Full-vehicle, End-to-End under-body blast (UBB) simulations with LS-DYNA have been common practice at the Tank Automotive Research, Development and Engineering Center (TARDEC) for the last several years to support Program Managers in the Army Acquisition of military ground vehicles. Soil, which is one of the four key components (with air, charge, and vehicle structure being the others), has been represented in these simulations by an Elastic-Plastic Hydrodynamic (EPH) model. EPH models has been available for the currently used Double-Sifted (DS) Topsoil since 2012, but not yet developed for a proposed Engineered Roadway Soil (ERS). This study describes a systematic method where EPH model fits were developed based on material characterization tests, and then validated against physical tests with two different types of large flat plates. The accuracy of this model has been shown in nine different comparisons of plate velocity (impulse) or deformation to be well within 11.3%. This model is therefore deemed to be accurate enough and acceptable for usage of UBB simulations in the Army.