Alla fine le osservazioni sono molto sintetiche:
The existence of the underground structure, with dimensions much smaller compared to the seismic wave lengths, has a limited effect on the aboveground buildings.  On the contrary, the existence of aboveground structures affect the underground structure response, increasing, in the majority of the cases, the seismic loading expressed in terms of racking deflections.  The presence of above ground structures has a strong effect on the flexibility to racking ratio (Fig.X.35), as a consequence of the complex wave propagation effects, i.e. reflections of the waves between underground structure and aboveground structures. In particular, in case of a rigid tunnel (F < 1.0), the seismic waves are trapped between the roof slab of the tunnel and the foundation of the building resulting in an increase of the racking distortion of the tunnel. This phenomenon is less important in case of flexible tunnels. The soil nonlinearity, expected during strong earthquake shaking, and the introduction of interfaces to better model the contact of the lining with the soil, might affect these first conclusions.
