should be assessed. Deformations accompanying
An example of liquefaction potential evaluation using
liquefaction may or may not be tolerable depending on
the simplified empirical procedure is presented in
the specific structure design and performance
Appendix G. The Navy has developed a computer
objectives. Guidance for allowable displacements due
program, LIQUFAC, for analyzing liquefaction
to liquefaction for different types of Navy facilities is
potential using the Seed-Idriss simplified procedure
presented by Ferritto (1997b). Guidelines for assessing
(Ferritto, 1997b). Figure F-12 is a graphic plot
consequences of liquefaction are presented in the
illustrating results of LIQUFAC analysis for a soil
following paragraphs.
profile.
(6) Consequences of liquefaction -- lateral spreads.
(2) Cone Penetration Test (CPT) data are also
Lateral spreads are ground-failure phenomena that can
utilized with the Seed-Idriss evaluation procedure by
occur on gently sloping ground underlain by liquefied
conversion of the CPT data to equivalent SPT
soil. Earthquake ground-shaking affects the stability of
blowcounts, using correlations developed among cone
sloping ground containing liquefiable materials by
seismic inertia forces within the slope and by shaking-
which N is the SPT blowcount (e.g., Seed and DeAlba,
induced strength reductions in the liquefiable materials.
1986; Robertson and Campanella, 1985). Direct
Temporary instability due to seismic inertia forces are
correlations of CPT data with liquefaction potential
manifested by lateral "downslope" movement that can
have also been developed. The most recent of these are
potentially involve large land areas. For the duration of
those by Robertson and Wride (1997) and Olsen
ground shaking associated with moderate-to large-
(1997) in the proceedings of the 1997 NCEER
magnitude earthquakes, there could be many such
workshop (Youd and Idriss, 1997). To date these are
occurrences of temporary instability, producing an
not as widely used as the Seed-Idriss correlation with
accumulation of "downslope" movement.
(N1)60 blowcount in Figure F-11.
(a) Various relationships for estimating lateral
(3) Shear wave velocity data have also been
spreading displacement have been proposed, including
correlated with liquefaction potential in a manner
the Liquefaction Severity Index (LSI) by Youd and
similar to the correlations with SPT and CPT data. A
Perkins (1978), a relationship incorporating slope and
recent correlation is presented by Andrus and Stokoe
liquefied soil thickness by Hamada et al. (1986), a
(1997) in the proceedings of the 1997 NCEER
modified LSI approach presented by Baziar et al.
workshop (Youd and Idriss, 1997).
(1992), and a relationship by Bartlett and Youd (1992,
1995), in which they characterize displacement
(4) Other approaches. The Becker hammer is a
potential as a function of earthquake and local site
larger-diameter penetrometer that has been used to
characteristics (e.g., ground slope, liquefiable layer
obtain penetration test data in gravelly soils. These
thickness, and soil grain size distribution). Equations
data are then correlated to SPT measurements so that
given by Bartlett and Youd (1992, 1995) for lateral
liquefaction potential of gravelly soils can be evaluated
spreading of sloping ground and free-face conditions
using Figure F-11. The approach is described by
are as follows:
Harder (1997). The threshold strain approach of
Dobry et al. (1981) utilizes shear wave velocity as a
for free-face conditions:
parameter to estimate a level of cyclic shear strain
below which excess pore water pressure will not be
LOG(DH+0.01) = - 16.366 + 1.178 M
generated and accumulated. If the cyclic shear strains
- 0.927 LOG R - 0.013 R + 0.657 LOG W
induced by an earthquake's ground shaking do not
+ 0.348 LOG T15 + 4.527 LOG (100-F15)
exceed the threshold level, liquefaction cannot occur
- 0.922 D5015
during that earthquake. National Research Council
(1985) notes that this is a conservative evaluation
because liquefaction may not occur even if the strains
do exceed the threshold.
(5) Consequences of liquefaction -- general. The
predicted occurrence of liquefaction does not
necessarily imply unacceptable adverse consequences
to a structure. If liquefaction is estimated to occur
under design ground motion levels, the consequences
F-24
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