liquefaction potential, the N-values are converted or

normalized to (*N*1)60 values. This involves adjusting the

values to a common effective overburden pressure of 1

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 (*N*1)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

(*N*1)60 values:

Holocene geologic age overlies dense (non liquefiable)

sands and stiff clays. The water table is at a depth of 20

( *N*1 ) 60*cs *= *α *+ *β *( *N*1 )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 (*N*1)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" (*N*1)60

Settlements are estimated using the Tokimatsu and Seed

values for the site, i.e. the (*N*1)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 / *σ*o ,

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

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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