عنوان مقاله [English]
Laboratory equipment is progressing, alongside advancements in technology, day by day. Along with these advancements in laboratory studies, the requirement to provide an appropriate model which can accurately predict soil behavior is felt more than at any other time. A great advancement was evolved in soil engineering by introducing the critical and steady state concept to geotechnical problems. This evolution was accompanied by an introduction to constitutive models in a critical state framework. In spite of successful predictions for clayey soil behavior, presented models failed to represent the behavior of granular soils. This weakness was significantly compensated for by introducing the concept of a state parameter in laboratory findings, and also by proposing the use of this parameter in constitutive models. In this paper, the effect of different state parameters on the improvement of model predictions has been shown by modifying an elasto-plastic model from the family of bounding surface models. For this purpose, the model of Manzari (1994) has been chosen. This model, which was developed in a bounding surface framework, is one of the elastoplastic constitutive models developed to predict sand behavior. The critical state concept was ignored by this model. In other words, the model of Manzari (1994) does not involve the concept of critical state and state parameters. Accordingly, this model predicts soil behavior without considering the current conditions of the sand, such as void ratio and mean effective stress. As a result, the model must be recalibrated when the state of the soil changes. In this paper, it has been attempted to modify the original model by adding different state parameters to it, in order to assess the effects of these parameters on the constitutive model predictions. Several state parameters have been introduced by some investigators, but there are three common ones used widely in a large number of papers. These state parameters ($psi$ ,$I_s$ , RSR), in three different ways, were added to the main model formulation, and the capabilities of the modified model were evaluated by the results of laboratory data on Toyoura sand.