earthquake engineering of tunnels close to ground surface

[AK92]

From what I know, there's two distinct methods of designing for earthquake resistance, one for underground structures and one for above-ground structures.

For underground structures you look at the soil displacement during the earthquake and assume that the soil "imposes" the displacement on the underground structure and the underground structure, depending on its rigidity will resist it, with corresponding actions (shear forces/bending moments/etc...).

For above-ground structures, you look at the inertia of the structure and the structural period to judge the design actions, using the design spectra.

Now I'm designing for a tunnel that is transitioning from underground (~10m below ground surface) to aboveground (at ground surface). What is the boundary between treating it as an above ground structure or as an underground structure? And what method should I use to assess the differential settlements between the aboveground/underground connection if that has to be designed for?

 

[racookpe1978]

Design for underground movement the whole way. Keep it consistent!!!!! If you don't -> You WILL get errors that will cause it to fail. (An "error" will never go "stronger" - only "weaker" ... )

Design "above ground" only for those areas where the "tunnel" is raised above grade and suspended on bridge piers or between abutments.