Geology Reference
In-Depth Information
montmorillonite, they are very responsive to changes in water
content, prone to
flow and liquefy when wet and to shrink and
crack when dry.
Atterberg limits are measured routinely in laboratories to de
ne the
nature of soils and to interpret the likely clay mineralogy by index
testing. If water is added to a clay or clay-silt mix, one can get it to
flow like a liquid. The moisture content at which it changes from a
plastic state to a liquid state is known as the liquid limit (LL) and can
be determined using standard tests where a groove cut in the surface
closes as the sample is tapped in a standard way or a cone is dropped
onto the sample and penetration measured. If the sample is then
dried out, the sample gets stronger and the water content at which a
sausage of 3mm diameter can be rolled without breaking is called
the plastic limit (PL). The difference in moisture content between
the liquid state (LL) and plastic semi-solid condition (PL) is the
plasticity index (PI). The PI is found to be a good indicator of the
type of material, species of clay and, hence, engineering behaviour
( Figures 3.12a and b).
Clay particles are carried by rivers to estuaries where a change in
salinity causes clay platelets to
flocculate. Because of their shape and
Figure 3.12 (a)
Chart of plasticity
index vs. liquid
limit used for
identifying the
nature of
(a)
Low
Medium
High
40
Plasticity
ne-
grained soil using
simple, standard
tests (Atterberg
limits). (b) Use of
plasticity index to
indicate clay
mineral in soil
sample (after
Skempton, 1953).
20
0
20
35
50
Liquid limit (%)
100
(b)
80
60
40
20
0
20
40
60
80
100
Percentage of clay
 
 
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