Development and initial applications of a sand plasticity model for nonlinear seismic deformation analyses involving liquefaction are described. The model follows the basic framework of the stress-ratio controlled, critical state compatible, bounding surface plasticity model for sand presented by Dafalias and Manzari (2004). Modifications were implemented to improve its ability to approximate stress strain responses important to geotechnical earthquake engineering applications. Initial applications of the model have included one-dimensional site response analyses of down-hole arrays where liquefaction was triggered and two-dimensional analyses of  dynamic centrifuge model tests involving liquefaction and lateral spreading. An overview of the sand model is presented, followed by  example results from these initial validation studies.