Constitutive models for geomaterials are typically based on the same mathematical plasticity theory framework used to model common metals. However, the constitutive behavior of geomaterials differs from that of metals in four important ways, geomaterials:

1. are (relatively) highly compressible, i.e., pressure-volume response;

2. yield strength depend on the mean stress (pressure), i.e., frictional response;

3. yield strength depend on the relative magnitudes of the principal stresses, i.e. third stress invariant, and

4. tensile strengths are small compared to their compressive strengths.

These basic differences between metals and geomaterials give rise to interesting aspects of constitutive modeling that may not be familiar to engineers trained in classical metal plasticity.

Unlike metals, whose entire constitutive response can often be described by one material characterization test, i.e. uniaxial stress tests, geomaterial material characterization requires a suite of material characterization tests. Geomaterial characterization data is needed to determine the various parameters in geomaterial constitutive models and to verify the material model response. For simulations involving geomaterials, that require a high degree of confidence, the analyst may need to specify the type and number of material characterization tests needed to adequately describe the material response and determine the constitutive model parameters. A knowledge of how these laboratory tests are performed, the form, and format, of typical laboratory test data, and the interpretation of the data for use with a geomaterial constitutive model, is essential to becoming a successful geomaterial modeler.

Figure 1.  Axial compression of a reinforced concrete column: left-to-right;  deformed mesh, rebar cage (DSF=3), ductile & brittle damage (DSF=3)