Hypoplasticity and Viscoplasticity
Numerical models can never be better than the "constitutive laws" employed for the soil. The aim is thus to describe the mechanical interaction between stresses and strains in each individual soil element as realistically as possible. For several decades, IBF has been one of the trailblazers in the development of such constitutive relations. A model called "hypoplasticity" describes granular soils, "viscoplasticity" cohesive soils under monotonic loading and unloading. In comparison with many widely used simple models, properties like shear strength, volume changes (dilatancy) and compressibility are no longer regarded as material constants but dependent on state variables and history (in terms of density, stress and overconsolidation ratio). The required parameters can be determined easily by means of standard tests.
Under smallest cyclic and dynamic strains, a grain skeleton is deformed in an almost elastic manner without grain redistribution. In comparison with large monotonic deformations, the stiffness of the material is increased. For this strain regime, the IBF has already proposed a special constitutive relation ("intergranular strain") and is about to develop a new even more realistic description ("paraelasticity").
High-cycle Accumulation Model
Numerical models as the ones mentioned are in general used for "implicit" calculation steps, which means that every new load step is applied in small portions and a new equilibrium has to be found each time. A calculation of many thousand cyclic load applications would, however, fail due to increasing numerical errors. This is why IBF developed an "explicit" calculation scheme which extrapolates the additional mean deformation generated in each cycle. The number of cycles is unlimited. The required parameters are derived sing cyclic triaxial tests. At present, a comprehensive database is built up for various soils.