Seismic Design of Tunnels - Parsons Brinckerhoff

Third, typically soil is backfilled above the structure and possibly between the in-situ
medium and the structure. Often, the backfill soil may consist of compacted material
having different properties than the in-situ soil. The properties of the backfill soil as well as
the in-situ medium should be properly accounted for in the design and analysis.
5.2 Racking Effect
During earthquakes a rectangular box structure in soil or in rock will experience
transverse racking deformations (sideways motion) due to the shear distortions of the
ground, in a manner similar to the ovaling of a circular tunnel discussed in Chapter 4. The
racking effect on the structure is similar to that of an unbalanced loading condition.
The external forces the structure is subjected to are in the form of shear stresses and
normal pressures all around the exterior surfaces of the box. The magnitude and
distribution of these external forces are complex and difficult to assess. The end results,
however, are cycles of additional internal forces and stresses with alternating direction in
the structure members. These dynamic forces and stresses are superimposed on the
existing static state of stress in the structure members. For rigid frame box structures, the
most critical mode of potential damage due to the racking effect is the distress at the top
and bottom joints.
Damages to shallow buried cut-and-cover structures, including regular tunnel
sections, were reported during the earthquakes of 1906 San Francisco and 1971 San
Fernando (Owen and Scholl, 1981). The damages included:
• Concrete spalling and longitudinal cracks along the walls
• Failure at the top and bottom wall joints
• Failure of longitudinal construction joints
For structures with no moment resistance — such as the unreinforced brick arch in
one of the cases during the 1906 San Francisco earthquake — total collapse is a
possibility.
The methods used in current design practice to counteract the seismic effects on
rectangular tunnel linings are described in the following two sections (5.3 and 5.4).
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