Environmental Engineering Reference
In-Depth Information
Table 11.3.
Coefficient of variation (COV) for available data on soil properties (Kulhawy, 1992 from
Kulhawy et al., 1991).
Property
No. of Studies
Mean COV without outliers (%)
Index
Natural water content, w
n
18
17.7
Liquid limit, w
L
28
11.3
Plastic limit, w
P
27
11.3
Initial void ratio, e
i
14
19.8
Unit weight,
12
7.1
Performance
Effective stress friction angle,
20
12.6
Tangent of
7
11.3
Undrained shear strength, S
u
38
33.8
Compression index, C
c
8
37.0
Table 11.4.
Estimates of in-situ test variability (Kulhawy, 1992; from Orchant et al., 1988).
COV
b
COV
c
(%)
COV
d
(%)
COV (%)
COV (%)
Test
a
Equipment
Procedure
Random
Total
Range
5
e
to 75
f
5
e
to 75
f
14
e
to 100
f
SPT
12 to 15
15 to 45
10
g
to 15
h
10
g
to 15
h
15
g
to 22
h
MCPT
5
15 to 25
5
g
to 10
h
7
g
to 12
h
ECPT
3
5
5 to 15
VST
5
8
10
14
10 to 20
DMT
5
5
8
11
5 to 15
10 to 20
i
PMT
5
12
10
16
15 to 25
i
SBPMT
8
15
8
19
a - Test notation: SPT Standard Penetration Test; CPT Cone Penetration Test; VST Vane Shear Test;
DMT Dilatometer Test; PMT Pressuremeter Test; SBPMT Self Boring Pressuremeter Test
(M
mechanical, E
Electrical).
b - COV
standard deviation/mean.
c - COV(Total)
1
⁄
2
d - Because of limited data and judgment involved in estimating COV, ranges represent probable
magnitudes of test measurement error.
e - Best case scenario for SPT test conditions.
f - Worst case scenario for SPT test conditions.
g - Tip resistance, q
c
, CPT measurements.
h - Side resistance, fs, CPT measurements.
i - Results may differ for P
o
, P
f
and P
L
, but data are insufficient to clarify this issue.
[COV(Equipment)
2
COV(Procedure)
2
COV(Random)
2
]
standard deviation/
mean) in soil properties and the variability in in-situ tests, excluding outliers in the data.
It should be noted that there is an inherent greater uncertainty in the undrained shear
strength than in the effective shear strength. As discussed below, this should be reflected
in the factor of safety used for undrained (total stress) analyses. The test variability shown
in Table 11.4 shows a greater uncertainty in SPT tests, than in CPT tests.
However it must be remembered that there is considerable further model uncertainty in
the assessment of strengths (and other properties) from the SPT, CPT and Vane Shear - see
Sections 6.1.7 and 6.1.9. For example the shear strength of clays can be estimated from
the CPT using the equation:
Tables 11.3 and 11.4 present the coefficient of variation (COV
q
c
vo
S
(11.6)
u
N
k
where S
u
undrained shear strength; q
c
cone resistance;
vo
total overburden stress;
N
k
Cone Factor.
