Seismic Design of Tunnels - Parsons Brinckerhoff

Use of this method will lead to a conservative design when a stiff structure is
embedded in a soft soil deposit. On the other hand, when the tunnel structure is
flexible relative to the surrounding ground, this method may also underestimate the
seismic racking response of the structure.
A proper design procedure that can avoid the drawbacks discussed above must
consider the soil-structure interaction effect. For this purpose, an in-depth study using
dynamic finite element soil-structure interaction analysis was conducted (Section 5.5). In
this study, many factors that might potentially affect the tunnel response to seismic effects
were examined. The results, however, indicate that the relative stiffness between the soil
and the structure is the sole dominating factor that governs the soil-structure interaction
effect.
Flexibility ratios, F , were defined to represent the relative stiffness between soils and
rectangular structures. Using these flexibility ratios, a well defined relationship was
established between the actual tunnel racking response and the free-field shear
deformation of the ground (Figures 34 and 35). This relationship allows engineers to
perform their design work by using conventional and simple frame analysis programs
without resorting to complex and time consuming finite element soil-structure interaction
analyses. A detailed step-by-step design procedure using these simplified frame analysis
models was given in Section 5.6 of Chapter 5.
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