The SOIL module performs a nonlinear site response analysis under vertically propagating Swaves using the Seed-Idriss iterative equivalent-linear model for the soil hysteretic behavior (included in SOIL-EQL).

The SOIL module is based on the SHAKE code methodology with some additional programming improvements done over years. The computed equivalent soil properties can be sequentially used in the SSI analysis. In addition to the output file, SOIL produces other that contain time histories for accelerations, soil layer strains and stresses for plotting and reporting purposes. SOIL also produces a file that contains the material soil curves that are used for the non-linear SSI analysis by the STRESS module, and another file with the iterated, equivalent-linear or effective soil properties that are used by SITE is non-linear SSI option is selected by the user.

In the last revision of the SOIL module, we also included a nonlinear time domain capability using a hyperbolic hysteretic soil model (included in the SOIL-NON submodule) based on the DEEPSOIL software theory developed by the Urbana-Champaign university researchers.