Geoscience Reference
In-Depth Information
2.3. EVALUATING THEPOTENTIAL FOR LIQUEFACTION-INDUCED
GROUND DISPLACEMENTS
Estimatesoffree-fieldsoildisplacementsareusedtorepresentlateralspreadingdemands.
The term “free-field” refers to the soil displacements that would occur if the pile group
wasnotthere,ortothesoildisplacementsthatwouldoccuroutsidethezoneofinfluence
ofthepilegroupwithallotherconditionsbeingthesame.Free-fieldlateralspreadingdis-
placementsmaybeestimatedinanumberofways,including:(1)theintegrationofshear
strain profiles that are estimated in conjunction with SPT and CPT based liquefaction
analyses, (2) empirical relationships based on case history data and broad site character-
istics, (3) Newmark sliding block analyses, and (4) nonlinear dynamic numerical analy-
ses. There is considerable uncertainty in the estimates of lateral spreading displacement
obtained from any of these methods, with the overall uncertainty including contributions
from the uncertainties in ground motion, site characterization, spatial heterogeneities,
soilpropertyestimation,andapproximationsinherenttoeachanalysismethods.Estimat-
ing ground displacements using more than one of these methods is often advisable, after
whichjudgmentcanbeusedinselectingabestestimateandadesignvaluethataccommo-
dates epistemic and aleatory uncertainties to an extent that is appropriate for the specific
bridge or structure.
The distribution of lateral spreading displacements versus depth must also be estimated.
Numerical and physical models have shown that shear strains may be larger at the top or
bottom of a liquefying soil layer depending on various factors, such as the initial relative
density distribution in the layer and details of the pore water pressure diffusion process
(e.g., presence of lower-permeability strata over a liquefied layer). For cases where the
piles are laterally stiff and strong enough to provide satisfactory performance, the pile
head displacement and maximum bending moment are often relatively insensitive to the
assumed soil displacement profile shape, such that a simplified profile with linear vari-
ations across layers can be assumed for design. For more flexible pile foundations, the
bending moment and curvature demands versus depth can be controlled by the assumed
shape of the free-field soil displacement profile, such that a range of soil displacement
profile shapes may need to be considered.
Timing of the lateral spreading displacements relative to the interval of strong ground
shaking is difficult to assess. Case histories, physical models, and numerical analyses
have shown that there are cases where the lateral spreading displacements will develop
primarily during shaking and cases where the lateral spreading displacements will con-
tinue to increase significantly after the end of strong shaking. For example, delayed lat-
eral movements can develop as a consequence of loosening associated with the trapping
of upwardly seeping pore water beneath a clay layer and the formation of a localized
dilating shear zone at the sand/clay interface (e.g., Malvick et al., 2006). This phenom-
enonwasobservedinaseriesofcentrifugemodelswithcross-sectionssimilartotheone
shown in Figure 12.2, wherein a clay crust layer spread laterally downslope when the
underlying saturated loose sand liquefied during shaking. The crust displacements were
often observed to progressively increase after the end of shaking, as illustrated by the
