Influence of Soil Stiffness on the Response of Piled Raft Foundations under Earthquake Loading

Piled raft foundations have been recognized as the most economical foundation systems, able to deal with heavy structural loads, as the bearing capacity of both the raft and pile components is considered. Due to the complexity of lateral load sharing between the raft and piles in a piled raft foundation, piles have been commonly designed for the entire lateral loads, which results in inefficient design. Therefore, the exact determination of load bearing proportion of each component, under both vertical and lateral loading and also in different soil types in terms of stiffness, leads to a more economical design. The present study aims to investigate the effect of soil stiffness on the performance of superstructure and foundation during an earthquake, by implementing a 3D finite element modeling. Three types of soil, including loose, medium, and dense sand, have been used to consider changes in the soil stiffness. Examined foundations include two types with 3 x 3 and 4 x 4 pile group arrays. Moreover, in order to take account of the influence of superstructure on the load sharing mechanism of the components, a ten-story building, consisting of moment frames, braces, and rigid floors, has been modeled. A dynamic load is then applied in the form of time history acceleration at the base of the soil mass. The results indicate that with an increase in soil stiffness, in spite of the rise in shear force at the base of the structure (base shear), the participation of the pile group in horizontal load bearing has been reduced (approximately by 50% in loose sand and 30% in dense sand).A similar trend is observed for the horizontal and vertical displacements of the foundation. Furthermore, bending moment and shear force in the pile gradually decrease with increasing soil depth, and their maximum occurs at the pile head.