Geoscience Reference
In-Depth Information
coarse-graded
sand gravel stone
fine medium coarse fine medium coarse cobbles boulders
0.1 0.3 0.5 2 6 20 60 0.15 1.00
mm
m
C
TTERBERG LIMITS
Atterberg limits are useful for classifying fine cohesive soils. They are based on
soil consistency as determined by its actual water content
w
, and four stages are
distinguished: liquid state, plastic state, semi-solid state and solid state, their
boundaries being the Atterberg limits: liquid limit, plastic limit and shrinkage limit.
The liquid limit is defined as the water content
w
l
at which a V-shaped groove in
a soil-cake in a standardised cup will just close after 25 drops from 10 mm height.
The soil behaves as clotted cream. The plastic limit is the water content
w
p
at which
it is just possible to roll a sample into a continuous thread with a uniform diameter
of 3 mm. The soil starts to break and crumble. An alternative method is the fall-
cone test, for the determination of the liquid and the plastic limit, while a
predetermined penetration is sought for a standardised cone of given weight and
apex angle. The shrinkage limit is the water content
w
s
during drying to the air at
which air begins to enter the pores and density does not increase further.
From these measures the following classification numbers are determined: the
plasticity index
I
p
=w
l
-
w
p
and the liquidity index
I
l
=
(
w
-
w
p
)
/I
p
. The particle's
surface, which attracts the water, is a dominating factor for these limits. Soils with
similar
I
p
and
I
l
show similar mechanical behaviour.
The plasticity chart, devised by Casagrande, plots
I
p
as a function of
w
l
. The A-
line separates clays (above) from silts (below), and the value of
w
l
itself determines
the degree of plasticity of the soil. Many clays plot on a line more or less parallel to
the A-line, approximately 5 - 15% above it. Organic soils and peat tend to have
extremely high plasticity and plot to the right of the A-line.
plasticity: low intermediate high very high extremely high
plasticity: low intermediate high very high extremely high
60
60
CLAY
CLAY
40
40
I
p
[%]
I
p
[%]
A - line I
p
= 0.73(w
l
20)
A - line I
p
= 0.73(w
l
20)
20
20
SILT
SILT
compressibility : low medium high very high
compressibility : low medium high very high
0
0
0
0
20
20
40
40
60
60
80
80
100
100
120
120
w
l
w
l
[%]
[%]
Figure 2.2 Plasticity chart after Casagrande
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