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seismic coefficient to use during these analyses and an 2.4 Conclusions
acceptable factor of safety.
Design of buried structures (that is, pipelines and cul- Conclusions from this task were that the methodologies
verts) is normally limited to a check on liquefaction available to design professionals within DOTs and consult-
potential, on the potential for flotation, and an evalua- ants for the DOTs are primarily limited either to pseudo-
tion of slope stability or lateral flow. Where lateral soil static methods, such as the M-O equations for estimating
movement was expected, the buried structure was either seismic earth pressures on retaining structures and the limit-
considered expendable or ground treatment methods equilibrium method of slope stability analysis, or to simplified
were used to mitigate the potential for lateral ground deformation methods (for example, Newmark charts or analy-
movement. ses). Although these methods have limitations, improvements
in these methodologies still offer the most practical approaches
An interesting observation from these contacts was that the to seismic design.
approach used by transportation agencies, specifically DOTs, A growing trend towards the use of more rigorous model-
seemed to lag the methodologies being used by many con- ing methods, such as the computer code FLAC (Itasca, 2007),
sultants. This is particularly the case for the seismic design for the evaluation of retaining structures, slopes and embank-
of slopes, where the common practice was to limit the seismic ments, and buried structures has occurred recently. While
stability analyses to the abutment fill using pseudo-static FLAC and similar software provide a more rigorous model-
methods. With the possible exception of some DOTs, such as ing of these problems and can be a very powerful method of
Caltrans and WSDOT, there was some hesitation towards analysis, these more numerically intensive procedures do not
using deformation methods. It also seemed that free-stand- appear to be suitable for development of design methodolo-
ing retaining walls and buried structures most often were not gies required by this Project. Rather they offer methodologies
designed for seismic loading. This was due in part to the lack either to check the simplified procedures appropriate for con-
of generally accepted design guidelines and the general costs ventional design or to evaluate special loading conditions and
associated with the implementation of additional design special geometries. Even in these special cases, these more rig-
requirements. orous procedures can be prone to significant inaccuracies
As a final note, it was commonly accepted by most practi- when the person using the software does not have a good
tioners involved in designing retaining walls and underground understanding of conditions that could affect results.
structures that earth structures have performed well in past As discussed in the next chapter, it also was apparent from
earthquakes, even for the higher ground shaking levels in the review of the literature that some areas of seismic design
WUS. These observations suggested that the seismic design were relatively mature, with design methods provided and gen-
requirement for earth structures should not burden the erally accepted. The design of slopes and embankments is
designer with overly complex and often over costly designed an example of this. But other areas were less well under-
systems. A very important part of the NCHRP 12-70 Project stood even for static loading. Design of geosynthetic walls falls
was to take advantage of recent seismological studies and into this category. This difference in "design maturity" added
seismic performance observations to avoid unwarranted to the complexity of the NCHRP 12-70 Project, as the intent of
conservatism and to reduce the region of the country requir- the NCHRP 12-70 Project was to have design guides consistent
ing seismic loading analyses. with and build upon static design methods.
