The equivalent linear nature of the dynamic Soil-Foundation-Superstructure Interaction (SFSI) of bridge-piers on caisson foundations

This study presents a simplified approach for evaluating the Foundation Input Motion (FIM) of embedded caissons, considering the complex interaction between the caisson and the surrounding soil. The proposed solution is based on a parametric study using the finite difference code FLAC3D. The analysis explores different embedment-to-radius aspect ratios while incorporating the nonlinear response of the surrounding soil. In FLAC3D the caisson was modelled as a cylindrical element with the mechanical properties of concrete, while the soil was modelled using both a linear viscoelastic and a nonlinear constitutive model. Numerical results in the nonlinear soil regime were compared with both the proposed solution and equivalent linear numerical results, where mobilized values of soil shear modulus and damping ratio (inferred from free-field analyses) were applied. These comparisons shed light on the equivalent linear nature of the soil-caisson interaction. Additionally, several soil-caisson-bridge pier system configurations were studied in linear viscoelastic, equivalent linear, and nonlinear soil regimes. A modified version of the “substructure approach”, in which the FIM was evaluated with the proposed solution, was applied to derive the maximum acceleration of the bridge deck and the drift between the deck and the caisson and the results were compared with those obtained with FLAC3D. The results confirm that the modified “substructure approach” captures the dynamic response of soil-caisson-bridge pier systems. Furthermore, as observed in the soil-caisson interaction case, the findings support the equivalent linear nature of the soil-caisson-bridge pier interaction. The proposed solution was also compared with other methodologies available in the literature.