For modeling a nonlinear spring representing the soil region, a nonlinear stress-strain curve of soil must be generated first, using soil programs, to be used in SAP2000 model.

This is a simplified approach, since we only consider the nonlinear properties of soil modulus of elasticity Es. Actually the nonlinear soil response depends on other factors as well, such as pore pressure, dilation angle, etc. Better solution can be achieved by using special 3D nonlinear FEM program specially designed for soil, such as PLAXIS.

I think, SAP2000 isn't proper for considering interaction soil and structure. I offer FEM program such ABAQUS.

 Exactly Mr. Nathan Madutujuh