ATTERBERG’S LIMIT/CONSISTENCY LIMIT IN SOIL
DEFINITION
•
The water content
at which the soil changes from one state to the other are known as consistency
limits or Atterberg’s limits.
•
These limits were
created by Albert Atterberg, a Swedish agriculturalist
Consistency
•
Consistency is a
term used to denote the degree of firmness of soil
•
Consistency of
soil is indicated by terms such as soft, stiff, very stiff and hard
•
Consistency is
mostly used for fined grained soil (clay)
Plasticity
of soil
•
Ability to undergo
deformation without cracking or fracturing
•
It is due to
presence of clay minerals
•
It is due to
adsorbed water on clay surface
•
The liquid should
be polarizing like water
Liquid state
•
A soil containing
high water content is in liquid state
•
It offers no
shearing resistance
•
Can flow like
liquid
•
No resistance to
shear deformation
•
Shear strength is
zero
Liquid limit
•
The water content
at which soil changes from liquid state to plastic state is known as liquid limit
•
It is denoted by
LL or Wl
•
Liquid limit is
the water content at which soil ceases to be liquid
•
It is determined
by
•
Casagrande's
apparatus or
•
cone penetration
method
Plastic
state
•
At some water
content soil becomes plastic from liquid state when water content is reduced
•
Soil in plastic
state can be moulded into various shape
•
As water content
is reduced, the plasticity of soil decreases
Plastic
limit
•
Water content at
which soil changes from plastic to semi-solid state
•
Water content at
which soil just fails to behave plastically
•
It is denoted by
PL or Wp
Plasticity
index
•
It is the
difference between liquid limit and plastic limit
•
It is denoted by
PI or Ip
•
PI = LL – PL
Where,
LL = Liquid limit
PL
= Plastic limit
Semi-solid
state
•
When water
content is reduced below plastic limit, soil attains a semi-solid state
•
The soil in this
state cracks when moulded
•
The volume of
soil decreases with the decrease in water content
Shrinkage
limit
•
The water content
at which soil changes from semi-solid state to solid state is called shrinkage
limit
•
It is denoted by
SL or Ws
•
SL = e/G; e is
void ratio and G is specific gravity of soil
•
SL is the water
content at which soil stops shrinking further and attains a constant volume
•
SL is the lowest
water content at which soil is fully saturated
Shrinkage
index
•
It is difference
between plastic limit and shrinkage limit
•
It is denoted by
Is
•
Is = PL - SL
Below
shrinkage limit(SL)
•
Soil is not
saturated
•
Air enters the
voids of soil
•
Due to capillary
tension developed, the volume of soil doesn’t change
Solid state
•
A stage is
reached when further reduction in water content doesn’t causes change in volume
of soil
•
The soil is then
said to have reached solid state from semi-solid state
•
In this state
there is no appreciable change in volume with the change in water content
Analogy
•
In liquid state,
soil is like Soup
•
In plastic state,
soil is like soft butter
•
In semi-solid
state, soil is like cheese
•
In solid state,
soil is like hard candy
Soil
according to PI = LL-PL
|
PI
|
0
|
<7
|
7-17
|
>17
|
|
Soil
|
Non-Plastic
|
Low plastic
|
Medium plastic
|
Highly plastic
|
Soil
according to liquidity index
•
Liquidity index
is denoted by Il
•
Il = (Wn-PL)/PI
•
Where, Wn =
natural water content
•
PL = plastic
limit
•
PI = Plasticity
Index
|
Il
|
-1
|
0
|
<1
|
1
|
>1
|
|
Soil
|
Solid
|
Very stiff
|
Soft
|
Very soft
|
liquid
|
Soil
according to consistency index
•
Consistency index
is denoted by Ic
•
Ic = (LL – Wn)/PI
•
Where, Wn =
natural water content
•
LL = Liquid Limit
•
PI = Plasticity
index
|
Ic
|
-1
|
0
|
<1
|
1
|
>1
|
|
Soil
|
Liquid
|
Liquid limit
|
Between LL and PL
|
Plastic limit
|
Semi-solid
|
Shrinkage
ratio
•
It is denoted by
SR
•
SR =[ ( (V1 –
V2)/Vd)/(W1 – W2) ] x 100
•
Where, V1 =
volume of soil mass at water content W1
•
V2 = volume of soil mass at water content W2
•
Vd = Volume of
dry soil mass
•
SR = dry density
of soil/density of water
•
Thus SR is equal
to mass gravity of soil mass in dry state
FLOW INDEX
•
Flow curve is the
graph between water content and logarithm of number of blows
•
The relation
between water content and logarithmic of number of blows is approximated as
straight line
•
The slope of flow
curve is known as Flow Index and is denoted by If
•
Flow Index (IF) =
(W1-W2)/(log(N2/N1)
•
Toughness Index =
Plasticity index/Flow index
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