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 (*N*1)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

=

for x < 30

low blowcounts, is illustrated in Figure F-11; the

1 + *bx *+ *dx * 2 + *fx*3 + *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

(*N*1)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

of a silty sand to an equivalent clean sand penetration

= 0.65

′d

′

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 *r*d 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

(*N*1 )60*cs *= *α *+ *β *(*N*1 )60

noted that the participants in the NCEER workshop

(Youd and Idriss, 1997) have not achieved consensus

regarding possible changes to the values for *r*d. The

where:

relationship for *r*d developed by Seed and Idriss (1971)

for FC#5%

and still in engineering usage is shown in the

for 5%<FC<35%

liquefaction potential evaluation example in Appendix

for FC$35%

G (Figure G-7). Using values of cyclic stress ratio

from the preceding equation and a plot such as Figure

for FC#5%

F-11 for the appropriate earthquake magnitude, a

for 5%<FC<35%

critical value of the (*N*1)60 blowcount can be

for FC$35%

determined, such that those (*N*1)60 blowcounts

exceeding the critical (*N*1)60 would likely not liquefy

where FC is the fines content (expressed as a

and those having a value less than the critical (*N*1)60

percentage) measured from laboratory gradation tests

would likely liquefy. By comparing the critical

from retrieved soil samples.

blowcount (*N*1)60 with the measured (*N*1)60 of the

material, it is possible to assess whether liquefaction

would be expected to occur or not at the site. The

critical blowcount (*N*1)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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