Civil Engineering Reference
In-Depth Information
Figure 7.1
The Atterberg limits tests.
or spatula. The tests are repeated with slightly different water contents until the precise
requirements of the tests are met.
Plastic limit test
The test consists of rollinga3mmdiameter thread of soil while the water evaporates
and the water content decreases until the thread splits and crumbles. The failure of
the thread corresponds to a strength of approximately 170 kPa. Notice that a strength
of 170 kPa corresponds to the division between stiff and very stiff clay in Table 5.2.
Remember the plasticity index
I
P
given by
I
P
=
w
L
−
w
P
(7.3)
This is an important material parameter. Because the Atterberg limits determine the
conditions of soil at certain well-specified strengths, the results can be used to esti-
mate a number of other important soil properties, as discussed in Chapter 18. Further
discussion of the Atterberg limits is given in Sec. 5.6.
7.4 Measurement of coefficient of permeability
Seepage of water through soil, discussed in Chapter 14, is governed by Darcy's law:
V
=
ki
(7.4)
where
k
, the coefficient of permeability, is a soil parameter. The value of
k
depends
principally on the grain size and specific volume (or more properly on the void size,
which is related to the grain size and specific volume). Permeability can be measured
