Lecture 3 consistncy of soil

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303322 - Soil Mechanics
Consistency of Soil
Dr. Abdulmannan Orabi
Lecture
2
Lecture
3

Dr. Abdulmannan Orabi IUST 2
Das, B., M. (2014), “ Principles of geotechnical
Engineering ” Eighth Edition, CENGAGE
Learning, ISBN-13: 978-0-495-41130-7.
Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil
Mechanics” Eighth Edition, Spon Press, ISBN: 978-
0-415-56125-9.
References

When clay minerals are present in fine-grained
soil, the soil can be remolded in the presence of
some moisture without crumbling. This cohesive
nature is caused by the adsorbed water
surrounding the clay particles. Swedish scientist
named Atterberg developed a method to describe
the consistency of fine-grained soils with varying
moisture contents.
Introduction

The physical properties of clays greatly differ
at different water contents. A soil which is very
soft at a higher percentage of water content
becomes very hard with a decrease in water
content.
Consistency is a term used to indicate the degree
of firmness of cohesive soils.
Consistency

However, it has been found that at the same
water content, two samples of clay of different
origins may possess different consistency. One
clay may be relatively soft while the other may be
hard. Further, a decrease in water content may
have little effect on one sample of clay but may
transform the other sample from almost a liquid
to a very firm condition.
Consistency

Consistency is an important characteristic
in the case of fine soil, the term consistency
describing the ability of a soil to undergo
unrecoverable deformation without cracking
or crumbing.
The consistency of clays and other cohesive
soils is usually described as soft, medium,
stiff, or hard.
Consistency

Consistency
Water Content Significantly affects properties
of Silty and Clayey soils (unlike sand and
gravel)
Strength decreases as water content increases
Soils swell-up when water content increases
Fine-grained soils at very high water content
possess properties similar to liquids

Consistency
As the water content is reduced, the volume
of the soil decreases and the soils become
plastic
If the water content is further reduced, the
soil becomes semi-solid when the volume
does not change

• The knowledge of the soil consistency is
important in defining or classifying a soil type
or predicting soil performance when used a
construction material
• A fine-grained soil usually exists with its
particles surrounded by water.
• The amount of water in the soil determines its
state or consistency
Consistency
9Dr. Abdulmannan Orabi IUST

The consistency of a fine-grained soil refers to its
firmness, and it varies with the water content of the
soil.
A gradual increase in water content causes the soil
to change from solid to semi-solid to plastic to liquid
states. The water contents at which the consistency
changes from one state to the other are
called consistency limits (or Atterberg limits).
Consistency

At a very low moisture content, soil behaves
more like a solid. When the moisture content
is very high, the soil and water may flow like
a liquid. Hence, on an arbitrary basis,
depending on the moisture content, the
behavior of soil can be divided into 4 basic
states: solid, semisolid, plastic, and liquid.
Atterburg Limits

Atterberg limits are the limits of water content used
to define soil behavior. The consistency of soils
according to Atterberg limits gives the following
diagram .
Atterburg Limits
Volume
Water content
Semi-
solid
Plastic Liquid
Solid
LLPLSL
PI
S = 100%

Consistency
The three limits are known as the shrinkage
limit (SL), plastic limit (PL), and liquid
limit (LL) as shown. The values of these
limits can be obtained from laboratory tests.
Volume
Water content
Semi-
solid
Plastic Liquid
Solid
LLPLSL
PI
S = 100%

If we know the water content of our sample is
relative to the Atterberg limits, then we already
know a great deal about the engineering response
of our sample.
Importance of Atterburg Limits
• As the water content is reduced, the volume of the
soil decreases and the soils become plastic.
• If the water content is further reduced, the soil
becomes semi-solid when the volume does not
change.

The Atterberg limits may be used for the following:
Importance of Atterburg Limits
1.To obtain general information about a soil
and its strength, compressibility, and
permeability properties.
2.Empirical correlations for some
engineering properties.
3. Soil classification

The liquid limit is defined as the water
content at which the soil changes from a
liquid state to a plastic state.
Liquid limit of soil is generally
determined by the Standard Casagrande
device.
The procedure for the liquid limit test is
given by ASTM D-4318
Atterberg Limits & Consistency indices
Liquid Limit

Casagrande- defined the liquid limit as a
water content at which a standard
groove cut in the remolded soil sample by
a grooving tool will close over a distance
of 13 mm (1/2”) at 25 blows of the
L.L cup falling 10 mm on a hard rubber
base. (See the figure below)
Liquid Limit

The device consists of a brass cup and a
hard rubber base.
Liquid Limit

Liquid Limit
To performed the liquid limit test, one must
place a soil paste in the cup. A groove is then
cut at the center of the soil pat with the
standard grooving tool.

By using the crank-operated cam, the cap is lifted
and dropped from a height of 10 mm. The water
content required to close a distance of 12.7 mm
along a bottom of the groove after 25 blows is
defined as the liquid limit.
Liquid Limit

Soil Pat after Groove Has ClosedGrooved Soil Pat in Liquid
Limit Device
Liquid Limit

It is difficult to adjust the moisture content in the
soil to meet the required 12.5 mm (0.5 in.) closure
of the groove in the soil pat at 25 blows. Hence, at
least three tests for the same soil are conducted at
varying moisture contents, with the number of
blows, N, required to achieve closure varying
between 15 and 35.
Liquid Limit

The moisture content of the soil, in percent,
and the corresponding number of blows are
plotted on semilogarithmic graph paper
The relationship between moisture content
and log N is approximated as a straight line.
Liquid Limit

Plot the number of drops, N, (on the log scale)
versus the water content (w). Draw the best-fit
straight line through the plotted points.
Liquid Limit
This line is referred to as the flow curve.
The moisture content corresponding to N
25, determined from the flow curve, gives
the liquid limit of the soil.

Watercontent,(%)
LL= 42 %
Number of Blows N
20 25
30
35
40
45
50
30 5040
Flow curve
10
Liquid Limit
Flow curve for liquid limit determination of clayey silt

U.S. Army Corps of Engineers proposed an empirical
equation of the form
Liquid Limit
=
25

(3 − 1)
where:
= ℎ " #
# $ % & 12.5 ( $ %
= % ) # ( & %
tanβ = 0.121 (but not that tan 3 " & 0.121
& )

Equation (3-1) generally yields good results for
the number of blows between 20 and 30.
This procedure is generally referred to as the one-point
method and was also adopted by ASTM under
designation D-4318.
The reason that the one-point method yields fairly
good results is that a small range of moisture content
is involved when N = 20 to N = 30.
Liquid Limit

Another method of determining liquid limit that is
popular in Europe and Asia is the fall cone method .
In this test the liquid limit is defined as the moisture
content at which a standard cone of apex angle 30°
and weight of 0.78 N (80 gf) will penetrate a distance
d = 20 mm in 5 seconds when allowed to drop from a
position of point contact with the soil surface
Fall-Cone Method (British Standard – BS1377)
Liquid Limit

Liquid Limit
d = 20 mm in 5 secondsCone position

Figure below shows the photograph of a fall cone
apparatus. Due to the difficulty in achieving the liquid
limit from a single test, four or more tests can be
conducted at various moisture contents to determine the
fall cone penetration, d.
3. Fall-Cone Method (British Standard – BS1377)

A semilogarithmic graph can then be plotted with
moisture content (w) versus cone penetration d. The plot
results in a straight line. The moisture content
corresponding to d = 20 mm is the liquid limit.
3. Fall-Cone Method (British Standard – BS1377)
35
40
45
50
30
10
MoistureContent
10020 30 50 70
Penetration , d (mm)
LL = 40 %

Plastic Limit
The plastic limit ( PL ) is defined as the water content
at which the soil changes from a plastic state to a semi-
solid state. At this state the mixture is deformed to any
shape under minor pressure.
Plastic State Liquid StateSemi Solid StateSolid State
Water content
LLPLSL

Casagrande defined the plastic limit as water at which a
thread of soil just crumbles when it is carefully rolled out
to a diameter of 3 mm(1/8”). It should break up into
segments about 3 – 10 mm (1/8 – 3/8 inch) long. If the
thread crumbles at diameter smaller than 3 mm, the soil is
too wet. If the thread crumbles at diameter grater than 3
mm, the soil past the P.L
Plastic Limit

The plastic limit test is simple and is performed by
repeated rollings of an ellipsoidal-sized soil mass by
hand on a ground glass plate
The procedure for the plastic limit test is given by
ASTM in Test Designation D-4318.
Plastic Limit

As in the case of liquid limit determination, the fall
cone method can be used to obtain the plastic limit.
This can be achieved by using a cone of similar
geometry but with a mass of 2.35 N (240 gf).
Three to four tests at varying moisture contents of
soil are conducted, and the corresponding cone
penetrations (d) are determined.
Plastic Limit

The moisture content corresponding to a cone penetration
of d = 20 mm is the plastic limit.
Plastic Limit
35
40
45
50
30
10
MoistureContent
10020 30 50 70
Penetration , d (mm)
PL = 40 %

The plasticity index (PI) is the difference between
the liquid limit and the plastic limit of a soil:
PI = LL - PL
Plasticity index indicates the degree of plasticity of
a soil. The greater the difference between liquid and
plastic limits, the greater is the plasticity of the soil
Plasticity Index (PI)
(3 − 2)

A cohesionless soil has zero plasticity index. Such soils are
termed non-plastic.
Fat clays are highly plastic and possess a high plasticity
index.
Report the liquid limit, plastic limit, and plasticity index to
the nearest whole number, omitting the percent designation.
If either the liquid limit or plastic limit could not be
determined, or if the plastic limit is equal to or greater
than the liquid limit, report the soil as nonplastic, NP.

The plasticity index is important in classifying fine-
grained soils. It is fundamental to the Casagrande
plasticity chart, which is currently the basis for the
Unified Soil Classification System.
If either the liquid limit or plastic limit could not be
determined, or if the plastic limit is equal to or greater
than the liquid limit, report the soil as nonplastic, NP.

Burmister (1949) classified the plasticity index in a
qualitative manner as follows:
Plasticity Index (PI) Description
0 Non- plastic
1-5 Slightly plastic
5-10 Low plasticity
10-20 Medium plasticity
20-40 High plasticity
> 40 Very high plasticity

In a recent study by Polidori (2007) that involved six
inorganic soils and their respective mixtures with fine
silica sand, it was shown that
4 = 0.04 + 0.26 89 + 10
4: = 0.96 − 0.26 89 − 10and
where CF clay fraction (<2 µm) in %. The experimental
results of Polidori (2007) show that the preceding
relationships hold good for CF approximately equal to or
greater than 30%.
(3 − 3)
(3 − 4)

Report the liquid limit, plastic limit, and plasticity
index to the nearest whole number, omitting the
percent designation.
If either the liquid limit or plastic limit could not
be determined, or if the plastic limit is equal to or
greater than the liquid limit, report the soil as
non-plastic, NP.

The relative consistency of a cohesive soil in the
natural state can be defined by a ratio called the
liquidity index (LI), which is given by:
Liquidity Index (LI)
: =
= − 4
4:
where : in situ moisture content of soil.=
(3 − 5)

The in situ moisture content for a sensitive clay may be
greater than the liquid limit. In this case: LI > 1.
Water content
LI = 1LI =0
SL
Soil deposits that are heavily overconsolidated may have
a natural moisture content less than the plastic limit. In
this case: LI < 1.
LLPL

Classification as per liquidity index is:
Classification Liquidity Index
Liquid > 1
Very soft 0.75 – 1.0
Soft 0.5-0.75
Medium stiff 0.25-0.50
Stiff 0-0.25
Semi-solid <0

