This document provides the procedure for determining the plastic limit of a soil sample. It describes preparing a soil sample that has been passed through a 425 micron sieve. The plastic limit is determined by rolling threads of the soil into 3 mm diameters and finding the minimum water content at which it will just begin to crumble. The given soil sample had a plastic limit of 22% and a plasticity index of 16% based on calculations provided. The conclusions state that since the plastic limit is less than 25%, the soil can be used as a fill material according to MoRTH specifications.

1. Department of Civil Engineering
P. G. Section in Transportation Engineering & Planning
Determination of Plastic Limit of Soil
Sample
Submitted by –
Adersh TV P17TP008
Manish Choudhary P17TP007
Shubham Sharma P17TP002
Vivek Wagh P17TP011

2. 2
Plastic Limit
The minimum water content at which a soil will just begin to crumble when it
is rolled into a thread of approximately 3 mm in diameter.
Reference IS : 2720 -Part V (1985)

3. Apparatus
3
1. Porcelain Evaporating dish –
about 12 cm in diameter
2. Spatula – Flexible with the blade
about 8 cm long and 2 cm wide
3. Glass plate about 20cm X 15 cm
4. Containers – Airtight to
determine moisture content
5. Balance sensitive to 0.01g
6. Oven – thermostatically
controlled with interior of non
corroding material to maintain
the temperature between 105 C
to 110 C
7. Rod – 3 mm in diameter and
about 10 cm long

4. Preparation of soil sample
 Soil sample passed through 425
micron IS –Sieve
 When both the liquid limit and the
plastic limit of soil are to be
determined, a quantity of soil
sufficient for both the tests shall be
taken for preparation of the soil
 At stage in the process of mixing soil
and water at which mass becomes
plastic enough to be easily shaped into
a ball, sample must be taken for test.
 In case of clayey soils the plastic soil
shall be kept for sufficient time(24
hours) for uniform distribution of
moisture within soil

5. Procedure
5
 A ball of about 8g of plastic mass is
formed
 Ball is rolled between fingers and
glass plate with sufficient pressure to
roll soil mass into thread of uniform
diameter throughout its length
 Rate of rolling = 80 to 90 Strokes/min
 1 stroke = complete motion of the
hand forward and back to starting
point again
 Rolling shall be done till the threads
are of 3 mm diameter

6. What if soil crushes at diameter more than
3 mm in first trial
 In such a case, Test must be conducted with a sample
having more water content than that of previous sample

7. What if soil crushes at diameter less than 3
mm in first trial, say 2mm
 In such a case, the soil shall be kneaded
together(crushed pieces) to a uniform mass
and rolled again.
 This process of alternate kneading and
rolling is continued till the thread crumbles
under pressure required for rolling and soil
can no longer be rolled into a thread
 Crumbling may occur when a thread has
diameter greater than 3 mm
 At no time shall an attempt to be made to
produce failure at 3 mm, by either reducing
the rolling or pressure or both, and
continuing the rolling without further
deformation until thread falls apart

8. Determination of moisture content
 IS 2720: Part II -1973 – Indian Standard Method of test for soils Part II,
Determination of water content
 Water content specimen should be discarded and should not be used for
any other use
 Plastic Limit as an average of result from 3 specimen passing through
425 micron IS Sieve must be reported to nearest whole number.
Size of particles more
than 90 percent passing
Minimum quantity of soil
specimen to be taken for
test (MASS in g)
425 micron IS Sieve 25
2 mm IS Sieve 50
4.75 mm IS Sieve 200
10 mm IS Sieve 300
20 mm IS Sieve 500
40 mm IS Sieve 1000

9. 9

10. Definition of Plasticity Index
10
 Plasticity Index is the numerical difference between the Liquid Limit w%
and the Plastic Limit w%
w% LLPL
PI = LL - PL
plastic State

11. 11
Calculations
Sample no Wt of wet soil (g) Wt of oven dried
soil (g)
Moisture content
(g)
Sample 1 25 20.53 21.77 %
Sample 2 25 20.53 21.77%
Plastic Limit as an average to nearest whole no. 22%
1. Moisture content = (Wt of Wet soil – Wt of Dry soil)*100/Wt of dry
soil
2. Plasticity index = 38 – 22 = 16%

12.  Plastic Limit of the given soil is 22 %
 Plasticity index of given soil is 16%
Results
Conclusions
Since the Plastic Limit of the soil is <25%, as per
MoRTH specifications for roads and bridges ,
the soil is fit to be used as a fill material,
provided it satisfies the specifications for other soil
parameters.

13. Factors affecting Plastic Limit
 Clay minerals in soil
 Rate of Rolling
 Non uniform mixing
 Clay content of soil
 Temperature

14. THANK YOU