A new macroscopic plasticity model for non-cohesive granular materials, with the focus on coarse-sized materials (railway ballast), is presented. The corresponding incremental relations are reformulated in the space of stress invariants that is extended with the internal (hardening) variables and the corresponding dissipative stresses. The model is calibrated using data from Conventional Triaxial Compression (CTC) tests. A function evaluation method is used for the optimization. A "multi-vector" strategy for choosing the appropriate start vector is proposed.
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