Modifying Duncan–Selig Soil Model for Plasticlike Behavior

Abstract

The variable-modulus Duncan–Selig soil model is very effective in capturing the nonlinear stress–strain behavior of most soils in loading environments, including the softening effect from increased shear stress as well as the stiffening effect from increased confining pressure. Accordingly, the Duncan–Selig model is used in many geotechnical applications, such as construction of culvert installations with incremental layers of soil, for which soil loading is the dominant condition. However, the Duncan–Selig model is a nonlinear elastic formulation that retraces the same stress–strain path upon unloading. Consequently, the model does not predict residual deformation upon unloading. In contrast, plasticity-based soil models are generally less accurate in simulating the nonlinear loading response of soil samples; however, they inherently include permanent deformation if plastic deformation occurs during loading. This paper introduces modifications to the Duncan–Selig model that result in permanent deformations upon unloading similar to those of advanced plasticity models. Although the modifications are based on concepts from incremental plasticity theory, the modified Duncan–Selig model remains a variable-modulus formulation without the need of defining any additional model parameters or evoking a plastic-flow rule. Thus, the large existing database of Duncan–Selig parameters remains valid for the modified formulation. Most important, the modified Duncan–Selig model is shown to satisfy thermodynamic and continuity requirements and to compare very favorably with triaxial and hydrostatic laboratory test data under load–unload–reload conditions. Finally, the modified Duncan–Selig model is used to simulate the effects of soil compaction for an installation of a long-span corrugated-steel culvert.