Numerical modeling of higher mode effects of adjacent tall buildings on seismic response of a tunnel

In an urban area, during large earthquakes, a tall building will generate large base shear forces that may be transmitted into the underground structure through the building's foundation and the surrounding soils. This study evaluates the impact of higher modes of the adjacent 13-and 42-degree of freedom (DOF) building models which represent 13-story midrise and 42-story high-rise buildings on the seismic response of the cut-and-cover tunnel in soil-superstructure-underground structure (SSUS) system using three-dimensional nonlinear finite element analysis. First, the seismic response of the tunnel adjacent to the equivalent 3-DOF midrise and 1-DOF high-rise models were compared to the corresponding centrifuge measurements, and good agreement was observed. Then, these calibrated numerical models are used to extend the building model into more realistic superstructure representation by including higher modes of the adjacent midrise and high-rise buildings. The results show that there is a significant increase in the spectral accelerations and base shear of the building due to the inclusion of the higher modes, which lead to the overall increase in dynamic earth pressure increments on the building-side wall of the tunnel. However, there are only small changes in the spectral accelerations and racking displacements on the tunnel wall. This study highlights that the seismic response of the building and tunnel might be underestimated if higher modes are ignored in the analysis.