Enquiry on SoilWorks Soft Ground Module

Question:

Hi Midas Support,

I am having following issues when using MIDAS SoilWorks Soft Ground Module.

1. The issue is on the consolidation rate, Cv. Apparently, when we are having several layers of soil with different Cv, MIDAS is averaging the Cv by layer thickness. Such calculation is appeared to be too rough for a computation software and giving imprecise result. The analysis result appeared to be very imprecise when I modeling thin layer of peat with thick layer of soft clay, where peat is having high void ratio and Cv while clay's are having very low Cv. Each of these layers are contributing significant level of settlement (said 1.5m each). However, as peat's Cv is apparently high, the settlement of 1.50m (said 100%) could be achieved within 1 year, while the clay might only settle for 0.3m (said 20%) in 1 year period. The 1 year settlement for the soils is approximately 1.80m if calculated separately, but is only about 1.20m (said 40%) if calculated using SoilWorks. The 0.6m (20% of Total Settlement) is too significant and might highly affect the design approach. Hence, I sincerely hope that MIDAS could show me the way to dissolve this issue and also improve the method of computing settlement rate using Cv. 

2. There are input of OCR, Preconsolidation pressure (Pc), Compression Index (Cc) and Swelling Index (Cr) in the clay model. According to Soil Mechanics, when OCR > 1, the soil is classified as Overconsolidated Soil, where Cc, Cr, Pc will take into consideration when calculating settlement. While OCR = 1, the soil is classified as Normal Consolidated Soil, where only Cc will take into consideration when computing settlement. However, when I input OCR =1, I realized Cr and Pc is still affecting my settlement result. It would be great if the support could explain how Cr and Pc affecting the settlement for normal consolidated soil. 

This would basically be my question on SoilWorks Soft Ground Module. Would really appreciate if the support could show me the way. Thanks.

Answer:

Hi,

1. As shown in the online help, there are three options about how to take consolidation coefficient, Cv. How about changing option and applying Cv of peat layer?

2. If you could send us the relevant model file, it would be helpful for us to fix this problem.

Regards,
DK Lee


Hi Mr Lee,

Appreciate your prompt response. I could not locate the option of toggling the Cv options in the material menu. Would be great if you can show me the way. Also, I still think it is much appropriate to have 2 Cv calculated separately and recommend MIDAS to include this option. Analysis using Top or Bottom Strata would contribute to a less accurate result as well. i.e. Cv for Peat is equal to 8, while Cv for Clay is equal to 2. Using either average, top or bottom strata would give a less accurate result with few hundred mms difference.

Also, I attached my model which having problem on Cr. However, I had changed the input  very small value for Cr to reduce the effect of Cr. Would be great if you can change to understand the effect of Cr on my model. Thanks.


Hi,

You can find the option from the image below. 

Also, midas SoilWorks provides FEM method for consolidation. Do you think this method would be a solution for this issue? Otherwise, it would be great if you could recommend any other software that provides accurate results using different Cv for the 1D consolidation method.


Regards,
DK


Hi Mr Lee,

I not sure if there is any other software providing solutions using different cv but it can be done easily using hand calculations. I believed the point is why should we standardize 1 Cv for the whole soil strata. As we can see in FEM analysis, soil of different properties are settle in a wide range. Calculating the settlement separately would give a much convincing result. By standardizing the Cv, we are assuming the soil settle in the same way which appears to be contradict with  the actual site condition. 

Also, would be great if you can assist me in the second question. Thanks. 


Thank you for your kind explanation.

As for the second question, I have run your original model and a modified model with different Cr=0.02 of peat layer and got the same settlement as shown below. Could you show me how different your results are?
- Original model

- Modified model

Regards,
DK


Sorry I dont have the license with me now and so I cannot run the analysis. Would you mind to change the Cr from 0.02 to 1.6? Cause last time it made a huge difference using Cr = 1.6 and 0.0001. Cr = 1.6 is obtained when I assume Cr = 1/5 of Cc.



It also gives the same settlement as shown below.




Really appreciate it for making the changes for me. If that is the case, it could be me modeling it in a wrong way previously. Would like to add one quick question, would the Pc making any difference to the settlement even I set the OCR into 1? If so, may I know how the Pc is affecting the NC Soil?

As for Pc, the settlements are different between Pc=10 kN/m2 and Pc=100 kN/m2. I will report this to the developer and let you know.

Regards,
DK


Thanks Mr Lee for clearing my doubt on the second question. Also, it would be great if you can reflect the suggestion on first item (where separate Cv should be applied instead of using a uniform Cv) to the developer. Thanks.

Sure. I will ask about it.

Regards,
DK


Hi,

Thank you for your patience. Here is the developer's reply.

1. Pc with OCR=1.0
Analysis manual says that when you select the Cc method, the settlement is calculated based on the amount of 'Po + delta P', instead of the value of OCR. This is why the settlement changed depending on the Pc when OCR=1.0. 
        
        

2. Cv
If you could provide us with the document/paper on how to apply Cv separately instead of one value, we would consider it for the future development. Thanks.

Regards,
DK
Creation date: 2/24/2019 8:30 PM (dklee@midasit.com)      Updated: 2/25/2019 7:01 AM (dklee@midasit.com)
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