Environmental Engineering Reference
In-Depth Information
Figure 6.20.
Coefficient m relating Ko and OCR vs. plasticity index (Ladd et al., 1977).
The SBPM method is preferred as it gives a direct measure of Ko, but is relatively
expensive and in many cases use of published relationships is sufficient. In any case SBPM
results should be checked against the other methods such as:
(a) For normally consolidated clays
Jaky (1944) derived the theoretical relationship
2
3
sin
1 in
1 in
(6.10)
Ko
1
NC
where
effective friction angle.
An approximation which is more commonly used is:
(6.11)
Ko
1
sin
NC
Ladd et al. (1977) showed that this latter relationship was accurate within
0.05
(b) For overconsolidated clays
Measure the preconsolidation pressure (
vp
) in oedometer tests and calculate the OCR
vp
/
vo
). Then calculate Ko
oc
from equation 6.12 (Ladd et al. (1977))
(
m
Ko
/Ko
OCR
(6.12)
OC
NC
where Ko
NC
is estimated from the Jaky formula, m is determined from the Figure 6.20.
Wroth (1984) shows that Ko varies in loading and unloading emphasising that the
Ko
OC
/Ko
NC
relationship is only applicable for reloading. On this basis the graphs would
not be applicable to soils subject to over-consolidation by desiccation. For these soils, Ko
should be determined by SBPM tests for important projects.
The undrained shear strength is dependent on the rate of shearing.
Figure 6.21
shows
the general effect, with the strength measured increasing with the strain rate.
Germaine and Ladd (1988) indicate that most laboratories (in their experience) use a
strain rate of 0.5% to 1.0% of the sample height per hour, for soft cohesive soils and that
this is reasonable based on case histories of undrained failures.
Note this is a lot slower than typically used in many laboratories. A strain rate of 1%
per minute, often used for undrained testing, will over-estimate the undrained strength by
about 20
10% for low OCR clays, and up to 50% for high OCR clays. In view of this,
it is recommended that a strain rate of 1% per hour be specified.
