Consistency Limits
Definition Soil Consistency
• Soil consistency is defined as the physical state of a fine-grained soil at a
given water content
• Field Test for Stickiness of wet soil
Non-sticky
Slightly Sticky
Sticky
Very Sticky
Soil Consistency according to Atterberg
• Relates to consistency of finegrained soil only.
• Develop system of measurement
to evaluate water content and
changes in soil.
Soil Consistency according to Atterberg cont…
• SOIL CONSISTENCY is defined as
the physical state of a fine-grained
soil at a given water content.
• A paste of fine soils passes through
four states of consistency as the
water content decreases:
•
•
•
•
Solid state
Semi-solid state
Plastic state
Liquid state
Soil Consistency according to Atterberg cont…
Semi solid state
Plastic state
Increase in moisture content
Liquid state
Atterberg Limits
• The percentage water contents at
which the soil passes from one state
to the next are its
Consistency/Atterberg Limits
• There are 3 consistency/Atterberg
limits:
• Shrinkage Limit (SL)
• Plastic Limit (PL)
• Liquid Limit (LL)
Atterberg Limits cont…
PL
SL
LL
Semi solid state
Eg. w=10%
Plastic state
Eg. w=20%
Liquid state
Eg. w=40%
Increase in moisture content
Difference between SL, PL and LL
SL
water content at which
further loss of water in soil
will not decrease the
volume of the soil any
more
PL
LL
lowest water content at
which soil remains in a
plastic state
lowest water content at
which soil behaves as a
liquid
• water content below it
changes from a plastic
material to a crumbly
semi-solid.
• can be deformed by an
external stress, remain
deformed when stress is
removed
water content above it
behaves like a viscous
mud and flows under its
own weight
Laboratory Test
Based on New Zealand Standard of Soil Testing NZS4402:1986
Linear Shrinkage Limit
Determination
NZS4402:1986 Test 2.6
Test demo min 0:00 – 3:13
https://www.youtube.com/watch?v=vrFKij8rHNk
Apparatus
Linear shrinkage mould
Procedure
• Use approx. 150g soil passing 425
micron sieve. Mix with water until it is
slightly above LL.
• Place soil in pre-greased mould of
known internal length. Remove air
bubbles by lightly tapping the base
of the mould.
• Slightly overfill the mould then level
off the excess material with spatula.
Procedure cont…
• Allow specimen to dry at
room temp for 24 hour until
a distinct change of colour
can be noticed.
• Transfer into oven and dry
between 105 and 110 deg.
Procedure cont…
• Allow soil to cool. Then measure
longitudinal shrinkage to the
nearest mm.
• If specimen cracks into pieces, hold
separate parts together and
measure the shrinkage.
• Calculate %LS = [1 – (Longitudinal
shrinkage/Length of mold)] x 100%.
Example Q2c(iii) S1 2021
A saturated sample of Soil B was placed in a standard 150 mm length linear
shrinkage mould. When oven dried at 105°C for 24 hours, the sample shrinks to
137 mm length. Calculate the linear shrinkage for Soil B.
Ans: 9%
Liquid Limit
Determination using
Casagrande Method
NZS4402:1986 Test 2.2
Test demo min 3:32 – 6:37
https://www.youtube.com/watch?v=EcXJ961qjGA
Casagrande Apparatus
Grooving tool
13mm
Soil to this level
Cam
Stiff rubber base
Procedure
• Approximately 250g of wet soil passing
425 micron sieve is mixed with water &
left overnight
• Next day approx. 30g of this soil is
placed in a standard metal dish and
divided into two halves by means of a
standard grooving tool
• there are at least 2 types of tool, one
curved rod-like, and the other flat, but
they both produce a standard 2mm
wide V-shaped groove
Procedure cont…
• The cup is placed into the
apparatus
• a rotating cam causes
the cup to rise and drop
10 mm onto a heavy
rubber base
Rotate counter clockwise at a rate
of 2 drops/sec
Procedure cont…
• The cup is raised and dropped twice a
second.
• number of taps needed to close the
groove over about 13 mm is recorded.
13mm
• the water content determined.
• The procedure is repeated with the
same soil at increasing water contents.
• A graph of water content against the
log of the number of taps is plotted.
Count no of drops requiring groove
to close at 13mm
Possible errors during Casagrande test
• Samples not “cured” or leave to rest overnight
• Incorrect sample size placed into LL device
• Groove cutting inappropriate
• Bowl dropping height set incorrectly
• Water content of samples are not accurate
How to plot LL graph?
36.2
No of blows
9
13
27
29.5
34
Moisture
content (%)
50
46
36
34
33
The Liquid Limit (LL) is the moisture content at which 25
taps are needed to close the groove over a length of 13mm
Liquid Limit Graph
36.2
The Liquid Limit (LL) is the moisture content at which 25
taps are needed to close the groove over a length of 13mm
Try Practice Tutorial Q1 in pairs.
Determine the Liquid Limit (LL) of
soil
Cone Penetration Limit
Determination
NZS4402:1986 Test 2.5
Test demo min 2:05 – 13:02
https://www.youtube.com/watch?v=yG784eXBj-Q
Dial
gauge
Cone Penetrometer
Apparatus
Release
button
40mm
Sample
55mm
Sample
cup
Procedure
• A sample of wet soil prepared as
per Casagrande method is placed
into a 40mm deep metal cup and
the surface levelled.
• The cone is lowered on to the
surface of the soil and the dial
gauge read.
• The cone is released for 5 sec and
its penetration into the soil is
measured.
How to plot CPL graph?
