Civil Engineering Reference
In-Depth Information
The usual approach for settlement analyses is to first estimate the amount of earthquake-
induced total settlement
max
of the structure. Because of variable soil conditions and struc-
tural loads, the earthquake-induced settlement is rarely uniform. A common assumption is
that the maximum differential settlement
of the foundation will be equal to 50 to 75 per-
cent of
max
(i.e., 0.5
max
0.75
max
). If the anticipated total settlement
max
and/or
the maximum differential settlement
is deemed to be unacceptable, then soil improve-
ment or the construction of a deep foundation may be needed. Chapters 12 and 13 deal with
mitigation measures such as soil improvement or the construction of deep foundations.
7.2 SETTLEMENT VERSUS FACTOR OF SAFETY
AGAINST LIQUEFACTION
7.2.1 Introduction
This section discusses two methods that can be used to estimate the ground surface settle-
ment for various values of the factor of safety against liquefaction. A liquefaction analysis
(Chap. 6) is first performed to determine the factor of safety against liquefaction. If FS is
less than or equal to 1.0, then liquefaction will occur, and the settlement occurs as water
flows from the soil in response to the earthquake-induced excess pore water pressures.
Even for FS greater than 1.0, there could still be the generation of excess pore water pres-
sures and hence settlement of the soil. However, the amount of settlement will be much
greater for the liquefaction condition compared to the nonliquefied state.
This section is solely devoted to an estimation of ground surface settlement for various
values of the factor of safety. Other types of liquefaction-induced movement, such as bear-
ing capacity failures, flow slides, and lateral spreading, are discussed in Chaps. 8 and 9.
7.2.2
Methods of Analysis
Method by Ishihara and Yoshimine (1992).
Figure 7.1 shows a chart developed by
Ishihara and Yoshimine (1992) that can be used to estimate the ground surface settlement
of saturated clean sands for a given factor of safety against liquefaction. The procedure for
using Fig. 7.1 is as follows:
1.
Calculate the factor of safety against liquefaction
FS
L
:
The first step is to calculate
the factor of safety against liquefaction, using the procedure outlined in Chap. 6 [i.e., Eq.
(6.8)].
2.
Soil properties:
The second step is to determine one of the following properties:
relative density
D
r
of the in situ soil, maximum shear strain to be induced by the design
earthquake
max
, corrected cone penetration resistance
q
c
1
kg/cm
2
, or Japanese standard
penetration test
N
1
value.
Kramer (1996) indicates that the Japanese standard penetration test typically transmits
about 20 percent more energy to the SPT sampler, and the equation
N
1
0.83(
N
1
)
60
can be
used to convert the (
N
1
)
60
value to the Japanese
N
1
value. However, R. B. Seed (1991) states
that Japanese SPT results require corrections for blow frequency effects and hammer
release, and that these corrections are equivalent to an overall effective energy ratio
E
m
of
0.55 (versus
E
m
0.60 for U.S. safety hammer). Thus R. B. Seed (1991) states that the
(
N
1
)
60
values should be increased by about 10 percent (i.e., 0.6/0.55) when using Fig. 7.1
to estimate volumetric compression, or
N
1
1.10(
N
1
)
60
. As a practical matter, it can be
