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Foundations behavior is affected by soil behavior which can vary from dilative to contractive depending on the stress level,

particularly in dense frictional soils. The Zero Extension Lines (ZEL) method has been generally developed to predict the

foundations behavior. Knowledge of soil behavior enables the ZEL method to predict the general and local shear failure modes.

In this paper, a relatively simple work hardening/softening soil constitutive model is developed to represent dense frictional soils

behavior under different stress levels. This model is based on the accumulation of the plastic work during a simple direct shear

test and its relationship to stress ratio to establish the hardening law. Verifications have been made for the developed soil model.

The model is then implemented into the ZEL method to theoretically investigate the bearing capacity and load-displacement

behavior of foundations over dense frictional soils. Utilization of this model enables the ZEL method to capture different modes

of failure depending on the foundation size. A numerical study on foundations behavior was performed showing the ability of the

presented approach in capturing both failure modes.

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To investigate the saturation induced collapse deformation behavior of rockfill material, a set of large-scale triaxial tests were conducted in saturated and dry-saturated conditions. Specimens were tested under various confining pressures. For dry-saturated tests, specimens were sheared in various stress levels. Results of all dry saturated tests indicate a sudden reduction in the specimen volume during the submerging process. The ratio of the minimum axial strength of a submerged specimen (at the end of the saturation process) to the shear strength of the specimen before saturation is defined as the coefficient of stress recovery, Csr. Results show that this ratio increases as the confining pressure increases, and decreases as the shear stress level increases. According to the results of dry-saturated tests, reduction values of the internal friction angle caused by saturation (c), the ratio of the elasticity modulus of the material after saturation to its elasticity modulus in dry condition, i.e., Ewet/Edry, and the saturation induced sudden volumetric strain (vc) decrease as the confining pressures increase. However the shear stress level does not have any meaningful effect on the variation of c, Ewet/Edry and (vc).