Civil Engineering Reference
In-Depth Information
Table 4.3
Sensitivity classification.
S
t
Classification
1
insensitive
1-2
low
2-4
medium
4-8
sensitive
8-16
extra sensitive
>
16
quick (can be up to 150)
Undisturbed undrained strength
Remoulded und
,
,
Sensitivity
= =
S
rained strength
Normally consolidated clays tend to have sensitivity values varying from 5 to 10 but certain clays in
Canada and Scandinavia have sensitivities as high as 100 and are referred to as quick clays. Sensitivity can
vary, slightly, depending upon the water content of the clay. Generally, overconsolidated clays have neg-
ligible sensitivity, but some quick clays have been found to be overconsolidated. A classification of sen-
sitivity appears in Table
4.3.
Thixotropy
Some clays, if kept at a constant water content, regain a portion of their original strength after remoulding
with time (Skempton and Northey,
1952
). This property is known as thixotropy.
Liquidity index I
L
The definition of this index has already been given in Chapter 1:
w w
I
−
p
I
=
L
p
where w is the
in situ
water content.
This index probably more usefully reflects the properties of plastic soil than the generally used consist-
ency limits w
P
and w
L
. Liquid and plastic limit tests are carried out on remoulded soil in the laboratory,
but the same soil, in its
in situ
state (i.e. undisturbed), may exhibit a different consistency at the same
water content as the laboratory specimen, due to sensitivity effects. It does not necessarily mean, there-
fore, that a soil found to have a liquid limit of 50% will be in the liquid state if its
in situ
water content w
is also 50%.
If w is greater than the test value of w
L
then I
L
is
>
1.0 and it is obvious that if the soil were remoulded
it would be transformed into a slurry. In such a case the soil is probably an unconsolidated sediment with
an undrained shear strength, c
u
, in the order of 15-50 kPa.
Most cohesive soil deposits have I
L
values within the range 1.0-0.0. The lower the value of w, the greater
the amount of compression that must have taken place and the nearer I
L
will be to zero.
If w is less than the test value of the plastic limit then If
L
<
0.0 and the soil cannot be remoulded (as it
is outside the plastic range). In this case the soil is most likely a compressed sediment. Soil in this state
will have a c
u
value varying from 50 to 250 kPa.
4.16 Residual strength of soil
In an investigation concerning the stability of a clay slope, the normal procedure is to take representative
samples, conduct shear tests, establish the strength parameters c
′
and
φ
′
from the peak values of the
