Dear community, I'm trying to find the ultimate bearing capacity of a shallow footing in PLAXIS 3D.
The analysis is made as Load Control: A weightless plate element (2x2) is created with a point load of 10,000 kN at its center (shear failure guaranteed).
The curve of Mstage vs IuI is drawn for a pre-calculated node at the footing's center, and the ultimate load is extracted as Mstage x 10,000 for the point at which the curve becomes parallel to the axis of IuI (shear failure occurs).
However, I'm having doubts about this method of reading the curve when dealing with a loose sand or soft clay, where Local or Punshing Shear Failure are expected to happen as shown the below picture.
I understand that the MC model does not includes softening of soil as a hypo-plastic model would, however there should be a method to get the ultimate capacity from the MC model given that even in the two cases above, the curve I'm having converges to a clear maximum load.
Would you recommend that I continue using the same approach for Dense and Loose soil?
Thanks a lot!
Below are curves extracted for a Loose Sand model with the following properties:
unit weight = 15 kN/m3, c = 0 kPa, phi = 30 degrees, E = 5000 kPa,
I've increased the number of steps simultaneously with the number of max unloading steps in order to force the solver to continue the iterations even after the curve had gone flat and the outcome is interesting:
After several steps failed to increase the loading, the model reached a breaking point and the curve became erratic as the literature suggests!
This is a close up look at the breaking point:
Now there are few things that I'd like to investigate and your help would be much appreciated:
1- For compromising in computational time, can we take the failure load as the point where the curve became perfectly flat? As setting the max unloading steps to a high number will significantly increase the time of run.
2- Why is the Arc Length control type affecting the point where the curve became flat? Shouldn't the two models reach the same value?
3- The same procedure done on this Loose Sand didn't yield a similar curve for Soft Clay modelled as Undrained (B) with the following parameters (unit weight = 12 kPa, su = 12, E = 2000 kPa). What do you suggest?
Thanks in advance!