Hi everyone,
I am working on a consolidation analysis using the Soft Soil Creep model to assess the long term settlement of a deep, normally consolidated clay layer due to the load of a building. I calibrated my const. model parameters based on a consolidation test data and I found that the presoncolidation pressure of the clay is about 350 kPa, which corresponds well to the vertical effective stress at that depth.
In Plaxis there are to ways to account for this - you either enter OCR = 1 for a NC clay or you apply the POP pressure. I noticed a very significant difference between the two approaches in terms of settlement.
Approach 1: I set up OCR = 1 and POP = 350 kPa. Total settlement ended up being around 200-300 mm after 75 years of consolidation.
Approach 2: I set up OCR = 1 and POP = 0. Total settlement ended up being close to 1300mm after 75 years of consolidation.
In theory, after the initial set up of the model with OCR =1, the preconsolidation pressure should be set up automatically to match the initial vertical effective stresses in the soil, which is about 350 kPa in the top part of the clay layer.
Why is there such a significant difference in these approaches and which one is the correct one to use in Plaxis?
Dear Boris,
I think you misinterpret the POP. The POP is not the preconsolidation pressure, it's the pre-overburden pressure. Hence, it's the difference between the preconsolidation pressure and the initial in-situ stress (the initial overburden). So when you define
By defining POP = 350 your soil is no longer normally consolidated, but overconsolidated. Which explains why it settles less.So for your case there are not two approaches, but only one: Approach 2.
With kind regards,
Dennis Waterman
Answer Verified By: Boris Kolev
Thank you Dennis. Below is what is included in the manual. From I understood from the manual, it seems that POP is the max preconsolidation stress, which led to this confusion.
Well, it doesn't say it's the maximum preconsolidation stress - it only says the POP is used to calculate the preconsolidation pressure, but not in what way. However, if you look at figure 171 in the Reference Manual there can be no doubt: