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The ovaling or racking deformations of a buried culvert or Each permanent ground deformation may be potentially cata-
pipe structure may develop when waves propagate in a direc- strophic to culvert or pipeline structures, although the dam-
tion perpendicular or nearly perpendicular to the longitudi- ages are usually localized. To avoid such damage, some sort of
nal axis of the culvert or pipe, resulting in a distortion of the ground improvement is generally required, unless the design
cross-sectional shape of the structure. Design considerations approach to this situation is to accept the displacement, local-
for this type of deformation are in the transverse direction. ize the damage, and provide means to facilitate repairs.
Figure 9-2 shows the ovaling distortion and racking deforma- Characteristics of permanent ground deformation and its
tion associated with a circular culvert or pipe and a rectangu- effects on culvert and pipes are extremely complex and must
lar culvert, respectively. The general behavior of the structure be dealt with on a case-by-case basis. It is unlikely that simple
may be simulated as a buried structure subject to ground de- design procedures or solutions can be developed due to the
formations under a two-dimensional, plane-strain condition. complex nature of the problem. In this chapter, detailed study
Ovaling and racking deformations may be caused by verti- of problems associated with permanent ground deformation
cally, horizontally, or obliquely propagating seismic waves of will not be conducted. Instead, only general guidelines and rec-
any type. Previous studies have suggested, however, that the ommendations on methodology for seismic evaluation under
vertically propagating shear wave is the predominant form of the effects of permanent ground deformation will be provided.
earthquake loading that governs the ovaling/racking behav-
ior for the following reasons: (1) except possibly in the very
9.4 Current Seismic Design Practice
near-source areas, ground motion in the vertical direction is
for Culverts or Other Buried
generally considered less severe than its horizontal compo-
Structures
nent, (2) vertical ground strains are generally much smaller
than shearing strain because the value of constrained modu- Currently there is no standard seismic design methodology
lus is much higher than that of the shear modulus, and (3) the or guidelines for the design of culvert structures, including
amplification of vertically propagating shear wave, particu- Section 12 within the current AASHTO LRFD Bridge Design
larly in the soft or weak soils, is much higher than vertically Specifications. The NCHRP Report 473 Recommended Specifi-
propagating compressional wave and any other type of waves cations for Large-Span Culverts, (NCHRP, 2002) does not ad-
traveling in the horizontal direction. Therefore the analysis and dress issues related to seismic evaluation of long-span culverts.
methodology presented in this chapter addresses mainly the ef- In the past, design and analysis procedures have been pro-
fects of vertically propagating shear waves on ovaling/racking posed by some researchers and design engineers for pipelines
behavior of the buried culverts or pipes. (for example, gas and water) or tunnel (that is, transportation
When subject to ovaling/racking deformations, a flexural or water) systems. While some of these procedures can be
type failure mode due to the combined effects of bending mo- used for the design and analysis of culverts and pipes (for ex-
ment and thrust force must be checked. The flexural failure ample, the transverse ovaling/racking deformation of the sec-
mode is typically the main concern for rigid culverts and pipes, tion, Figure 9-2), others cannot be directly applied because
such as those constructed with reinforced concrete. For flex- they are only applicable for buried structures with a long
ible culverts and pipes (typically, thin-walled conduits con- length, or with a deep burial depth. Furthermore, significant
structed with steel, aluminum, or thermoplastic such as HDPE disparity exists among engineers regarding the appropriate
or PVC), they are likely to be controlled by buckling, which design philosophy and methods of analysis applicable to var-
can occur in the elastic range of stresses. For buckling, thrust ious types of culvert structures.
is the key factor and conservative assumption must be made The following two paragraphs provide a brief description
regarding interface condition (slip or nonslip) between the of procedures and methodologies proposed in the past for
exterior surface of the conduit and the surrounding ground. seismic evaluation of buried structures in general:
An elastic buckling criterion for circular conduits in uniform
soil was proposed by Moore (1989) and may be used for buck- · O'Rourke (1998) provides a general overview of lifeline
ling potential evaluation purpose. earthquake engineering, including the treatment of seismic
evaluation of pipelines. O'Rourke and Liu (1996) present a
detailed methodology for evaluating response of buried
9.3.2 Ground Failure
pipelines subject to earthquake effects. Pipelines responses to
Ground failure broadly includes various types of ground in- both transient ground deformation and permanent ground
stability such as faulting, landslides, liquefaction (including deformation were addressed in these two studies. How-
liquefaction-induced lateral spread, settlement, flotation, etc.), ever, the focus of these studies was on pipeline behavior in
and tectonic uplift and subsidence. These types of ground the longitudinal direction which is more suitable for a long
deformations are called permanent ground deformations. continuous buried pipeline structure. Although failure
