Environmental Engineering Reference
In-Depth Information
Zone C
: Recent alluvium of loose sands with high water table experienced heavy
damage.
●
The primary difference between zones B and C appears to lie in soil density as revealed
by SPT
N
values. Heavy damage implies that buildings suffered large settlements or
tilted, such as illustrated in
Figure 11.33,
a building supported on spread footings.
Reinforced concrete buildings in zone C were supported either on shallow spread foot-
ings or on piles.
Damage vs. SPT values
: Japanese engineers found that buildings supported on shallow
foundations suffered heavy damage where
N
values were less than 15, and light to no
damage where
N
values were between 20 and 25. For buildings supported on pile foun-
dations (lengths ranged from 5 to 18 m), damage was heavy if the
N
value at the tip was
less than 15. A relationship among foundation depth, SPT value, and extent of damage,
developed from Seed's (1975) study, is given in
Table 11.11.
Seed concluded that two
important factors vary with depth: the SPT value as affected by soil confinement (see
Section 3.4.5),
and ground acceleration, which is usually considered to be larger at the
ground surface and to decrease with depth.
Predicting the Liquefaction Potential
General
Various investigators have studied locations where liquefaction did not occur in attempts
to obtain correlations for predicting liquefaction potential on the basis of material density,
initial effective overburden stress, and earthquake-induced cyclic horizontal shear stress
(Castro, 1975; Christian and Swiger, 1975; Seed et al., 1975; Seed, 1976).
Cyclic Stress Ratio
The cyclic stress ratio has been proposed as a basis for anticipating liquefaction potential
(Seed et al., 1975; Seed, 1976; Seed et al, 1985). It is defined as the ratio of the average
horizontal shear stress (
λ
h
) induced by an earthquake to the initial effective overburden
pressure (
'
o
).
For sites where liquefaction occurred, a lower bound was plotted in terms of the cyclic
stress ratio vs. corrected SPT values (N1)
60
, where (N1)
60
is the measured
N
value corrected
to an effective overburden pressure of approximately 100 kPa (1 tsf) and other factors
(Section 3.4.5).
The correlation is given in
Figure 11.35.
The cyclic stress ratio (CSR) at any depth in the ground causing liquefaction can be cal-
culated with reasonable accuracy from the relationship given by Seed et al. (1985):
σ
CSR
λ
hav
/
σ
'
o
0.65 (
α
max
/
g
)(
σ
vo
/
σ
'
vo
) r
d
(11.21)
TABLE 11.11
Relationship between Foundation Depth, SPT, and Damage at Niigata
a
Foundation Depth Range (m)
N
Value at Foundation Base
Damage Relationship
0-5
14
Apparently adequate to prevent damage by
settlement or overturning
5-8
14-28
Required to prevent heavy damage
8-16
28
Required to prevent heavy damage
a
After Seed, H. B.,
Foundation Engineering Handbook,
Winterkorn and Fang., Eds., Van Nostrand Reinhold, New
York, 1975.
