Evaluation of Viscoplastic Cap Model

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Abstract
A time‐dependent plasticity model for rocks and soils based on Perzyna's elastic/viscoplastic theory is presented, wherein the plasticity yield function is patterned after the inviscid cap model developed over the last decade by Sandler et al. Three aspects are addressed: (1) Theoretical review and development; (2) numerical solution strategies suitable for a displacement‐based finite element method; and (3) illustrative examples of model behavior and comparisons with experimental data. The numerical solution algorithm employs a one‐parameter time integration scheme which provides options for explicit or implicit methods. Nonlinear equations resulting from the implicit method are solved by a Newton‐Raphson iterative technique, and efficiency versus accuracy studies are reviewed. Parameter identification and comparison of the model with experimental data is demonstrated for hard limestone in a variable loading history, soft sedimentary rock in triaxial creep tests, and well‐graded sand in slow and rapid loading (ground shock). It is concluded that the viscoplastic cap model is capable of adequately representing the time‐dependent behavior of soil and rocks for a wide range of loading environments. Limitations of the model are noted, and recommendations for improvement are offered.

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