Civil Engineering Reference
In-Depth Information
TABLE 6.2
Magnitude Scaling Factors
Anticipated earthquake magnitude
Magnitude scaling factor (MSF)
8
1
⁄
2
0.89
7
1
⁄
2
1.00
6
3
⁄
4
1.13
6
1.32
5
1
⁄
4
1.50
Note:
To determine the cyclic resistance ratio of the in situ soil, multiply the
magnitude scaling factor indicated above by the cyclic resistance ratio determined
from Fig. 6.6.
Source:
Seed et al. (1985).
values, the moment magnitude
M
w
tends to significantly deviate from the other magnitude
scales, and the moment magnitude
M
w
should be used to determine the magnitude scaling
factor from Table 6.2 or Fig. 6.7.
Two additional correction factors may need to be included in the analysis. The first cor-
rection factor is for the liquefaction of deep soil layers (i.e., depths where
v
0
100 kPa,
in which liquefaction has not been verified by the Seed and Idriss simplified procedure, see
Youd and Idriss 2001). The second correction factor is for sloping ground conditions,
which is discussed in Sec. 9.4.2.
As indicated in Secs. 4.6.1 and 5.6.4, both the peak ground acceleration
a
max
and the
length of ground shaking increase for sites having soft, thick, and submerged soils. In a
sense, the earthquake magnitude accounts for the increased shaking at a site; that is, the
higher the magnitude, the longer the ground is subjected to shaking. Thus for sites having
soft, thick, and submerged soils, it may be prudent to increase both the peak ground accel-
eration
a
max
and the earthquake magnitude to account for local site effects.
6.4.4
Factor of Safety against Liquefaction
The final step in the liquefaction analysis is to calculate the factor of safety against lique-
faction. If the cyclic stress ratio caused by the anticipated earthquake [Eq. (6.6)] is greater
than the cyclic resistance ratio of the in situ soil (Fig. 6.6), then liquefaction could occur
during the earthquake, and vice versa. The factor of safety against liquefaction (FS) is
defined as follows:
FS
CRR
_____
CSR
(6.8)
The higher the factor of safety, the more resistant the soil is to liquefaction. However,
soil that has a factor of safety slightly greater than 1.0 may still liquefy during an earth-
quake. For example, if a lower layer liquefies, then the upward flow of water could induce
liquefaction of the layer that has a factor of safety slightly greater than 1.0.
In the above liquefaction analysis, there are many different equations and corrections
that are applied to both the cyclic stress ratio induced by the anticipated earthquake and the
cyclic resistance ratio of the in situ soil. For example, there are four different corrections
(i.e.,
E
m
, C
b
, C
r
,
and
v
0
) that are applied to the standard penetration test
N
value in order
to calculate the (
N
1
)
60
value. All these different equations and various corrections may pro-
vide the engineer with a sense of high accuracy, when in fact the entire analysis is only a
