Geoscience Reference
In-Depth Information
2. Seismic displacement analysis
2.1. CRITICALDESIGN ISSUES
Two critical design issues must be addressed when evaluating the seismic performance
of an earth structure. First, are there materials in the structure or its foundation that will
lose significant strength as a result of cyclic loading (e.g., soil liquefaction)? If so, this
should be the primary focus of the evaluation, because large displacement flow slides
couldresult.ThesoilliquefactionevaluationproceduresinYoudetal.(2001)arelargely
used in practice; however, recent studies have identified deficiencies in some of these
procedures. For example, the Chinese criteria should not be used to assess the lique-
faction susceptibility of fine-grained soils. Instead, the recommendations of recent stud-
ies such as Bray and Sancio (2006) based on soil plasticity
(
PI
<
12
)
and sensitivity
(w
c
/
should be followed. Flow slides resulting from severe strength loss due
toliquefactionofsandsandsiltsorpost-peakstrengthreductioninsensitiveclaysarenot
discussed inthispaper.
LL
>
0
.
85
)
Second, if materials within or below the earth structure will not lose significant strength
as aresult ofcyclic loading, willthestructureundergo significant deformations that may
jeopardize satisfactory performance? The estimation of seismically induced permanent
displacementsallowsanengineertoaddressthisissue.Thisisthedesignissueaddressed
inthis paper.
2.2. DEVIATORIC-INDUCED SEISMIC DISPLACEMENTS
The Newmark sliding block model captures that part of the seismically induced per-
manent displacement attributed to deviatoric shear deformation (i.e., either rigid body
slippage along a distinct failure surface or distributed deviatoric shearing within the
deformable sliding mass). Ground movement due to volumetric compression is not
explicitly captured by Newmark-type models. The top of a slope can displace downward
duetodeviatoricdeformationorvolumetriccompressionoftheslope-formingmaterials.
However, top of slope movements resulting from distributed deviatoric straining within
the sliding mass or stick-slip sliding along a failure surface are mechanistically different
thantopofslopemovementsthatresultfromseismicallyinducedvolumetriccompression
of the materialsforming theslope.
Although a Newmark-type procedure may appear to capture the overall top of slope dis-
placement for cases where seismic compression due to volumetric contraction of soil or
wasteisthedominantmechanism,thisismerelybecausetheseismicforcesthatproduce
largevolumetriccompressionstrainsalsooftenproducelargecalculateddisplacementsin
a Newmark method. This apparent correspondence should not imply that a sliding block
model should be used to estimate seismic compression displacements due to volumetric
straining. There are cases where the Newmark method does not capture the overall top
of slope displacement, such as the seismic compression of large compacted earth fills
(e.g., Stewart et al., 2001). Deviatoric-induced deformation and volumetric-induced
