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Granular materials in their natural state have an inter particle boning that is resulted from natural cementation. These bonds form a relatively strong structure in the soil mass that is called soil structure and consequently these types of material are called structured soils. Structured soils could also be produced artificially by cement or lime treatments. Volumetric compression and the stress-strain behavior of the structured materials after virgin yielding are highly nonlinear that cannot be expressed by a single line in semilogarithmic scale. The natural or artificial structure of the soil retains the void ratio of the soil in higher levels than the void ratio of the same soil in remolded state at the same stress levels. Increasing the stress level from the threshold stress of the virgin yielding initiates the crashing of the soil structure that results large amounts of volumetric strains with a small value of volumetric stiffness. Further crashing the structure of the soil and decreasing its void ratio increases the volumetric stiffness of the soil. Although this procedure is highly nonlinear, however it is a continuous phenomenon and can be formulated mathematically. Since the structure losing behavior of structured soils occurs between two known states, therefore, it could be explained based on the disturbed state concept (DSC). According to the DSC, the behavior of complex phenomena between two reference states could be described based on their behaviors in two reference states using an appropriate state function. The state function or interpolating function relates the response of the material at any level to its responses at two reference states. In this paper a constitutive model base on hierarchical single surface model (HISS) and the disturbed state concept was proposed to describe the stress-strain and the failure behavior of structured soils. The behavior of the soil at the beginning of the virgin yielding was considered as initial, relatively intact (RI), state and its behavior after fully crashed state was considered as fully adjusted (FA) state. The disturbance function derived based on the isotropic compression behavior of the material in the laboratory. A power form state function was proposed to describe the variation of the bulk modulus of the soil. The variable compression model was implemented in HISS model to capture the volumetric behavior of the structured soil. The proposed model verified based on the data from literature. The verification of the proposed constitutive model showed the ability of the model to predict the stress-strain and failure behavior of structured soils. The proposed model could be employed with any other constitutive models to introduce the effect of the structure destruction on the stress-strain and failure behavior of the soil. In the proposed model, if the initial and end modulus of elasticity are equal, the strain stress relationship is linear, and if the initial and final values of the modulus of elasticity are different, then the nonlinear stress-strain behavior is simulated. Hence the behavior of a wide range of materials can be predicted by this model. The proposed model could be utilized to predict the behavior natural structured soils, artificially cemented soils.

Keywords: Disturbed state concept, Structured soil, Compression, Constitutive model, HISS model

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