Seismic Design of Tunnels - Parsons Brinckerhoff

5.0 RACKING EFFECT ON RECTANGULAR TUNNELS
This chapter first addresses some of the conventional methods used in seismic
racking design of cut-and-cover tunnels and the limitations associated with these
methods. To provide a more rational design approach to overcoming these limitations, an
extensive parametric study was conducted using dynamic finite-element soil-structure
interaction analyses.
The purpose of these complex and time consuming analyses was not to show the
elegance of the mathematical computations. Neither are these complex analyses
recommended for a regular tunnel design job. Rather, they were used to generate sets of
data that can readily be incorporated into conventional design procedures. At the end of
this chapter, a recommended procedure using simplified frame analysis models is
presented for practical design purposes.
5.1 General
Shallow depth transportation tunnels are often of rectangular shape and are often built
using the cut-and-cover method. Usually the tunnel is designed as a rigid frame box
structure. From the seismic design standpoint, these box structures have some
characteristics that are different from those of the mined circular tunnels, besides the
geometrical aspects. The implications of three of these characteristics for seismic design
are discussed below.
First, cut-and-cover tunnels are generally built at shallow depths in soils where seismic
ground deformations and the shaking intensity tend to be greater than at deeper locations,
due to the lower stiffness of the soils and the site amplification effect. As discussed in
Chapter 2, past tunnel performance data suggest that tunnels built with shallow soil
overburden cover tend to be more vulnerable to earthquakes than deep ones.
Second, the dimensions of box type tunnels are in general greater than those of
circular tunnels. The box frame does not transmit the static loads as efficiently as the
circular lining, resulting in much thicker walls and slabs for the box frame. As a result, a
rectangular tunnel structure is usually stiffer than a circular tunnel lining in the transverse
direction and less tolerant to distortion. This characteristic, along with the potential large
seismic ground deformations that are typical for shallow soil deposits, makes the soilstructure
interaction effect particularly important for the seismic design of cut-and-cover
rectangular tunnels, including those built with sunken tube method.
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