Geoscience Reference
In-Depth Information
correlation to an S
r
/
σ
vo
-q
c1Ncs
-
Sr
correlation; (2) convert the SPT data in Table 1.1,
measured or estimated, to corresponding values of CPT penetration resistance, as neces-
sary; and (3) derive an adjustment for fines content for CPT penetration resistances that
is consistent with the adjustment recommended by Seed (1987) for SPT blow counts.
The resulting correlation and case history data points are then compared to ensure that
consistency was maintained.
4.1. CONVERTING THE SPT CORRELATION
Correlations between D
R
,
N
1
)
60
and q
c1N
in clean sands, as summarized below, were
used to convert the S
r
/
σ
vo
-
(
N
1
)
60cs
-
Sr
correlation in Figure 1.5 to a S
r
/
σ
vo
-q
c1Ncs
-
Sr
correlation. The D
R
of clean sand has been expressed in terms of SPT
(
(
N
1
)
60
using the
following form:
(
N
1
)
60
C
d
D
R
=
(1.4)
where D
R
is expressed as a ratio, rather than a percentage. Meyerhof's (1957) original
observations indicated a C
d
value of about 41. Skempton (1986) suggested, based on a
review of field and laboratory data, that the best average C
d
values for normally con-
solidated natural sand deposits were about 55 for fine sands and 65 for coarse sands.
Skempton further noted that C
d
varied significantly with the age of a deposit, such that
thetypicalC
d
valueforfinesandscouldrangefrom35inlaboratorytests,to40inrecent
fills,to 55 in natural deposits.
Cubrinovski and Ishihara (1999) showed that C
d
is affected by the grain size character-
istics and the type of soil under consideration. Cubrinovski and Ishihara included data
forhighqualityundisturbedsamples(obtainedbyin-situfreezing)forcleansandandfor
siltysand.TheD
R
,
valuesforthe
high quality undisturbed samples tabulated by Cubrinovski and Ishihara are summarized
in Figure 1.9. Note that the N
1
values reported by Cubrinovski and Ishihara were most
likely obtained with a delivered energy of about 78%, and were therefore multiplied by
the ratio 78/60 to obtain equivalent values of
(
N
1
)
60
,finescontent(FC),andmediangrainsize
(
D
50
)
N
1
)
60
. The average values of C
d
for the
soils included in Figure 1.9 are about 51 for the clean sand samples, about 26 for the
silty sand samples, and about 39 for all samples. It is interesting to note that the value of
C
d
=
(
39 for all samples is very close to the value of 41 originally obtained by Meyerhof
(1957).
CPT tip resistances have also been correlated to D
R
in various forms. Based on the work
of Boulanger (2003) and the solutions by Salgado et al. (1997a, b) for clean sands, the
following approximate relationships can be used to relate D
R
toq
c1N
:
3D
R
K
o
m
−
0
.
077
C
1
25
q
c1N
=
.
7
+
39
.
7D
R
+
212
.
(1.5)
0
.
45
