Grouped-drilled shaft foundations are one of the most common foundation types in highway bridge foundations, and it is expected to develop a nonlinear numerical model to predict their deformation capacities during earthquakes. In this paper, numerical simulations of model single and grouped drilled-shafts are conducted using a fiber model that considers the effects of material nonlinearity including lateral confinement from surrounding soil to pile concrete and reinforcement swelling. Numerical

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... results agree well with experimental results in single drilled shaft cases. However, the capability of the numerical simulation for grouped-drilled shafts is worse than that for single drilled shafts, while the calculation captures overall tendencies as observed in the experiment. The numerical results indicate that the compressive stress-strain model for concrete can strongly affect calculated foundation behavior, because, in groupedpile foundations, the axial load in each pile varies with an increase in the horizontal displacement of the foundation, and the piles become subjected to large axial loads.