Liquefaction Analysis on Plaxis

Question

Hello, 
 Good afternoon. 
 I am working on my Master's dissertation with Liquefaction Analysis from Earthquake Loading and I have three questions about my modeling. 
 1) What should be the correct dynamic boundaries to use on this modeling? I am simulating a field without any lateral constraint. Should I use for xmin and xmax, free field? And for ymin: should i use Compliant Base or Viscous? 
 2) From simulations that I already did: it took a lot of time to be concluded. Is there any way to decrease the time needed to calculate? And, in most of the cases, the value of pore pressure peaked positive values. What is the reason about it? 
 3) How can I install drains on this soil to decrease the excess of pore pressure? I already tried to use the Drain tool but it was in vain. 
 
 Thank you so much.

Dennis Waterman · Accepted Answer

Dear Vu Ngo, 
 I fully agree with you, I just wanted to add on 3) that in PLAXIS we can nowadays do a Dynamics analysis with Consolidation, which was introduced because for liquefaction consolidation can make a difference. I agree with you that an earthquake is a short time, but on the other hand liquefiable soils are usually quite permeable so there is a consolidation effect in the 30-60 seconds an earthquake may last and that effect can sometimes be the difference between complete liquefaction or not... But using drains in a normal "Dynamics" analysis won't work as such analysis doesn't include groundwater flow nor consolidation, it has to be done in a "Dynamics with Consolidation" analysis. 
 With kind regards, 
 Dennis Waterman

Vu Ngo · Answer

/cfs-file/__key/communityserver-discussions-components-files/343671/PLAXIS_5F00_Site_5F00_response_5F00_analysis_5F00_liquefaction_5F00_evaluation.pdf 
 This article may help to review your model. 
 1) the new boundaries are now available in recent Plaxis versions. What I understand is that we don&rsquo;t need to extend the boundaries too far with free field and compliant base. 
 2) there are different ways to reduce calculation time: mesh size selection, length of the model, calculation parameters, etc. Observations in an earthquake damage we did the assessment before is that filling sand boiled up like under the vacuum. Different with surcharging, earthquake will induce positive pore pressure. That&rsquo;s can be understandable. 
 3) I don&rsquo;t think drain can help to decrease excess pore pressure since earthquake is in short time undrained behavior. 
 This is only my experience, I also wanna hear from another experienced Plaxis users. 
 Kindly regards,

Liquefaction Analysis on Plaxis

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