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· Slopes and Embankments. This topic was assigned a lower problems. Results of this work were summarized in the third
priority for several reasons. First, many times the seismic Interim Report.
design of slopes and embankments is ignored, as the cost of Specifications and commentaries were presented in three
mitigating potential problems is often far more than the cost sections:
of repairing damage after an earthquake. A second reason
is the factor of safety (FS) used for the static design of slopes · Section X: Retaining Walls. This section provided proposed
(for example, FS = 1.3 to 1.5 for permanent slopes) is often specifications and commentaries for six types of retaining
observed to be sufficient to cover stability during small to walls: (1) rigid gravity and semi-gravity (conventional)
medium seismic events (where liquefaction is not an issue). walls, (2) nongravity cantilever walls, (3) anchored walls,
Finally, failure of a slope often involves minimal risk to the (4) mechanically stabilized earth (MSE) walls, (5) prefabri-
highway users and the failed slope can usually be quickly cated modular walls, and (6) soil nail walls. With the excep-
repaired. tion of soil nail walls, design methods for gravity loads for
· Buried Structures. This topic is given a lower priority pri- each of these wall types were covered within the current
marily because the consequences of failure are often limited. AASHTO LRFD Bridge Design Specifications.
Nevertheless, the current AASHTO LRFD Bridge Design · Section Y: Slopes and Embankments. This section pro-
Specifications is deficient in that no guidelines are provided, vided proposed specifications and commentaries for the
even for those designers who might want to consider seismic seismic design of slopes and embankments. The specifica-
loading. tions covered natural slopes and engineered fills. A method-
ology for addressing sites with liquefaction potential was
One of the other important considerations during the sec- included in the specifications. Current AASHTO LRFD
ond phase of work was developments that were occurring in Bridge Design Specifications do not provide specific guid-
the area of ground motions. At the time of the work, current ance on the methods used to evaluate the stability of slopes
AASHTO LRFD Bridge Design Specifications (2006) provided under gravity and live loads. In this case the specifications
guidance on the determination of ground motions required and commentaries used the standard of geotechnical prac-
for design; however, the guidance was being modified as part tice as the starting point for design.
of a separate NCHRP project to update the current LRFD · Section Z: Buried Structures. This section covered the
seismic provisions. This work was being performed within seismic design of culverts and drainage pipes. The discus-
NCHRP 20-07 Project being conducted by Imbsen & Associates sion focused on the design for transient ground displace-
(Imbsen, 2006). Part of the recommended update involved ments (TGD) and included mention of the requirements
changing from the then current 500-year earthquake (that is, for design for PGD. Generally, the ability of the culvert or
10 percent probability of occurrence in 50 years) to a 1,000 year drainage pipe to withstand PGD depends on the amount
design basis (approximately 7 percent in 75 years). (Various of permanent ground movement that occurs during the
probabilities of occurrence are associated with the nominal seismic event. Procedures given in Section Y provide a
1,000-year return period. For a 75-year exposure period, the means for estimating these displacements. Culverts and
exceedance probability is approximately 7 percent. This ex- drainage pipes will generally move with the ground; there-
ceedance probability is also approximately 5 percent for a fore, movement of more than a few inches to a foot will
50-year exposure period.) Included within the proposed up- often damage the pipe or culvert.
date was a focus on using the spectral acceleration at 1 second
(S1) as a basic proxy for ground motion. Realizing the plans Also included within the third Interim Report were (1) an
within the NCHRP 20-07 Project, as well as a fundamental appendix presenting charts for estimating seismic active and
need for velocity information for some of the methodologies passive earth pressure coefficients that included the contri-
being proposed as part of the NCHRP 12-70 Project, a signif- butions from cohesion and (2) an appendix summarizing the
icant focus was given to the development of a set of rational design of nongravity cantilever walls using a beam-column
ground motion parameters to use during the seismic design displacement method.
and analysis of retaining walls, slopes and embankments, and Contents of the third Interim Report were reviewed with the
buried structures. NCHRP 12-70 Oversight Panel. The focus of the panel discus-
sions was on the organization of the specifications and the ex-
ample problems that needed to be completed to support the
1.2.4 Overview of Conclusions
development of the specifications. This feedback was used to
from Third Phase of Work
modify the specifications and commentaries and to update the
The third phase of work involved Tasks 9 and 10: the de- example problems. A fourth Interim Report was prepared to
velopment of specifications, commentaries, and example document this information. The NCHRP Oversight Panel
