0.3 m (1 foot) at a 60 percent hammer energy
preceding equation) to the cyclic resistance ratio (CRR)
efficiency, with correction to an effective overburden
(see Figure F-11) that defines the boundary between
pressure of 96 kPa (2 ksf). The procedure is based on
liquefaction and non-liquefaction behavior.
the empirical correlation between cyclic stress ratio
(computed from the peak ground surface acceleration)
To facilitate the use of electronic computational aids,
Youd and Idriss (1997) present equations that may be
and (N1)60 blow count that differentiates the observed
used to approximate the CRR curves given in Figure F-
occurrence or non-occurrence of liquefaction in sand
11. The clean sand curve (fines content < 5 %) is
deposits during earthquakes. The basic correlation
approximated by the following equation:
presented by Seed et al. (1985) for magnitude 7.5
earthquakes for materials with different fines contents
(FC), and adjusted in Youd and Idriss (1997) for very
a + cx + ex2 + gx3
=
CRR7.5
for x < 30
low blowcounts, is illustrated in Figure F-11; the
1 + bx + dx 2 + fx3 + hx 4
correlation may be adjusted to other earthquake
magnitudes using adjustment factors developed by
where:
Seed and Idriss (1982) given in Table F-2. Youd and
a
=
0.048
Idriss (1997) present several alternative magnitude
b
=
-0.1248
scaling factors; however, at present, consensus has not
c
=
-0.004721
been attained on revisions to these factors.
d
=
0.009578
e
=
0.0006136
(a) For a given value of peak ground surface
f
=
-0.0003285
acceleration (PGA) (in g units) and the total and
g
=
-0.00001673
effective overburden pressures at the depth of interest
h
=
0.000003714
′
( σo and σo , respectively), a value of the average
(N1)60 cs
x
=
induced cyclic stress ratio (CSR) can be computed
using the expression (Seed and Idriss, 1971):
The curves for silty sands in Figure F-11 may be
approximated by correcting the penetration resistance
τa
PGA σo
of a silty sand to an equivalent clean sand penetration
CSR =
= 0.65
r
′d
′
σo
g σo
blowcount may then be used in the preceding equation
to estimate liquefaction resistance. The equivalent
in which τa is the induced average cyclic shear stress at
clean sand blowcount is approximated by the following
the depth of interest, and rd is a stress reduction factor
equation:
that decreases from a value of 1 at the ground surface to
a value of 0.9 at a depth of about 10.7 m (35 feet). It is
(N1 )60cs = α + β (N1 )60
noted that the participants in the NCEER workshop
(Youd and Idriss, 1997) have not achieved consensus
regarding possible changes to the values for rd. The
where:
α =0
relationship for rd developed by Seed and Idriss (1971)
for FC#5%
α = exp[1.76-(190/FC2)]
and still in engineering usage is shown in the
for 5%<FC<35%
liquefaction potential evaluation example in Appendix
α = 5.0
for FC$35%
G (Figure G-7). Using values of cyclic stress ratio
from the preceding equation and a plot such as Figure
β = 1.0
for FC#5%
F-11 for the appropriate earthquake magnitude, a
β = [0.99+(FC1.5/1000)]
for 5%<FC<35%
critical value of the (N1)60 blowcount can be
β = 1.2
for FC$35%
determined, such that those (N1)60 blowcounts
exceeding the critical (N1)60 would likely not liquefy
where FC is the fines content (expressed as a
and those having a value less than the critical (N1)60
percentage) measured from laboratory gradation tests
would likely liquefy. By comparing the critical
from retrieved soil samples.
blowcount (N1)60 with the measured (N1)60 of the
material, it is possible to assess whether liquefaction
would be expected to occur or not at the site. The
critical blowcount (N1)60 condition corresponds to a
factor of safety against liquefaction equal to unity (i.e.,
1.0). Factor of safety is defined as the ratio of the
ground-shaking induced cyclic stress ratio (from the
F-21
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