Hardening soil model

Question

Hi reader, 
 I am budding design engineer, working with plaxis 2d for deep excavations. Most of the times, the geotechnical soil report doesn't include stiffness parameters with respect to depth. In such cases, I just keep on imposing MC model for almost all projects. Now, I'm in need of how to model HS model and how to corelate stiffness parameters from SPT "n". Kindly drop me some inputs, please.

Dennis Waterman · Answer

Dear Kousik, 
 Well, in short, you can't get stiffness parameters from an SPT. An SPT is just too crude, one can get an idea of undrained shear strength or friction angle from SPT and an idea about density, but that's about it. So if you have a lot of historic correlations between density and stiffness from the past soil testing and projects, you could try to determine a density from the N_spt and then a stiffness from that density. The error of such a procedure would probably be somewhere in the order over 100%. Hence, the in situ stiffness can be only have of the determined value, but also double. Alternatively (and probably better), you find an experienced colleague that has done some projects in that area and check with him/her what stiffness data was used and how the final calculation results matched the measured settlements and calibrate the stiffness data a bit more. 
 With kind regards, 
 Dennis Waterman