Civil Engineering Reference
In-Depth Information
FIGURE 6.5
Reduction factor
r
d
versus depth below level or gently sloping ground surfaces. (
From Andrus
and Stokoe 2000, reproduced with permission from the American Society of Civil Engineers.
)
6.4.3
Cyclic Resistance Ratio from the Standard Penetration Test
The second step in the simplified procedure is to determine the cyclic resistance ratio of the
in situ soil. The cyclic resistance ratio represents the liquefaction resistance of the in situ
soil. The most commonly used method for determining the liquefaction resistance is to use
the data obtained from the standard penetration test. The standard penetration test is dis-
cussed in Sec. 5.4.3. The advantages of using the standard penetration test to evaluate the
liquefaction potential are as follows:
1.
Groundwater table:
A boring must be excavated in order to perform the standard
penetration test. The location of the groundwater table can be measured in the borehole.
Another option is to install a piezometer in the borehole, which can then be used to moni-
tor the groundwater level over time.
2.
Soil type:
In clean sand, the SPT sampler may not be able to retain a soil sample.
But for most other types of soil, the SPT sampler will be able to retrieve a soil sample. The
soil sample retrieved in the SPT sampler can be used to visually classify the soil and to esti-
mate the percent fines in the soil. In addition, the soil specimen can be returned to the lab-
oratory, and classification tests can be performed to further assess the liquefaction
susceptibility of the soil (see item 3, Sec. 6.3).
3.
Relationship between N value and liquefaction potential:
In general, the factors
that increase the liquefaction resistance of a soil will also increase the (
N
1
)
60
from the stan-
dard penetration test [see Sec. 5.4.3 for the procedure to calculate (
N
1
)
60
]. For example, a
