These Slides describe about consistency of soil. Consistency is a very important index property of fine grained soil. Consistency actually is a term used to describe the degree of firmness of a fine grained soil. It helps in Classification and identification of fine grained soil.

1. GEOTECHNICAL
ENGINEERING
CHAPTER-II
CONSISTANCY OF SOIL

2. Plasticity
Plasticity :
Causes of Plasticity of Soils :

3. Plasticity contd....
• Behaviour of soil with Non polarising Liquid : The
presence of adsorbed water is necessary to impart
plasticitry characteristics to a soil.The soil does not
become plastic when it is mixed witha non polarising
liquid like kerosene or parafine oil etc. These liquids do
not have electromagnetic properties to react with clay
minerals.
• Behaviour of Non clay mineral soil particles :The soil
become plastic only when it has clay minerals. If the soil
contains only non clay minerals such as quartz, it would
not become plastic whatever may be the fineness of the
soil particles. When such soils are ground to very fine
size these can not be rolled into threads when in contact
with water.

4. Consistency
• Consistency is a term which is used to
describe the degree of firmness of a soil in
a qualitative manner by using descriptions
such as soft, firm, stiff or hard. It
represents the relative ease with which the
soil may be deformed.
• The consistency of a fine-grained soil
refers to its firmness, and it varies with the
water content of the soil.
• It is the property of a material which is
manifested by its resistance to flow

5. Consistency Limits
• Atterberg, a Swedish Soil Scientist, in
1911, formally distinguished the following
stages of consistency–liquid, plastic, semi-
solid, and solid.
• The boundary water contents at which the
soil passes from one of these states to the
next have been arbitrarity designated as
‘consistency limits.’ These are called
‘Atterberg limits’ in honour of the originator
of the concept.
• These limits are expressed as percentage
water content.

6. Atterberg Limits
• These limits were created by Albert Atterberg, a
Swedish agriculturist. They were later refined by
Arthur Casagrande.
• The three limits are known as the shrinkage limit
(WS), plastic limit (WP), and liquid limit (WL) as
shown. The values of these limits can be
obtained from laboratory tests.
• Two of these are utilized in the classification of
fine soils:
• Liquid limit (WL) - change of consistency from
plastic to liquid state
• Plastic limit (WP) - change of consistency from
brittle/crumbly to plastic state

7. Moisture Content Volume Relationship
 As a dry, clayey soil takes on increasing amounts
of water, it undergoes distinct changes in behavior
and consistency.
 Depending on the water content of the soil, it may
appear in four states: solid, semi-solid, plastic and
liquid.
 In each state, the consistency and behavior of a
soil are different and consequently so are its
engineering properties.
 Thus, the boundary between each state can be
defined based on a change in the soil's behavior.

8. Graphical Representation of Moisture content
Volume Relationship
The figure shows the moisture
content and volume relationship.
Upto semisolid state the soil
remains fully saturated and any
reduction in the volume of water
will result in an almost equal
reduction in volume of soil mass. A
further reduction in the water
content will bring a state when with
a decrease in moisture content the
soil mass does not change any
further in volume and the sample
changes from semi solid to solid
state.

9. Liquid limit
• The liquid limit (LL) is conceptually
defined as the boundary water
content at which the behavior of a
clayey soil changes from plastic to
liquid.
• The precise definition of the liquid
limit is based on standard test
procedures described below. The
liquid limit is defined as the water
content at which a groove cut in a
part of a wet soil by a grooving tool
of standard dimentions will flow
together for a distance of 12 mm
under the impact of 25 blows in a
standard liquid limit test device.

10. Plastic limit
• Plastic limit is the water
content corresponding to a
arbitary limit between the
plastic and semi solid state of
consistency of soil.
• The plastic limit is defined as
the moisture content at which
the thread just begins to
crumble at an approximate
diameter of 3.0 mm (about 1/8
inch). A soil is considered non-
plastic if a thread cannot be
rolled out down to 3.2 mm at
any moisture possible.

11. Shrinkage limit
• The shrinkage limit (SL) is the boundary
water content where further loss of
moisture will not result in any more volume
reduction. . The shrinkage limit is much
less commonly used than the liquid and
plastic limits.

12. Derived Limits
• The values of consistency limits are used in a
number of ways.
• There is a close relationship between the limits
and properties of a soil such as compressibility,
permeability, and strength.
• This is thought to be very useful because as limit
determination is relatively simple, it is more difficult
to determine these other properties.
• Thus the Atterberg limits are not only used to
identify the soil's classification, but it allows for the
use of empirical correlations for some other
engineering properties.

13. Derived Limits
• Plasticity index
• Liquidity index
• Consistency index
• Shrinkage Index
• Flow index (FI)
• Toughness Index

14. Plasticity index
• The plasticity index (PI) is a measure of the plasticity of a
soil.
• The plasticity index is the size of the range of water
contents where the soil exhibits plastic properties.
• The PI is the numerical difference between the liquid
limit and the plastic limit (PI = LL-PL). Soils with a high PI
tend to be clay, those with a lower PI tend to be silt, and
those with a PI of 0 (non-plastic) tend to have little or no
silt or clay.
Soil descriptions based on PI:
• (0)- Nonplastic
• (<7) - Slightly plastic
• (7-17) - Medium plastic
• >17 - Highly plastic

15. Liquidity index
• The liquidity index (LI) is used for scaling
the natural water content of a soil sample
to the limits.
• It can be calculated as a ratio of difference
between natural water content, plastic limit,
and liquid limit: LI=(W-PL)/(LL-PL) where
W is the natural water content.The value
of LI varies from 0 to 1.

16. Consistency index
• The consistency index (CI) indicates the
consistency (firmness) of a soil.
• It is calculated as CI = (LL-W)/(LL-PL),
where W is the existing water content.
• Soil at the liquid limit will have a
consistency index of 0, while soil at the
plastic limit will have a consistency index
of 1.

17. Shrinkage Index,Flow index & Toughness
Index
• Shrinkage Index : Shrinkage Index(SI) is defined as the
difference between the plastic and shrinkage limits of soil.
In other words it is the range of water content within which
a soil is in a semi solid state of consistency. SI = PL-SL
• Flow index (FI) : It is the slope of the water content versus
log10 of the blows number plot for the Casagrande cup
method. It is the measure of the rate at which soil mass
loses its shear strength with increased water content.
• Toughness Index: Toughness index is defined as the ratio
of plasticity index (PI) of the soil to the flow index (FI) of
the soil. Toughness index varies between 0 to 3. This gives
us an idea of shear strength of soil at its plastic limit.