We are using Plaxis to assess the settlement of an embankment on firm to stiff clay soils in which the long term settlement is critical for an acceptable design. We are adopting the Hardening Soil Model with Small Strain Soil Model and are a bit confused on the value of power m to adopt for are soil due to varied understanding of the use of this parameter.
Understanding 1: this parameter is used to represent the increasing stiffness Eur with depth and cam be adjusted to mimic the actual variation in stiffness parameter with depth. Where there is no variation with depth a very small value of 0.001 can be adopted. True or false
Understanding 2: Adopt a value between 0.5 and 1 (or default value 0.5) to inform the variation in unload reload stiffness values based on the stress variation. This does not have anything to do with stiffness variation with depth but all to do with the stiffness values to be adopted at different stress levels. If so how do we model the variation in stiffness with depth.
My advice to you is to check the manual or just type google the hardening soil model.You will see a pdf regarding each parameter. As per my understanding, I am with understanding 2 nad partly with understanding 1. That is for cohesionless soils a value of 0.5 is usually adopted. For clay soils, soft clays usually take the value in the order of 1 while stiff clays generally assume a value of 0.7. Of course, this will change with the change in stress paths and the type of soil you are dealing with. If you have a triaxial test, then you can calibrate it easily. I have a document but it is in Turkish and I even dont know the language. I hope someone would elaborate further. These are my own views, I hope it helps and someone well equipped with the matter can assist.
In the file below the author is explaining step by step, even has some information regarding embedded beams, which confuses me a bit, like the input values of a pile should be the ones divided by a factor of safety or the maximum values? for a pile in clay, which input values should be adopted when calculating using the alpha method (alpha*cu*perimeter*L or what exactly)?
source: www.researchgate.net/.../Basitlestirilmis-Kazikli-Temel-Hesaplama-Yoentemlerinin-Plaxis-3D-ile-Karsilastirilmasi-Hardening-Soil-Modelinin-Detaylarinin-Incelenmesi.pdf
You can find the English version here: berkdemir.github.io/.../
Thank You Berk for the reply. What about the embedded beam row? Do you input the bearing capacity divided by the factor of safety? And for side resistance do you take the allowable side resistance divided by length and input it in kN/m?
What I tried to tell in that slide was always enter ultimate (characteristic values), because pile behaviour is non-linear, and you have to use characteristic values to simulate piles in Plaxi - to get the correct deformations.
Thank You Mr. Berk. I got the english article and other publications from the same source. They are very informative. I have now understood that it is the characteristic values.
1. What about the side resistance input value. For example, for a 10 metres thick homogeneous clay soil with 100 kPa as cu value and the alpha approach has been seemed suitable for the site. The ultimate side resistance is, 0.5*100*3.142*1*10=1570. The input value can be suggested as linear with, 1570/10 which is around 0 kN/m top and 157 kN/m bottom. Is this okay?
2. When you have pile load test results to calibrate the embedded beam value characteristics, how does one go about this?
Hi Jamal, you have some problems with the understanding of embedded pile and I would strongly recommend you to take a look at the reference manual again. In the example you give, you have to enter 157 kN/m all along the length, not linear. Plaxis will multiply this value with the length of embedded beam. If you have 10 m pile, 1570 kN.
Since this question is on a different topic, I think we should stop discussing these here. Have a nice day.