I think I understood what is the notation to represent the soil stiffness in the SSI input file, but I would like to be certain of it.
I looked for these info in here and here, but I did not find them
So, this is my interpretation:
Kxx = reaction force in the x direction for a displacement in the x direction
Kxty = reaction force in the x direction for a rotation around the y axis
Ktytx = reaction moment around the y axis for a rotation around the x axis
Is this correct?