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Reinforcing soils with tension resisting elements is an alternative soil improvement technique confirmed by a series of experimental studies. Fiber reinforcement has advantages in increasing the soil strength and improving the stability of geo-structures. In order to describe and assess the effect of fiber inclusions in soils a suitable constitutive equation is necessary with considering different testing conditions (drainage characteristics, density, applied stress, etc.). In this study, the super/subloading yield surface Cam-clay model (SYS Cam-clay) was used to reproduce the experimental results of drained triaxial compression tests performed under different confining pressures with varying initial relative densities of both unreinforced and fiber-reinforced sand. The reproduced results were in good agreement with experimental results. Both simulated and experimental results showed that an increase in fiber content led to decrease in initial stiffness and increase in initial volumetric compression. This was attributed to the lower initial anisotropy of reinforced sand due to the fiber inclusions. Furthermore, the simulation results indicated that the fiber inclusions inhibited the development of anisotropy compared to unreinforced sand. Consequently, a higher peak and post-peak stresses with increased critical state parameter M, and less volumetric expansion at higher strain rates observed in fiber-reinforced sand. [ABSTRACT FROM AUTHOR]

This paper proposes a combined-loading elastoplastic constitutive model (CL model) for a unified description of the mechanical behavior of soil materials, including clay, sand, and intermediate soil. This mathematical model combines the super-subloading yield surface Cam-clay model and the non-associated Drucker–Prager (DP) model. The first model introduces the skeleton soil structure concept into the Cam-clay model based on critical-state soil mechanics. It describes the mechanical behavior of naturally deposited clay more accurately than the existing model does. The non-associated DP model considers the frictional properties of granular materials and is widely used to describe the undrained shear behavior, particularly that of sandy soil. The newly proposed model is one in which these two independently existing models simultaneously or independently work in loading states according to the loading criterion, which can be derived as a natural result from plastic multipliers. This framework allows for a unified description of a wide range of soil materials, including naturally deposited clayey soils and sandy soils. [ABSTRACT FROM AUTHOR]