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CHAPTER 10
Recommendations for Future Work
During completion of the NCHRP 12-70 Project, it be- · Simple but rational methods for estimating site factors at
came apparent that additional work would be required to de- locations should be developed for locations where NEHRP
velop simplified recommendations for the seismic design of site factors may not be appropriate. These locations in-
retaining walls, slopes and embankments, and buried struc- clude deep soil sites located in CEUS, where the frequency
tures. The required work generally occurs in two categories: characteristics of ground motions in combination with the
(1) fundamental research into seismic performance related to depth and shear wave velocity of the soil profile make the
specific issues, and (2) testing of recommended procedures NEHRP factors inaccurate in some situations. Likewise, lo-
described in this Final Report and as set forth in the specifi- cations where thin soil layers (for example, less than 50 feet)
cations and commentaries contained in Volume 2. The fol- occur over rock also may not be adequately modeled by the
lowing discussions summarize some of the topics that will NEHRP site factors.
require further research or evaluation. · An approach for introducing the effects of liquefaction
into ground motion computations is needed. Although
10.1 Ground Motions one-dimensional, nonlinear effective stress computer pro-
and Displacements grams are available, use of these methods is relatively lim-
ited. Either simple ground motion adjustment procedures
Applicable ground motion criteria have been established by
that account for liquefaction should be developed, or easier-
the AASHTO decision to adopt the 1,000-year ground motion
to-use, commercially available, effective stress computer
maps and the NEHRP-type site factors as a basis for seismic de-
programs are needed. In the absence of these methods, it is
sign. This decision on the part of AASHTO resolves many of
difficult to properly account for changes in ground motion
the uncertainties that existed during this Project and should
above sites where liquefaction is predicted.
provide a sufficient basis for the seismic design of retaining
· Revised equations are needed for estimating the site-adjusted
walls, slopes and embankments, and buried structures. The re-
PGA in Equations (5-7) and (5-9) at a predetermined per-
vised Newmark displacement charts given in this Final Report
manent displacement. The current equations cannot be
also provide an up-to-date method of estimating permanent
ground displacements suitable for WUS and CEUS. Height- applied by a designer within a spreadsheet analysis procedure
dependent coherency, or wave scattering, factors also were to estimate limiting PGA values if the displacement (d ) is
introduced in this Final Report, and these will be useful for specified.
· Additional evaluations should be conducted to confirm
seismic design of walls over 20 to 30 feet in height.
The following topics in the areas of ground motions and that the wave scattering factors described in Chapters 6 and
displacement determination appear to warrant either future 7 are applicable for a variety of site, retaining wall, and slope
consideration or development: conditions.
· Maps are needed from the USGS that provide PGV for the
10.2 Retaining Walls
1,000 year return period. These maps would eliminate the
need to use empirical equations based on the 1-second spec- A relatively simple methodology was identified during this
tral ordinates for making the PGV determination and could work for the seismic evaluation of retaining walls. This method-
contribute to simpler estimates of permanent ground ology was based on either M-O equations for cases where
displacements. soil is homogenous behind the retaining structure, or a more
