G-3.
Example 3 - Liquefaction Potential
liquefaction potential, the N-values are converted or
Evaluation
normalized to (N1)60 values. This involves adjusting the
values to a common effective overburden pressure of 1
a.
Introduction
tsf (96 kPa) using the relationship in Figure G-4. The
calculations for each of the five borings drilled at the
This example illustrates the steps involved in evaluating
site are shown in Table G-1. The sands at the site
liquefaction potential using the Seed-Idriss empirically
contain varying amounts of silty fines (i.e., the
based methodology described in paragraph F-4. This
percentage of minus No. 200 sieve material). The
procedure is a widely used procedure that would
percentage of fines influences the liquefaction
typically be employed for a site for which there remains
susceptibility, as shown in Figure G-5 (Youd and Idriss,
the potential for a liquefaction hazard after applying the
1997; Seed et al.,1985), which will be used to assess the
screening criteria in paragraph F-3. Also included in
liquefaction potential. For this evaluation, it is desired
this example is an assessment of the consequences of
to use the correlation curve for clean sands (# 5 percent
liquefaction in terms of settlements.
fines) in Figure G-5. Therefore, it is necessary to
further adjust the (N1)60 values of the silty sands (> 5
(1) The site conditions are illustrated in Figure G-
percent fines) to a clean sand condition. The following
2. Approximately 50 feet (15 m) of predominantly
equations are utilized to make this adjustment in the
loose to medium dense sand with lenses of clay of
(N1)60 values:
Holocene geologic age overlies dense (non liquefiable)
sands and stiff clays. The water table is at a depth of 20
( N1 ) 60cs = α + β ( N1 )60
feet (6.1 m). The site cannot be screened out as having
an insignificant potential for liquefaction using any of
the three criteria given in paragraph F-3; therefore, the
where:
soils below 20 feet (6.1 m) depth are evaluated for their
α =0
for FC#5%
liquefaction potential and consequences. The soils
α = exp[1.76-(190/FC2)]
for 5%<FC<35%
above the water table cannot be screened out for
α = 5.0
for FC$35%
differential compaction using the criteria in paragraph
F-3; therefore, settlements in the upper 20 feet (6.1 m)
β = 1.0
for FC#5%
are evaluated also.
β = [0.99+(FC1.5/1000)]
for 5%<FC<35%
β = 1.2
for FC$35%
(2) The proposed structure is a light, two-story
structure to be supported on isolated spread footings
where FC is the fines content (expressed as a
bearing at a depth of 2 feet (0.6 m) below the ground
percentage) measured from laboratory gradation tests
surface. Because the foundation loads are light and the
from retrieved soil samples.
footings are well above the water table, there is not a
potential for liquefaction to result in a foundation
The adjusted (N1)60 clean-sand values are shown in the
bearing capacity failure. Rather, the primary concern is
right-hand column of Table G-1 and plotted versus
settlement due to consolidation of the liquefied sand as
depth in Figure G-6.
pore pressures dissipate following liquefaction. The
sands above the water table may also densify due to the
ground shaking and contribute to the overall settlement.
(1) The next step is to assess the "critical" (N1)60
Settlements are estimated using the Tokimatsu and Seed
values for the site, i.e. the (N1)60 values dividing
(1987) methods. The site and vicinity is flat, with a
expected liquefaction and non-liquefaction behavior.
slope gradient of less than 0.1 percent, and there are no
′
To accomplish this, the cyclic stress ratio, τ a / σo ,
free faces within thousands of meters of the site. The
induced in the soil by the earthquake ground shaking is
potential for lateral spreading movements is therefore
calculated as a function of depth for depths below the
judged to be negligible.
ground water table. The simplified procedure
developed by Seed and Idriss (1971) is used to
b.
Liquefaction potential
A plot of the Standard Penetration Test (SPT)
blowcounts (N-values) in sands versus depth is shown
in Figure G-3. These blowcounts were obtained in
borings using recommended methods described in Seed
et al. (1985) and Youd and Idriss (1997) and no energy
correction to the values is required. For assessment of
G-6
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