Civil Engineering Reference
In-Depth Information
magnitude scaling factor
1.5, and thus the corrected CSR is equal to 0.34 divided by
1.5, or 0.23. Entering Fig. 7.2 with the modified CSR
7.7, the volu-
metric strain is still equal to 3.0 percent. Thus, provided the sand liquefies for both the
magnitude 5
1
⁄
4
and magnitude 7.5 earthquakes, the settlement of the liquefied soil is the
same.
●
Summary of values:
Based on the two methods, the ground surface settlement of the
1.0-m-thick liquefied sand layer is expected to be on the order of 3 to 4 cm.
0.23 and (
N
1
)
60
Silty Soils.
Figures 7.1 and 7.2 were developed for clean sand deposits (fines
5 per-
cent). For silty soils, R. B. Seed (1991) suggests that the most appropriate adjustment is to
increase the (
N
1
)
60
values by adding the values of
N
corr
indicated below:
Percent fines
N
corr
5
0
10
1
25
2
50
4
75
5
7.2.3 Limitations
The methods presented in Figs. 7.1 and 7.2 can only be used for the following cases:
●
Lightweight structures:
Settlement of lightweight structures, such as wood-frame
buildings bearing on shallow foundations
●
Low net bearing stress:
Settlement of any other type of structure that imparts a low net
bearing pressure onto the soil
●
Floating foundation:
Settlement of floating foundations, provided the zone of lique-
faction is below the bottom of the foundation and the floating foundation does not impart
a significant net stress upon the soil
●
Heavy structures with deep liquefaction:
Settlement of heavy structures, such as mas-
sive buildings founded on shallow foundations, provided the zone of liquefaction is deep
enough that the stress increase caused by the structural load is relatively low
●
Differential settlement:
Differential movement between a structure and adjacent appur-
tenances, where the structure contains a deep foundation that is supported by strata below
the zone of liquefaction
The methods presented in Figs. 7.1 and 7.2 cannot be used for the following cases:
●
Foundations bearing on liquefiable soil:
Do not use Figs. 7.1 and 7.2 when the foun-
dation is bearing on soil that will liquefy during the design earthquake. Even lightly
loaded foundations will sink into the liquefied soil.
●
Heavy buildings with underlying liquefiable soil:
Do not use Figs. 7.1 and 7.2 when the
liquefied soil is close to the bottom of the foundation and the foundation applies a large
net load onto the soil. In this case, once the soil has liquefied, the foundation load will
cause it to punch or sink into the liquefied soil. There could even be a bearing capacity
type of failure. Obviously these cases will lead to settlement well in excess of the values
obtained from Figs. 7.1 and 7.2. It is usually very difficult to determine the settlement for
these conditions, and the best engineering solution is to provide a sufficiently high static
