Environmental Engineering Reference
In-Depth Information
in
Figure 12.19
represents a probability of liquefaction of between 20% and 50% for SPT
(N
1
)
60
values between 5 and 25. That is, to have a CRR greater than CSR is not a guar-
antee of no liquefaction.
It is emphasised that the methods need to be used with reasonable engineering judge-
ment and should not be regarded as giving precise outcomes.
12.4.3.3
Evaluation of Cyclic Stress Ratio (CSR)
The Cyclic Stress Ratio, CSR, is calculated from:
CSR
(
/
)
0.65(a
/g)(
/
)r
′
′
(12.10)
av
vo
max
vo
vo
d
where a
max
peak horizontal acceleration at the ground surface generated by the earth-
vo
and
vo
are total and effective vertical overburden
quake; g
acceleration of gravity;
stresses, respectively and r
d
stress reduction coefficient, which can be calculated from:
r
.0
0.00765z
for z
9.15 m
(12.11a)
d
r
1.174 .0267z
for 9.15m
z
23m
(12.11b)
d
where z
depth below ground surface in metres.
It should be recognized that there is some variability in r
d
, as shown in Figure 12.18.
This uncertainty is greater at depth. The peak horizontal acceleration at the ground sur-
face should be calculated allowing for amplification or de-amplification of the peak hori-
zontal bedrock acceleration estimated in the absence of increased pore pressure or the
onset of liquefaction. The peak horizontal bedrock acceleration is the geometric mean of
the two horizontal components (N-S and E-W) where they are available.
Stress Reduction Coefficient,
r
d
0.0
0
0.2
0.4
0.6
0.8
1.0
Average values
by Seed &
ldriss (1971)
5
Approximate average
values
10
Range for different
soil profiles by
Seed & ldriss (1971)
15
Simplified procedure
not verified with
case history data
in this region
20
Figure 12.18.
Stress reduction coefficient factor r
d
versus depth curves developed by Seed and Idriss
(1971) with added mean-value lines plotted from Eq. 12.11 (Youd et al., 2001, repro-
duced with permission of ASCE).
