PLAXIS 2D - ANALYSE SPEED

Question

HI 
 
 1) AM NEW TO PLAXIS, PLAXIS MODEL TAKES A 10 MINUTES AVERAGE TIME TO COMPLETE ANALYSIS, IS THERE ANY POSSIBILITES TO INCREASE SPEED OF ANALYSIS 
 
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 I HAVE ATTACHED MY SCREENSHOT, PLEASE CLARIFY

Nitharshan · Accepted Answer

Mr. Arun, 
 
 The reason for that triangle green colour mark will be some stresses exceed your strength parameters. Do you use clay layers with Su? 
 You can see the stress points by looking plastic points plot and stresses. 
 
 For example i provided Su = 15 kPa, so all the shear strength in the soil need to be less than or almost equal to 15 kPa. so eventhough the stress increases shear strength limit to 15kPa so after the relative shear stress takes the value 1 all points below become exceed the failure points. You can easily see this from plastic points.

When i increase the Su=30, failure points further reduce as below.

so you need check the stress and shear strength and need to change the soil parameters (clay layer strength properties increases along the depth so you can use cinc option). 
 
 for water conditions, what is the purpose of the analysis, whether you are going to reduce the water level based on permeabilities or keep the water level as it is without reduction. Steay state ground flow automatically adjust the pore pressure distribution based on permeabilities. 
 
 i hope i provide the correct answers, if wrong please tell me Rushan Gizzatullin 
 
 Thanks.

Stefanos Papavasileiou · Answer

Dear Arun, 
 Great to hear that you are starting to work with PLAXIS. 
 A calculation requires time and that time can be related to different parameters. 
 First of all, the type of analyses determines how complex the computation will be. For instance a dynamic analysis is more demanding to a plastic as more degrees of freedom are included in the analysis. 
 Then, the number of elements included in the finite element model can increase significantly the calculation time. That is why it is important to have an efficient mesh generated, meaning have fine elements where needed and large element far away on model boundaries. 
 Another aspect is the amount of plasticity generated. The mode plasticity generated the longer it takes the model to converge. 
 The latter depends on the constitutive model used and how well it can "describe" the soil response. 
 Note that we cannot give advise on material parameters as this is the job of the geotechnical engineer that has all the data at hand (field and/or laboratory) as well as experience. 
 A general recommendation, I would not try to "force" or "trick" a model to calculate with a green checkmark, as there might a real failure that occurs which is the reason why you run this analysis. In this case, in order to find out what the mechanism for failure is, and so possibly determining the cause, these Output plots should be inspected: 
 
 deformed mesh 
 total displacements (shaded plot) 
 incremental displacement (shaded plot and/or arrows) 
 total deviatoric strain (shaded plot) 
 incremental deviatoric strains (shaded plot) 
 plastic points 
 when applicable: excess pore pressures (pexcess) 
 
 A combination of these plots will usually give insight into what is happening.

Nitharshan · Answer

Oh how can the soil has this type of parameters? Su=1 is very very small. Please check the parameters.

Nitharshan · Answer

Phreatic level is just the user defined one, if you draw at what level then pore pressures generated based on that level. this can be adjusted based on the permeabilities values like steady state flow.

Nitharshan · Answer

Hi Mr. Arun, 
 
 1)Do you know the SPT of the clay, normally for clay Cu can be correlated using 4 to 5 N (where N is the spt values), you can refer the correlations in web also. Further in your country sometimes the particular soil some range values might be used. So check that one also. In my analysis part i didn't use any clay with Su=1 kPa. I used very weak clay, peat with Su=14 to 16 kPa. Better please check with the values. 
 
 2) I really don't know. I hope system performance depend (amount of ram etc), number of phases in calculation, type of analysis (when i used steady groundflow analysis it might take little bit more time than phratic). I will add 2 resource people here whom may help in this. Rushan Gizzatullin Stefanos Papavasileiou 
 
 Thanks.

PLAXIS 2D - ANALYSE SPEED

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