Geoscience Reference
In-Depth Information
foundation characteristics (structural and geotechnical capacities). Other parameter vari-
ations can investigate factors such as the shape of the soil displacement profile and the
p-multipliers for liquefied layers, although these factors were generally of lesser impor-
tance in determining pile-cap displacements and loads at the pile-cap connection. If the
bending moment distribution beneath the ground surface is important, then the soil dis-
placement profile and p-multipliers for liquefied layers can be of greater importance.
For most parameter variations, it is important to consider best estimates with high and
low ranges because it is not always evident which will result in a conservative estimate
of foundation response. For example, a conservative estimate of pile response for the
nonliquefaction case might correspond to a softer load transfer relationship between the
pile cap and surrounding crust (e.g., weaker crust strengths, assumption of zero shear on
thebaseandsidesofthepilecap,largerrelativedisplacementstomobilizethecrustloads)
whereas a conservative estimate of pile response for the liquefaction case might instead
correspondtoastifferestimateofthesameloadtransferrelationship(e.g.,strongercrust
strengths, inclusion of base shear on the pile cap, etc.).
The equivalent static BNWF analysis method that uses imposed limit pressures in the
lateral spreading soils (BNWF LP in Figure 12.4b) has serious limitations relative to
the BNWF method that uses imposed soil displacements (BNWF SD in Figure 12.4a).
Theuseoflimitpressuresmaybereasonableforcaseswherethelateralspreadingground
displacements are large, the displacements of the underlying nonliquefied soils are very
small, and the pile foundation is stiff enough that the limit pressures are truly mobilized.
If the lateral spreading displacements are insufficient to mobilize passive pressures from
thecrust,theuseoflimitpressurescanbeoverlyconservative.Ifsignificantshearstrains
developintheunderlyingnonliquefiedlayers,thelimitpressuresapproachmaystillneed
to include free-field soil displacements for the underlying nonliquefied layers to avoid
significantly under-predicting pile and pile-cap displacements. Conditions that affect the
lateral soil pressures, when they are below their limit values, are sufficiently compli-
cated that it is impractical to develop simple guidelines for their estimation. For this rea-
son,theBNWF SDapproachispreferabletotheBNWF LPapproachforgeneraldesign
purposes.
3. Pinning effects for approach embankments
3.1. DEFINITIONAND BACKGROUND
Thissectionconsidersthelocalanalysisofabridgeabutmentinanapproachembankment
(Figure 12.1) for the case where liquefaction has been triggered in the underlying soils.
As the embankment soils spread longitudinally, the piles and bridge superstructure can
develop reaction forces that are significant relative to the mass of a finite-width embank-
ment.These“pinning”forcesreducetheembankmentdisplacementsrelativetothosethat
would occur in the absence of any pinning force. The result is a coupled system wherein
demands imposed on the bridge depend on embankment displacements, which in turn
depend on the degree to which the piles and bridge superstructure pin the embankment.
