Reconsolidation problem

[Rain]

Hi,

I have built a model to simulate the undrained simple shear test under earthquake loading. At the end of the loading process, the water pressure reached to the peak level. However, as I changed the boundary condation in the following reconsolidation state, the pressure ramained unchanged.

I changed the boundary condation to drain by fixing the DOF of pressure. Is there anyone could help me?

Thanks.

[skamalzare]

Hi,

If you could post a simplified version of your could, I might be able to help you. For now, you might be looking at the hydrostatic PWP, not the excess one!

[Rain]

Hi,

The simplified code is as follows:

# Create a 3D model with 4 Degrees of Freedom
model BasicBuilder -ndm 3 -ndf 4

# Create nodes
node 1 1.0 0.0 0.0
node 2 1.0 1.0 0.0
node 3 0.0 1.0 0.0
node 4 0.0 0.0 0.0
node 5 1.0 0.0 1.0
node 6 1.0 1.0 1.0
node 7 0.0 1.0 1.0
node 8 0.0 0.0 1.0

# Create Fixities
fixZ 0.0 1 1 1 1
fixZ 1.0 0 1 0 1

equalDOF 5 6 1 3
equalDOF 5 7 1 3
equalDOF 5 8 1 3

nDMaterial CycLiqCP 1 ...
element brickUP 1 ...

# Apply confinement pressure
pattern Plain 1 {Series -time {0 100 1e6} -values {0 1 1} -factor 1} {
load 5 0.0 0.0 $pNode 0.0
load 6 0.0 0.0 $pNode 0.0
load 7 0.0 0.0 $pNode 0.0
load 8 0.0 0.0 $pNode 0.0
}

#Create analysis
...
analyze 10 10
updateMaterialStage -material 1 -stage 1
analyze 10 10
puts "Consoladation analysis finished"

# Close drainage valves
for {set x 1} {$x<9} {incr x} {
remove sp $x 4
}

# Keep the reference time being zero
setTime 0.0
remove loadPattern 1
pattern Plain 2 Constant {...} ;# the same as the confinement pressure
pattern UniformExcitation 3 1 -accel 1 ;# earthquake loading in x-dir
analyze...

There is no problem in the above analysis. The pressure, strain and stress change normally during the analysis. At the end of the earthquake loading, the pore pressure reach to almost the same as the initial vertical pressure(4*$pNode) which means the liquefaction occor.
After this, I want to make the pore pressure decrease gradually to simulate the following drain test. It turns out that, however, simply fixing the pore pressure DOF(as the following command shows) cannot let the pressure drain normally.

# Open drainage valves
fixZ 0.0 0 0 0 1
fixZ 1.0 0 0 0 1

The result shows that the pore pressure will remain stable. So how can I make this reconsolidation analysis run successfully?
I have found out that fixing the P-DOF means fix the pore pressure at zero before loading or certain level after loading. It can not make the pressure dissipate. Is there any other command to make the pressure dissipate?

[gaziz]

Hi Rain,

Have you eventually figured out the solution?

I need to do the same for my simulation and have discussed with a friend about the issue.

When you fix pore pressure (PP) DOF, you force PP=0. Therefore, if after undrained loading you fixed PP DOF, then it should force PP=0 suddenly. In your example you fix PP DOF at all nodes, which sets PP=0 at all of them but for dissipation you should leave some of the nodes without PP DOF fixed.

Honestly, I'm not sure even if we need to fix PP DOF. I tried not changing fixities for PP DOF and, instead, run analysis further (with larger time step) and see if PP starts dissipating. So far I had no luck but at least I see that effective stresses start changing.

If you solved the problem please let me know. Would be nice if someone could help with it!

[Rain]

Hi gaziz,

I think the PP DOF should be unfixed.
I have found two methods to dissipate. One is to change the permeability parameter to a bigger value, like 10 m/s. The other is to use pattern command to impose zero PP to the nodes after undrained loading.
However, neither of these are the real dissipation process, which is much slower and more stable. Final strains and stresses may be wrong.