RE: Soil column not experiencing any load.

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ryancwl95
Posts: 42
Joined: Mon Sep 25, 2023 12:42 am

RE: Soil column not experiencing any load.

Post by ryancwl95 » Wed Oct 11, 2023 10:37 pm

Hi all,

I am trying to model a 1d consolidation problem of a soil column (1m by 10m) in stko following the example of:

https://opensees.berkeley.edu/wiki/inde ... solidation

This example uses PressureIndependMultiYield Material and a 9-4 up quad element,

The analysis ran smoothly and the mesh generated seems alright. However during post processing, the results are not as expected following the example.
It seems as though the soil column is not experiencing any load, and no excess pore pressure is generated.

I tried simplifying the model by using a linear elastic isotropic material model, but the results is the same. Could it be due to my modelling of the column & analysis steps? could you please advise.

below attached is the 2 files i created this model for
1) using PressureIndependMultiYield Material
1D consolidatioon PIMY.zip
(7.39 MiB) Downloaded 176 times
2) Linear elastic isotropic material model
1D Consolidation LE.zip
(6.04 MiB) Downloaded 181 times
It would be super helpful if you could help me take a look and advice me on this. Thank you very much in advance:)

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: RE: Soil column not experiencing any load.

Post by STKO Team » Fri Oct 13, 2023 10:32 am

I do see results. Are you saying you don't see any displacement/stress/etc?

ryancwl95
Posts: 42
Joined: Mon Sep 25, 2023 12:42 am

Re: RE: Soil column not experiencing any load.

Post by ryancwl95 » Tue Oct 17, 2023 8:18 pm

Hi,

Sorry for the trouble, I realized that I might have put the wrong load in the wrong units, therefore the results is as though no load is added because it is small.

Also, after processing the results i have some questions about pore pressure contours produced.

Below is a screenshot of the pore pressure contours at the loading stage of the soil column:
Screenshot 2023-10-17 211554.png
Screenshot 2023-10-17 211554.png (67.03 KiB) Viewed 2664 times
It shows porepressure (pp) generated at only the nodes with porepressure degree of freedom. However, nodes that do not have porepressure dof shows porepressure=0.

How do I obtain results at nodes without porepresure dof or at depths with no nodes at all. Is there some sort of bi-linear porepressure interpolation between the the pwp nodes, so that the pwp readings & profile are consistent with depth? How can I produce a continuous profile?

For example, in the case of Uy displacement profile:
Screenshot 2023-10-17 211627.png
Screenshot 2023-10-17 211627.png (56.41 KiB) Viewed 2664 times
The profile is continuous with depth, indicating that there is some displacement interpolation between the nodes allowing for a continuous Uy displacement profile even for positions where there are no nodes.

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: RE: Soil column not experiencing any load.

Post by STKO Team » Wed Oct 18, 2023 8:35 am

It's because the 9-node up element used pressure dofs only at corner nodes. That's why you see 0 on dofs without pressure.
You can try to convert your mesh into a 4-node mesh and use a SSPQuadUP. It uses a reduced integration + stabilization and the accuracy should be close to the 9-node. but all nodes have pressure dofs

ryancwl95
Posts: 42
Joined: Mon Sep 25, 2023 12:42 am

Re: RE: Soil column not experiencing any load.

Post by ryancwl95 » Mon Mar 18, 2024 12:47 pm

Hello,

I appreciate your previous response; it was beneficial.

Currently, I'm attempting to simulate the same 1-D consolidation problem (of a soil column) using a brick element. Specifically, I've utilized a high-order element, the 20-8brickup element, for this purpose.

However, I'm encountering the same issue during post-processing. There appears to be a lack of bi-linear interpolation between the pore pressure nodes, resulting in discontinuous pore pressure contours when viewing the profile. Is there a method to address this and achieve a continuous profile?

Thank you for your assistance!

A screenshot of the situation (lower part of soil column) is shown below :
Screenshot 2024-03-18 123911.png
Screenshot 2024-03-18 123911.png (197.05 KiB) Viewed 1970 times

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: RE: Soil column not experiencing any load.

Post by STKO Team » Wed Mar 27, 2024 9:04 am

Did you try this?
It's because the 9-node up element used pressure dofs only at corner nodes. That's why you see 0 on dofs without pressure.
You can try to convert your mesh into a 4-node mesh and use a SSPQuadUP. It uses a reduced integration + stabilization and the accuracy should be close to the 9-node. but all nodes have pressure dofs

ryancwl95
Posts: 42
Joined: Mon Sep 25, 2023 12:42 am

Re: RE: Soil column not experiencing any load.

Post by ryancwl95 » Wed Apr 03, 2024 10:06 pm

Hi,

Yes i did. however this time is for the 3d brick element and my intention is to use a higher order element 20-8brickup. I have tried the the 8noded SSpbrickup elements bu they require element size (h) and alpha (a) input. My final model that im using does not have perfectly square bricks, so im not too sure what values of element size (h) values i should be using. Please advise.

Sincerely,
RYan

STKO Team
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Joined: Tue Oct 29, 2019 8:45 am

Re: RE: Soil column not experiencing any load.

Post by STKO Team » Thu Apr 04, 2024 9:21 am

In the documentation they suggest this:
alpha = h^2/(4*(Ks + (4/3)*Gs))
where h is the element size, and Ks and Gs are the bulk and shear moduli for the solid phase. The $alpha parameter should be a small number. With a properly defined $alpha parameter, the SSPbrickUP element can produce comparable results to a higher-order element such as the 20_8_BrickUP Element at a significantly lower computational cost and with a greater ease in mesh generation.
The h is the mesh size. It should not be exactly the size of each element, just the average mesh size.

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