On the limitations of decoupled approach for the seismic behaviour evaluation of shallow multi-propped underground structures embedded in granular soils

The evaluation of the seismic behaviour of shallow multi-propped underground structures (e.g. metro stations, parking, etc.) can be considered as one of the most important and actual research topics in structural earthquake engineering. Over the last decades, different types of analysis approaches have been proposed, but several issues are still open, especially for mull-propped structures embedded in granular soils. In this paper, the main limitations of the decoupled approach are investigated and discussed through a large set of numerical simulations involving: i) a mull-propped underground structure; ii) five natural and one synthetic accelerograms; iii) four different soil profiles characterized by the same mechanical properties but different values of the shear modulus that are related to the shear wave velocity ranging between 360 m/s and 750 m/s. The results, in terms of bending moment acting on RC retaining walls, obtained through the decupled approach are discussed in comparison with those obtained through the coupled approach (non-linear dynamic analysis), highlighting the main differences and limitations. This study shows that the decoupled approach provides consistent results only for soil profiles characterized by low values of stiffness due to the main assumptions underlying the approach.