Seismic Design of Tunnels - Parsons Brinckerhoff

general, flexibility can be achieved by using ductile reinforcement at critical joints. In
contrast, increasing the thickness of the members makes the structure less flexible. The
special structural details suggested by Kuesel and the plastic-hinge design specified by
Monsees and Merritt are in fact based on this philosophy.
Another design concept that can increase the flexibility of the cut-and-cover box
structure is to specify pinned connections at walls/slabs joints. This design detail
becomes attractive when cofferdam retaining structures are used as permanent walls
because pinned connections are less difficult to build than fixed connections in this case.
Applicability of the Free-Field Racking Method
The free-field deformation method serves as a simple and effective design tool when
the seismically induced ground distortion is small, for example when the shaking intensity
is low or the ground is very stiff. Given these conditions, most practical structural
configurations can easily absorb the ground distortion without being distressed. The
method is also a realistic one when the structure, compared to its surrounding medium, is
flexible.
Cases arise, however, when this simple procedure leads to overly conservative
design for box structures. These situations generally occur in soft soils. Seismically
induced free-field ground distortions are generally large in soft soils, particularly when they
are subjected to amplification effects. Ironically, rectangular box structures in soft soils are
generally designed with stiff configurations to resist the static loads, making them less
tolerant to racking distortions. Imposing free-field deformations on a structure in this
situation is likely to result in unnecessary conservatism, as the stiff structure may actually
deform less than the soft ground. An example to demonstrate the effect of structure
stiffness on racking deformation is given below.
Examples
Soil Parameters. In this example a simplified subsurface profile is used in the free-field
deformation analysis and the soil-structure interaction analysis. Figure 22 shows the soil
stratigraphy of this profile. Shear wave velocities are used to represent the stiffness of the
soil layers overlying the bedrock. For parametric study purposes, the analysis is
performed for two cases with the silty clay layer being represented by a shear wave
velocity of:
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