This dissertation evaluates the behavior of Geosynthetic-Reinforced Soil (GRS) retaining structures under various soil stress states, with specific interest in the development and distribution of soil and reinforcement stresses within these structures. The stress distribution within the GRS structures is the basis of much of the industry’s current design. Unfortunately, the stress information is often not directly accessible through most of current physical testing and full-scale monitoring methods. Numerical simulations like the finite element method have provided good predictions of conservatively designed GRS structures under working stress conditions. They have provided little insight, however, into the stress information under large soil strain conditions. This is because in most soil constitutive models the post-peak behavior of soils is not well represented. Also, appropriate numerical procedures are not generally available in finite element codes, the codes used in geotechnical applications. Such procedures are crucial to properly evaluating comparatively flexible structures like GRS structures. Consequently, this study tries to integrate newly developed numerical procedures to improve the prediction of performance of GRS structures under large soil strain conditions. There are three specific objectives: 1) to develop a new softening soil model for modeling the soil’s post-peak behavior; 2) to implement a stress integration algorithm, modified forward Euler method with error control, for obtaining better stress integration results; and 3) to implement a nonlinear reinforcement model for representing the nonlinear behavior of reinforcements under large strains. The numerical implementations were made into a finite element research code, named Nonlinear Analysis of Geotechnical Problems (ANLOG). The updated finite element model was validated against actual measurement data from centrifuge testing on GRS slopes (under both working stress and failure conditions). Examined here is the soil and reinforcement stress information. This information was obtained from validated finite element simulations under various stress conditions. An understanding of the actual developed soil and reinforcement stresses offers important insights into the basis of design (e.g., examining in current design guidelines the design methods of internal stability). Such understanding also clarifies some controversial issues in current design. This dissertation specifically addresses the following issues: 1) the evolution of stresses and strains along failure surface; 2) soil strength properties (e.g., peak or residual shear strength) that govern the stability of GRS structures; 3) the mobilization of reinforcement tensions. The numerical result describes the stress response by evaluating the development of soil stress level S. This level is defined as the ratio of the current mobilized soil shear strength to the peak soil shear strength. As loading increases, areas of high stress levels are developed and propagated along the potential failure surface. After the stress levels reach unity (i.e., soil reaches its peak strength), the beginning of softening of soil strength is observed at both the top and toe of the slope. Afterward, the zones undergoing soil softening are linked, forming a band through the entire structure (i.e., a fully developed failure surface). Once the band has formed and there are a few loading increments, the system soon reaches, depending on the tensile strength of the reinforcements, instability. The numerical results also show that the failure surface corresponds to the locus of intense soil strains and the peak reinforcement strain at each reinforcement layer. What dominates the stability of GRS structures is the soil peak strength before the completed linkage of soil-softening regions. Afterward, the stability of GRS structures is mainly sustained by the soil shear strength in the post-peak region and the tensile strength of reinforcements. It was also observed that the mobilization of reinforcement tensions is disproportional to the mobilization of soil strength. Tension in the reinforcements is barely mobilized before soil along the failure surface first reaches its peak shear strength. When the average mobilization of soil shear strength along the potential failure surface exceeds approximately 95% of its peak strength, the reinforcement tensions start to be rapidly mobilized. Even so, when the average mobilization of soil strength reaches 100% of its peak shear strength, still over 30% of average reinforcement strength has not yet been mobilized. The results were used to explain important aspects of the current design methods (i.e., earth pressure method and limit equilibrium analysis) that result in conservatively designed GRS structures. / text