e. (pwp-gravitypwp)/(stress y). I could notice that the value thus computed is way above 1.0, which is the critical value, the upper limit of ru. I was expecting this to be either 1.0 or less. Is this normal? Can anyone help explain the reason for this? Is there anything wrong in the approach?

[jinlu]

Can you try 9_4_QuadUP element? This 9-node element has a better formulation for the solid-fluid fully coupled mechanism.
http://opensees.berkeley.edu/wiki/index ... -p_Element

You can also check the 4-node quadUP element for stratified cases (i.e., no embedded stone column).