Seismic Design of Tunnels - Parsons Brinckerhoff

• When tunnels are embedded in unstable ground (e.g., landslides and liquefiable
sites)
• When tunnels are locally restrained from movements by any means (i.e., “hard spots”)
Ovaling Effect on Circular Tunnels
Ovaling of a circular tunnel lining is caused primarily by seismic waves propagating in
planes perpendicular to the tunnel axis. Usually, the vertically propagating shear waves
produce the most critical ovaling distortion of the lining.
The conventional simplified free-field shear deformation method was first reviewed,
through the use of several design examples in this study, for its applicability and
limitations. Then a more precise, equally simple method of analysis was developed to
assist the design. This method takes into account the soil-lining interaction effects and
provides closed form solutions (Equations 4-9 through 4-13) to the problems.
Numerical finite difference analyses using the computer program FLAC were
performed to validate the proposed method of analysis. A series of design charts (Figures
10 through 16) was developed to facilitate the engineering design work.
Racking Effect on Rectangular Tunnels
The racking effect on a cut-and-cover rectangular tunnel is similar to the ovaling effect
on a mined circular tunnel. The rectangular box structure will experience transverse
sideways deformations when subjected to an incoming shear wave travelling
perpendicularly to the tunnel axis. The most vulnerable part of the rectangular frame
structure, therefore, is at its joints.
Conventional approaches to seismic design of cut-and-cover boxes consist of:
• The dynamic earth pressure method (Section 5.3), originally developed for
aboveground retaining structures. Its applications in the seismic design of
underground structures are limited only to those built with very small backfill cover,
and those with structural characteristics that resemble the characteristics of
aboveground retaining structures (e.g., a depressed U-section).
• The free-field shear deformation method (Section 5.4), which assumes that the racking
deformation of a tunnel conforms to the shear deformation of the soil in the free-field.
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