Liquid Limit Plastic Limit

1. Geotechnical Engineering–I [CE-221]
BSc Civil Engineering
Atterberg Limits
by
Engr. Mudassar Munir Shah
Qualification: MSc Geotechnical Engineering (2022)
Institute: UET Lahore
Teaching Exp: AUG-2018 to
OCT-2019 at JCET
Email:mudassarmunirshah461@gmail.com
1

2. SOIL CONSISTENCY
Is a property of soil which…
 Indicates the degree of firmness of
fine-grained soil.
 Soil generally expressed in terms
of very soft, soft, stiff, very stiff,
and hard.
Soil behave like:
2
 SOLID
 LIQUID at
at very low moisture
content
very high moisture
content

3. ATTERBERG/CONSISTENCY LIMITS
(Swedish Agriculturist, Define boundaries b/w states of soil)
Solid Semi-Solid Plastic Viscous Liquid
Moisture content, w (%)
Soil
volume,
v
or
e
Shrinkage Limit
SL
Plastic Limit
PL
Liquid Limit
LL
Plasticity
Index
3

4. Liquid Limit (LL) is defined as the moisture content at which
soil begins to behave as a liquid material and begins to flow.
(LL of a fine-grained soil gives the moisture content at which the shear
strength of the soil is approximately 2.5kN/m2)
LIQUID LIMIT (LL or wLL)
4

5.  250g air dry sample
passing through
No.40 sieve.
 Add some water to
make paste of soil.
 Add water carefully
not much more nor
much less then LL.
 Place soil paste in
pat of apparatus.
 Make a groove in
soil.
 Repeat test three
time for 3 point
method & one time
for one point method
5
LIQUID LIMIT DETERMINATION Casagrande Apparatus , ASTM D-4318

6. LL is the moisture content required to close a 2-mm wide
groove in a soil pat a distance of 12.7 mm (1/2”) along the
bottom of the groove after 25 blows.
6
LIQUID LIMIT DETERMINATION
A) Three-Point Method

7. 25
7
 
 
 N 
tan 
LL  w
N  number of blows
wn  corresponding moisturecontent
tan  0.121
Assumes a constant slope of the flow curve.
The slope is a statistical result of 767 liquid limit tests (Norman, 1959).
Limitations:
tan  is an empirical coefficient, so it is not always 0.121.
Good results can be obtained only for the blow number between 20 to 30.
n
B) One-Point Method
LIQUID LIMIT DETERMINATION

8. Flow Index
N2=30
N1=20
w1=44
w2=39
FLOW INDEX (IF) (Rate at which soil mass loses its
shear strength with an increase in moisture content)
 Larger the IF, smaller will be
shear strength of soil.
 Soil Flows like liquid material
IF=28.39
8

9. Plastic Limit (PL) is defined as the moisture content at which soil begins to
behave as a plastic material.
Plasticity Index (PI or IP) = LL - PL
PLASTIC LIMIT (PL or wPL)
9
 PI is a difference between
numerical values of LL & PL
 PI gives a range of moisture
content for which soil behaves like
a Plastic Material.

10. Moisture content at which the soil when rolled into
threads of 3.2mm (1/8 in) diameter, will crumble.
PL = w% at 3.2 mm (1/8 in) dia.
10
PLASTIC LIMIT DETERMINATION
ASTM D-4318
 50g air dry sample
passing through
No.40 sieve.
 Add some water to
make small ball of
soil.
 Add water
carefully not much
more nor much less
then PL.
 Make thread (1/8’’
dia) with the help of
your palm
 Repeat test three
time for better
results

11. Shrinkage Limit (SL) is defined as the moisture content at
which no further volume change occurs with further reduction
in moisture content.
SHRINKAGE LIMIT
11

12. SHRINKAGE LIMIT
12

13. PLASTICITY/A-LINE CHART
13

14. PLASTICITY/A-LINE CHART
High plasticity
Low plasticity
LL=50%
14

15. PLASTICITY/A-LINE CHART
High Plastic
Silts
Silts
Cohesionless
Soils
15

16. where,
IL= Liquidity index
wn= Natural moisture content
PL= Plastic limit moisture content
LL= Liquid limit moisture content
0<IL<1 → Soil is in plastic state
IL < 0 → Soil is in semi-plastic or solid state
IL > 1 → Soil is in liquid state (quick clays or ultra sensitive clays)
LIQUIDITY INDEX (IL)
IL
LL PL

wn PL
16
PI

wn PL
 Also known as water
plasticity ratio

17. Toughness index (It) is useful to distinguish between soils
(types) of different physical properties such as clays, silts
etc.
TOUGHNESS INDEX (It)
F
17
t
Flow Index I
I 
Plasticity Index

PI
Soil Type It
Clayey soils 0-3
Soils which are friable at plastic
limit (e.g. Silts)
<1
 it varies between 0 to 3

18.  Helpful to predict the dominant clay type/mineral in soil
sample.
 V of clay during shrinkage & swelling depends on A
 It measures water holding capacity of clay soil.
 High activity signifies large volume change when wetted
and large shrinkage when dried.
Clay Type Activity
Inactive Clays < 0.75
Normal Clays 0.75 < A <1.25
Active Clays > 1.25
ACTIVITY, A
PI
18
%clay fraction smaller than  0.002mm
A 

19. The following data were recorded from a LL test on a silty
clay;
19
If PL=23.4%, determine LL, flow index, and the toughness
index.
Practice Problem#1
No. of blows Water content (%)
35 41.1
29 41.8
21 43.5
15 44.9

20. LL = 42.8%
PL = 23.4%
PI = 42.8-23.4
PI = 19.4%
Practice Problem
1 100
w
(%)
10
No. of blows (N)
N=25
20
48
47
46
45
44
43LL=42.8%
42
41
40

21. IF= 10.22
Practice Problem
40
41
42
43
44
48
47
46
45
1 10
No. of blows (N)
100
w
(%)
2 
log
 N1 
 N 
w1 w2
I 
F

21
 20
 30
log

43.5 41.7
It= IP/IF
I =19.4/10.22
t
It=1.898
N2=30
N1=20
w1=43.5%
w2=41.7%

22. CONCLUDED
22
REFERENCE MATERIAL
An Introduction to Geotechnical Engineering (2nd Edition)
By R. D. Holtz, W. D. Kovacs and T. C. Sheahan
Chapter #2
Principles of Geotechnical Engineering – (7th Edition)
Braja M. Das
Chapter #4
Norman, L. E. J. (1959). The one-point method of determining the value of
the liquid limit of a soil. Geotechnique, 9(1), 1-8.