CONSISTENCY LIMITS OF SOILS.
CONSISTENCY
LIMITS OF SOILS.
( IS : 2720
– PART – 5 )
INTRODUCTION:
The
physical properties of fine-grained soils, especially of clay differ much at
different water contents. Clay may be almost in liquid state, or it may show
plastic behavior or may be very stiff depending on the moisture content.
Plasticity is a property of outstanding importance for clayey soils, which may
be explained as the ability to undergo changes in shape without rupture.
Liquid limit: defined as the minimum water content at which the
soil will flow under the application of a very small shearing force.
Plastic limit: defined as the minimum moisture content at which the
soil remains in a plastic state.
Plasticity Index (PI): is defined as the numerical difference between the
liquid limit and plastic limits. PI thus indicates the range of moisture
content over which the soil is in a plastic condition.
Consistency
limits and the plasticity index vary for different soil types. Hence these
properties are generally used in the identification and classification of
soils.
Liquid
limit test:(Mechanical liquid limit device)
Object:
Determination
of the liquid limit of soil by mechanical liquid limit device.
Apparatus:
i)
Mechanical liquid limit device consists of a cup and arrangement for raising
and dropping through a specified height and standard grooving tools.
ii)
Balance of 200 g capacity and sensitive to 0.01 g.
iii)
Oven to maintain 1050 to 1100C.
Procedure:
About
120 g of dry pulverized soil sample passing 425 micron IS sieve is weighed, and
mixed thoroughly with distilled water in the evaporating dish to form a uniform
thick paste. The liquid limit device is adjusted to have a free fall of cup
through 10 mm. A portion of the paste is placed in the cup above the lowest
spot, and squeezed down with the spatula to have a horizontal surface. The
specimen is trimmed by firm strokes of spatula in such a way that the maximum
depth of soil sample in the cup is 10 mm. The soil in the cup is divided along
the diameter through the center line of the cam followed by firm strokes of the
grooving tool so as to get a clean sharp groove. Grooving tool (b) may be used
for all soils, where as grooving tool (a) may be used only in clayey soils free
from sand particles or fibrous materials.
The
crank is rotated at the rate of two revolutions per second (either by hand or
electrically operation) so that the cup is lifted and dropped. This is
continued till the two halves of the soil cake come into contact at the bottom
of the groove along a distance of about 10 mm, and the number of blows given is
recorded. A representative soil is taken, placed in the moisture container, lid
placed over it and weighed. The container is dried in oven and the dry weight
determined the next day for finding the moisture content of the soil. The
operations are repeated for at least three more trials with slightly increased
moisture contents each time, noting the number of blows so that there are at
least four uniformly distributed readings of number of blows between 10 and 40 blows.
Calculations:
Taking
the number of blows in the log scale on the X-axis, and the water content in
arithmetic scale on the Y-axis plots the flow curve. The flow curve is straight
line drawn on this semi-logarithmic plot, as nearly as possible through three
or more plotted points. The moisture content corresponding to 25 blows is read
from this curve, rounding off to the nearest whole number and is reported as
the liquid limit (LL or wl) of the soil.
The
slope of the straight-line flow curve is the flow index. It may be calculated
from the following formula:
w1 –
w2 w10
– w100 Where,
w10 = water content at 10 blows.
Flow
Index (If) = = w100 = water content at 100 blows.
log n2 – log n1 log 100 – log 10
Liquid
limit test: (Cone penetration)
Object:
To determine the liquid limit of the soil by cone penetration
apparatus.
Apparatus:
i) Cone penetration apparatus confirming to IS: 11196-1985 (cone angle
300+/-0.50) and weight of assemble is 80 +/- 0.5 g
including all).
ii)
Balance of 200 g capacity and sensitive to 0.01 g.
iii)
Oven to maintain 1050 to 1100C.
Procedure:
About
150 g of dry pulverized soil sample passing 425 micron IS sieve is weighed, and
mixed thoroughly with distilled water in the evaporating dish to form a uniform
thick paste. The soil paste shall then be transferred to the cylindrical mould
of the cone penetrometer apparatus and leveled up to the top of the cup. The
penetrometer shall be so adjusted that the cone point just touches the surface
of the soil paste in the trough. The scale of the penetrometer shall then be
adjusted to zero and the vertical rod released so that the cone is allowed to
penetrate into the soil paste under its weight. The weight should be 80 +/- 0.5
g and the penetration shall be noted after 5 seconds from the release of the
cone.
If
the difference in penetration lies between 14 and 28 mm the test is repeated
with suitable adjustments to moisture either by addition of more water or
exposure of the spread paste on a glass plate for reduction in moisture
content. The test shall be repeated at least to have four sets of values of
penetration in the range 14 and 28 mm. The exact moisture content of each trial
shall be determined.
Calculations:
A
graph representing water content on the Y-axis and the cone penetration value
on the X-axis shall be prepared. The best fitting straight line is then drawn.
The moisture content corresponding to cone penetration of 20 mm shall be taken
as the liquid limit of the soil and shall be expressed to the nearest first
decimal place.
Liquid Limit Graph (Mechanical Device
Method):
Liquid Limit Graph (Cone Penetration
Method):
Plastic
limit test:
Object:
Determination of the plastic limit of the soils.
Apparatus:
Evaporating dish, spatula, glass plate, moisture containers, rod of 3
mm diameter, balance sensitive to 0.01 g and oven controlled at temperature 1050
to 1100C.
Procedure:
About 20 g of dry pulverized soil passing 425 micron IS sieve is
weighed. The soil is mixed thoroughly with distilled water in the evaporating
dish till the soil paste is plastic enough to be easily moulded with fingers. A
small ball is formed with the fingers and this is rolled between the fingers
and the glass plate to a thread. The pressure just sufficient to roll into a
thread of uniform diameter should be used. The rate of rolling should be
between 80 to 90 strokes per minute counting a stroke as one complete motion of
hand forward and back to the starting position again. The rolling is done till
the diameter of the thread is 3 mm. Then the soil is kneaded together to a ball
and rolled again to form thread. This process of alternate rolling and kneading
is continued until the thread crumbles under pressure required for rolling and
the soil can no longer be rolled into a thread.
If the crumbling starts at diameter less than 3 mm, then moisture
content is more than plastic limit and if the diameter is greater while
crumbling starts, the moisture content is lower. By trial, the thread that
starts crumbling at 3 mm diameter under normal rolling should be obtained and
this should be immediately transferred to the moisture container, lid placed
over it and weighed. The container is kept in the oven for about a day and dry
weight found to determine the moisture content of the thread. The above process
is repeated to get at least three consistent values of the plastic limit (PL or
wp).
Calculations:
Plasticity Index (PI or Ip) = Liquid limit – Plastic limit.
=
LL - PL
=
wl - wp
Toughness Index (TI or IT) = Ip / If
Liquidity Index ( LI or Il) = (w – wp) / Ip where, ‘w’ is the natural moisture
content of the soil.
Consistency Index (CI or Io) = (wp – w) / Ip
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