40
Penetration
(mm)
14
15
17
25
27
29
Moisture
content (%)
33
34
36
46
50
53
The Cone Penetration Limit (CPL) is the moisture content
corresponds to a cone penetration of 20mm
Cone Penetration Limit Graph
• The test is repeated with the
same soil at increasing water
contents.
40
• it is easier to wet the soil evenly
than dry it evenly.
• A graph of cone penetration vs
moisture content is plotted.
The Cone Penetration Limit (CPL) is the moisture content
corresponds to a cone penetration of 20mm
Try Practice Tutorial Q2 in pairs.
Determine the Cone Penetration Limit
(CPL) of soil
LL and CPL
Too wet
Dry the soil
Too dry
Add water
Note: CPL = LL for low values of LL (less than 50)
Liquid Limit vs Cone Penetration Limit
LL correlates well with CPL up to values of ±50
The CPL test is starting to replace the Casagrande test because:
• easier and cheaper to perform
• less sensitive to design of apparatus & manner in which it is used
• test is more precise & does not rely on the experience / subjective opinion of the
user
• applicable to wide range of soils
Plastic Limit
Determination
NZS4402:1986 Test 2.3
Test demo min 6:39 – 8:10
https://www.youtube.com/watch?v=EcXJ961qjGA
Procedure
• Use approx. 30g soil passing 425
micron sieve. Mix with water until it
is workable.
• Allow to dry until easily shaped
into ball without sticking to fingers.
Procedure cont…
• Divide the sample into
several smaller samples.
Then, roll each sample on a
glass plate using palm of
hand to reduce to 3mm dia.
3mm dia, 100mm long steel rod is often used
as a guideline.
Procedure cont…
• Continue rolling until thread
shears longitudinally and
transversely at 3mm dia.
• Repeat procedure for other
samples.
Procedure cont…
• Place the crumbled soil in the dish
to determine moisture content.
• If the value of PL is >5% of each
other than the test has to be
repeated.
• Eg 1st attempt: 15, 2nd attempt: 16
The Plastic Limit (PL) of a soil is the moisture content below
which it changes from a plastic to a semi-solid state
Try Practice Tutorial Q1 in pairs.
Determine the Plastic Limit (PL) of soil
Plastic and Non-Plastic Soils
Plastic Soils
Non-Plastic Soils
• The plastic limit is controlled by clay content:
• If a silt or sand soil cannot be
rolled into thin threads at any
moisture content they have no
plastic limit and are termed NonPlastic.
• Non-expansive clay minerals, eg. kaolinite
and illite have lower Plastic Limits.
• Expansive clays, eg. montmorillonite have
higher Plastic Limits.
• NZS 3604:2011, Section 17 Expansive soils in
NZ: LL > 50% and SL > 15%
Plasticity Index
PL
Semi solid state
PI = LL - PL
Plastic state
LL
Liquid state
The PLASTICITY INDEX (PI) is the range of water contents over which a soil behaves
plastically. PI = LL-PL
Engineering Implications of PI
• If PI of a soil is small, eg. 5% - the soil will be very sensitive to moisture. This is
undesirable for a foundation material.
• If PI is large, eg. 20% - considerable water can be added before the soil
becomes liquid. The soil is a desirable foundation material.
• HOWEVER soils with very high PI (> 35%) may have a high swelling
capacity.
Try Practice Tutorial Q1 in pairs.
Determine the Plasticity Index (PI) of
soil
Plasticity Chart
A-line
Plasticity Index
60
50
40
CH
Soils with both high LL and PI are
highly plastic.
30
20
CL
MH or OH
10
ML or OL
0
0
10
20
30
40
50
60
70
Liquid Limit
80
A graph of liquid limit vs plasticity
index is used to classify fine-grained
soils.
90 100
Soils with both low LL and PI are
slightly plastic.
Eg: LL=60, PI=40. Therefore soil is CH
From Practice Tutorial Q1
List down the LL, PL and PI. Then, classify the soil using
Plasticity chart
Engineering Implications of Atterberg Limits
• Soils with high Atterberg limits, especially high liquid limits > 50%, generally
have:
• low shear values
• high clay content
• low load carrying capacity
• Atterberg limits also give an indication of clay mineral composition.
• liquid and plastic limits are higher for montmorillonite clays than for kaolinites or
illites.
Other Indices
Liquidity Index
• The Liquidity Index (LI) compares the in-situ water
content with its plasticity
LI = (w-PL) / PI = (w-PL)/ (LL- PL)
where w = natural field moisture content
Implications of LI
• If LI < 0, the water content is less than the Plastic Limit and the soil acts like a
semi–solid or solid
• (i.e., (w-PL) is negative)
• If 1 < LI > 0, then the in situ water content is at or below the Liquid Limit and
soil is in a plastic state.
• If LI > 1, the soil is extremely weak
• fluid-like debris flows have LI > I
Eg: Liquidity Index (LI)
• The laboratory test on a soil sample yields the following results:
• Natural moisture content = 18%
• Liquid limit = 60%
• Plastic limit = 25%
• The liquidity index of the soil is
Ans: -0.2
Activity
• The Activity of a clay is ratio of PI to the clay content.
• This parameter is a fair indicator of the clay minerals present in
a soil as shown in table below.
Activity = Plasticity index / % clay particles
Mineral
Activity value
Na-montmorillonite
4–7
Ca-montmorillonite
1.5
Illite
0.5 – 1.3
Kaolinite
0.3 -0.5
Halloys (hydrated)ite
0.1
Calcite
0.2
Active > 1.25
Normal 0.75 – 1.25
Inactive < 0.75
Eg: Activity
The plasticity index and the percentage of grain size finer than 2 microns of
a clay sample are 25 and 15, respectively. Its activity is ______.
Is the soil active?
Ans: 1.67