Environmental Engineering Reference
In-Depth Information
(ER); C
B
correction factor for borehole diameter; C
R
correction factor for rod
length; C
S
correction for samplers with or without liners.
C
N
should be calculated from:
C2.2/(1.2
′
/P )
(12.13)
N
vo
a
where
atmos-
pheric pressure (100 kPa). Youd et al. (2001) note that there is some uncertainty in C
N
,
and that equation 12.13 only applies for
vo
effective vertical stress at the time of doing the SPT test (kPa); P
a
300 kPa.
The hammer energy ratio C
E
varies between hammer types and for individual hammers
and drill rig set ups. It varies between 0.5 and 1.0 for rope and pulley donut hammer, 0.7
and 1.2 for rope and pulley US Safety hammers and 0.8 and 1.3 for USA Safety hammers
with automatic trip/free fall (Youd et al., 2001). Tests on Australian free fall SPT hammers
at Hume Dam surprisingly gave low C
E
of about 0.6. It is clear that C
E
should be meas-
ured for each project. If it is not, conservative assumptions will have to be made which
may prove costly in overly conservative assessments of liquefaction potential.
Correction for borehole diameter (C
B
) is seldom needed. C
B
vo
1.0 for holes 65-125 mm
diameter. C
B
1.15 for holes of 200 mm diam-
eter. Correction for rod length, C
R
is necessary for rod length
1.05 for holes 150 mm diameter and C
B
3 m, where C
R
0.75. No
other corrections are needed as they were not applied to develop the database.
SPT tests should be done with the split inner tubes in place, in which case C
S
1.0. If
the inner tube was left out, C
S
1.1 to 1.3.
Correction for fines content, FC, (% passing 0.075 mm sieve) should be calculated from:
(N )
(N )
(12.14)
1 60CS
1 60
where
and
coefficients determined from the following relationships:
0
for FC
5%
(12.15a)
2
(12.15b)
exp[1.76
190/FC ]
for 5%
FC
35%
5.0
for FC
35%
(12.15c)
1.0
for FC
5%
(12.16a)
[0.99
(FC
1.5
/1,000)]
for 5%FC
35%
(12.16b)
1.2
for FC
35%
(12.16c)
It should be recognized that
Figure 12.19
was originally developed by Seed et al.
(1985b) from
Figure 12.20
,
which shows three curves showing different cyclic strains
l
.
The 3% line is equivalent to the SPT clean sand base curve in Figure 12.19.
Sites which are just to the left of the 3% strain curve are likely to experience cyclic
mobility or cyclic liquefaction; those to the left of the 20% are likely to experience flow liq-
uefaction conditions.
Baziar and Dobry (1995), Ishihara (1993) and Cubrinovski and Ishihara (2000a, b)
have further investigated the boundary of flow liquefaction conditions.
Figure 12.21
sum-
marizes the outcomes. Readers should read the original papers before applying this figure
to serious decision making.
