Geotechnical Engineering

DHARM

108 GEOTECHNICAL ENGINEERING

(c) Which soil has better strength as a function of water content?
(d) Which soil has better strength at the plastic limit?
(e) Could organic material be present in these soils?
Plot the positions of these soils on the Casagrande’s plasticity chart and try to classify
them as per IS Classification.
(a) Plasticity index, Ip for soil S 1 = wL – wP = (38 – 18) = 20
Ip for soil S 2 = wL – wP = (60 – 20) = 40
Obviously, Soil S 2 is the more plastic.
As per Burmister’s classification of the degree of plasticity, S 1 borders between low-to-
medium plasticity and S 2 between medium-to-high plasticity.
(b) Consistency index,

Ic for soil S 1 =

()()ww

I

L

p

− = 38 40−

20

= – 0.1

Ic for soil S 2 = ()60 50

40

− = 0.25

Since the consistency index for soil S 1 is negative it will become a slurry on remoulding;
therefore, soil S 2 is likely to be a better foundation material on remoulding.

(c) Flow index, If for soil S 1 = 10
If for soil S 2 = 5
Since the flow index for soil S 2 , is smaller than that for S 1 , soil S 2 has better strength as
a function of water content.
(d) Toughness index, IT for soil S 1 = Ip/If = 20/10 = 2
IT for soil S 2 = 40/5 = 8
Since toughness index is greater for soil S 2 , it has a better strength at plastic
limit. (e) Since the plasticity indices are low for both the soils, the probability of the presence of
organic material is small.
These conclusions may be mostly confirmed from the following:
The soils are marked on Casagrande’s plasticity chart as shown in Fig. 4.7.

CH

w=w=50LL 50

SS 22

SS 11

w=wLL=35 35

MHMH or OHorOH

CICI

CL

CLCL–ML–ML Mor

O

I

I

Mor

O

I

I

100908070605040302010

wL

60

50

40

30

20

10

0

IP

A-line

IPL= 0.73 (w – 20)

Fig. 4.7 Plasticity chart, soils S 1 and S 2 plotted (Example 4.1)