Impact problems in geotechnical engineering typically encountered by the Defense Nuclear Agency (DNA) can be modeled by DYNA, the general purpose finite element codes of Lawrence Livermore National Laboratory (LLNL). The DYNA cap model is one of the geological material models in DYNA codes. This cap model is based on classical incremental theory of work hardening plasticity. It was developed as part of a constitutive model to control the excessive dilation that is predicted by the earlier elastic perfectly-plastic models. Several versions of cap model have been successfully developed with a number of features which can be included or excluded in specific problems. However, the DYNA cap model has deficiencies and can give deceptive results. Fixing or replacing the DYNA cap model was the main objective of this study.

In this study, a modified cap model with new features such as Koiter’s generalized flow rule, Kuhn-Tucker optimization conditions, and recent developments in return mapping alogotithms was developed. It is based mainly on Hofstetter, Simo, and Taylor’s 1993 cap model. A subroutine containing the modified cap model was written that can fit the general purpose finite element code DYNA-3D. Furthermore, two applications were carried out to check the applicability of the modified cap mode and compare it to the existing one in DYNA-3D. In the first application uniaxial strain tests were simulated using the Material Model Driver of DYNS-3D. In the second application the results of finite element analysis were compared with laboratory experiments of a free-falling weight penetrating a soil specimen in a confined model. Two results wee compared; the maximum penetration depth caused by the falling weight and the deceleration of the falling weight while penetrating the soil. When compared with the existing model, the modified cap model is shown to be faster, requires less computer time, is simple and easy to follow from programming point of view, contains new features, and predicts heave, which the existing cap model could not do. In general, the modified cap model is shown to be an improvement over the existing model. Some of the bugs in the general purpose finite element code DANA-3D are described. However, more detailed and various laboratory and/or field applications may be required.