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Modeling of Dry and Saturated Soil-Foundation Interfaces

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Modeling and simulation of earthquake soil-structure interaction (ESSI) require a number of sophisticated modeling and simulation approaches to reduce modeling uncertainty and improve the accuracy of results. The superstructure can be supported by either a shallow or deep foundation and in dry, partially saturated, or fully saturated soil. An interface element is thus required to accurately model the interaction of dry as well as partially saturated soil with the foundation. The current modeling techniques mostly assume a hard normal contact behavior i.e. normal contact stiffness is constant with penetration. However, more physical contact stress expected between the soil-foundation interface is non-linear. The normal contact stiffness increases with penetration until the soil surface becomes hard. At this stage, any further penetration can be assumed to be of hard contact. In this thesis, a soft contact formulation is presented to model the non-linear stiffness at the soil-foundation interface. The cyclic shear behavior of the soil-structure interface is highly non-linear and sophisticated. It includes hysteresis, hardening, softening (dilation), and particle breakage. Depending upon the normal stress or confinement, the shear behavior of the interface can have hardening until a peak shear strength is attained and then soften to the critical or residual shear strength. In this thesis, apart from the most popular Elastic Perfectly Plastic shear model, two additional shear models with nonlinear hardening and non-linear hardening/softening are proposed with minimum modeling parameters to model the monotonic as well as cyclic shear behavior at the soil-foundation interface. In partially or fully saturated conditions, during dynamic events (seismic shaking) pore fluid pressures in the soil adjacent to foundations will change dynamically. Moreover, for strong shaking, the structure might rock, and foundation-soil interface might develop gaps and create suction pressure pulling the water up in tension. A coupled element is developed to model the changes in dynamic pore-fluid pressures and effective stress at the soil-foundation interface for submerged conditions. An extensive verification for all the components of the proposed elements is also performed. 

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