• The plasticity model for silt (PM4Silt) was developed for geotechnical earthquake engineering applications by Professors Ross W. Boulanger (homepage) and Katerina Ziotopoulou (homepage) at the University of California at Davis.
  • PM4Silt is a stress-ratio controlled, critical state-based, bounding surface plasticity model for clays and plastic silts that has been implemented as a user defined dynamic link library for use with the programs FLAC and FLAC2D by Itasca (homepage). The model was developed and implemented to approximate stress-strain responses of specific importance to geotechnical earthquake engineering and cyclic softening problems.
  • PM4Sand, which provided the framework upon which PM4Silt was built, is available (homepage) for modeling sand and other purely nonplastic granular soils in geotechnical earthquake engineering applications.
  • Changes in Version 2 relative to Version 1 were modifications to (1) the initial back-stress ratio initialization routine and (2) the stress ratio term in the elastic shear modulus equation. The only change in Version 2.1 relative to Version 2 was restricting the stress ratio term to less than or equal to unity in the elastic shear modulus equation. These changes did not significantly affect model responses, but calibrations need to be checked/revised when updating from Version 1 to 2 or from Version 2 to 2.1.


  • effective stress
  • liquefaction
  • pore pressure
  • seismic loading


Boulanger, R. W., and Ziotopoulou, K. (2023). “PM4Silt (Version 2.1): A silt plasticity model for earthquake engineering applications.” Report No. UCD/CGM-23/02, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, CA. [February 2023; revised June 2023] [PDF available here]


UDM Version: 2.1
Release Date: Jun 12, 2023

Category: Low-Plasticity Silt and Clay

Software: Download: User Guide: Contact: Developer:

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