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24 Specifications, at least within the scope of this Project. Rather, Current design and analysis methodologies for pipeline and the seismic design of rock slopes would be more accurately tunnel systems were developed typically for long, linear treated on a case-by-case basis. structures. For most highway applications, the culvert or For rock slope stability evaluations, geologists and geo- pipe, however, is typically with limited length. The effect of technical engineers will be required to define the potential the short length of the culvert or pipe on seismic response, mechanisms of failure, the strength parameters representing as well as on the analysis procedure, had to be evaluated. the failure mechanisms, and the seismic loads. With this Current design and analysis methodologies for pipeline and information an assessment of available computer software is tunnel systems were developed typically for level-ground required to investigate seismic stability. In some cases where conditions. Culverts and pipes, however, are typically con- two-dimensional conditions are predominant, conventional structed within a built-up embankment. There was a lack of stability software similar to programs used for soil slopes data of how to determine the appropriate TGD parameters could be used. Otherwise, more complete or specialized pro- for culverts and pipes embedded in embankments, especially grams, involving two- and three-dimensional wedge-failure in high embankments. surfaces would be needed. The effect of soil overburden thickness (or embedment depth) and the effect of vertical components of the ground shaking on culvert or pipe performance was not well under- 3.3 Buried Structures stood. Further studies in these aspects were required. Almost all highway culverts and buried pipes have been When subjected to the TGD effect, the response of a buried designed and built without regard to seismic effects. Cur- linear structure can be described in terms of three principal rently, there are no seismic provisions in AASHTO LRFD types of deformations: (1) axial deformations, (2) curvature Bridge Design Specifications for culverts and buried structures, deformations, and (3) ovaling (for circular cross section) or except for a general requirement stating that "earthquake racking (for rectangular cross section) deformations. The loads should be considered only where buried structures first two types, axial and curvature deformations, are cross active faults." Unless there is a global slope stability induced by components of seismic waves that propagate problem within the embankment through which the culvert along the culvert/pipe axis. The ovaling/racking deforma- of pipeline passes, it is unlikely that existing highway culverts tions are induced along the transverse cross section when or buried structures (other than tunnels) have been designed seismic waves propagate perpendicularly to the culvert/ and built with the consideration of fault displacements. While pipe axis. Previous observations have suggested that smaller this approach may be acceptable for drainage culverts and diameter pipes (or small diameter highway culverts) are most pipelines, it may not be an acceptable approach for a more resistant to ovaling deformations than the tunnel well-used pedestrian tunnel. structures (and large diameter/size culverts). On the other In recent years, a great deal of attention has been given to hand, tunnels and large-size highway culverts have per- the study of seismic performance of underground structures formed better than small diameter pipes under the effects to improve the understanding of factors influencing the seis- of axial/curvature deformations. A further understanding mic behavior of underground structures. Design and analysis of the factors resulting in this different performance procedures also have been proposed by some researchers and between large and small buried structures was important. design engineers, but they are generally developed either for Once identified, these factors were considered in the design pipelines (for example, gas and water) or tunnels (that is, and analysis procedures. transportation or water) systems. These procedures have not Simplified ovaling and racking analysis procedures devel- been directly applied to culvert installations. oped for tunnel structures (for example, mined circular The potential problems and knowledge gaps associated tunnels and box type cut-and-cover tunnels) can be applied with the current seismic design and evaluation procedures for to large-span circular and rectangular culverts, respectively. buried structures were considered. Simplified procedures for noncircular and nonrectangular sections (for example, ellipse, arch, arch top 3-sided, etc.) Culverts and buried pipes have performed much better were nonexistent. Numerical analysis was required in this than other highway structural components (for example, case and specific procedures related to performing this type bridges and foundations). The "no-analysis required" cri- of analysis were needed. terion proposed for the bridge structures may not be appli- Various approaches for analysis or design of pipeline sys- cable to the culvert structures. A separate and less stringent tems (for gas and water) have been proposed, particularly screening criterion, taking into account both the ground under the effect of PGD, including fault displacements, lat- shaking intensity and the project geological site conditions, eral spread, and slope deformations (slump). Significant was needed. disparity exists among these approaches. There are also