Consistency Index (LI)
Water content
LLPL
CI = 1 CI = 0
SL
CI is the ratio of the liquid limit minus the natural
water content to its plasticity index:
8: =
− =
4:
where : in situ moisture content of soil.=
(3 − 6)

"Clayey soils" necessarily do not consist of 100% clay size
particles. The proportion of clay mineral flakes (< 0.002
mm size) in a fine soil increases its tendency to swell and
shrink with changes in water content. This is called
the activity of the clayey soil, and it represents the degree
of plasticity related to the clay content.
Activity
PI
Percentage by weight finer than 2µm
A =
Activity of clays is the ratio of plasticity index to the
percentage of particle sizes finer than 2µm
( 3- 7 )

Activity
Activity , A Classification
< 0.75 Inactive clay
0.75 to 1.25 Normal clay
> 1.25 Active clay
Classification as per activity is:
Based on Eqs. (3-3) and (3-4), Polidori (2007) provided
an empirical relationship for activity as (for CF equal to
or greater than 30%)
4 =
0.96 − 0.26 89 − 10
89
(3 − 8)

Shrinkage Limit (SL)
The moisture content, in percent, at which the volume of
the soil mass ceases to change is defined as the shrinkage
limit.
Shrinkage limit tests [ASTM (2007)—Test Designation
The shrinkage limit ( SL ) is defined as the water
content at which the soil changes from a semi-solid to a
solid state.

The shrinkage limit is determined as follows. A mass of
wet soil, M1, is placed in a porcelain dish 44.5 mm in
diameter and 12.5 mm high and then oven-dried.
The volume of oven-dried soil is determined by using
mercury to occupy the vacant spaces caused by shrinkage.
The mass of mercury is determined and the volume
decrease caused by shrinkage can be calculated from the
known the density of mercury.

The shrinkage limit is calculated from:
where M1= initial wet mass of soil
M2 = final dry mass of soil
V1 = initial volume of soil
V2 = final volume of dry soil
@ =
AB − AC − B − C DE
AC
(3 − 9)

Soil pat before drying Soil pat after drying
M1= initial wet mass of soil
V1 = initial volume of soil
M2 = final dry mass of soil
V2 = final volume of dry soil

Another parameter that can be determined from a
shrinkage limit test is the shrinkage ratio, which is
the ratio of the volume change of soil as a
percentage of the dry volume to the corresponding
change in moisture content, or
Shrinkage ratio (SR)
@F =
AC
C ∗ DE
(3 − 10)

It can also be shown that
Shrinkage ratio (SR)
where: Gs = specific gravity of soil solids.
H =
1
1
@F
−
@
100
(3 − 11)

An A-line separates the inorganic clays from
the inorganic silts
Inorganic clay values lie above the A-line, and
values for inorganic silts lie below the A-line.
Organic silts plot in the same region (below the
A-line and with LL ranging from 30 to 50) as
the inorganic silts of medium compressibility.
Plasticity Chart

Organic clays plot in the same region as inorganic
silts of high compressibility (below the A-line and
LL greater than 50).
The information provided in the plasticity chart is
of great value and is the basis for the
classification of fine-grained soils in the Unified
Soil Classification System
The U-line is approximately the upper limit of the
relationship of the plasticity index to the liquid
limit for any currently known soil.
Plasticity Chart

0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100
LL
PI
Liquid Limit ( % )
PlasticityIndex
Plasticity Chart
Dr. Abdulmannan Orabi IUST
Plasticity Chart
57

LL
PI
PlasticityIndex
Inorganic clays of
high plasticity
Inorganic
clays of
medium
plasticity
Inorganic silts of
high compressibility
and organic clays
Inorganic
clays of low
plasticity
Inorganic silts of medium
compressibility and organic silts
Inorganic silts of low
compressibility
cohesionless
soil
Plasticity Chart

Fine – grained soils can exist in one of
four states: solid, semisolid, plastic, and
liquid.
Water is the agent that is responsible for
changing the states of soils.
A soil gets weaker if its water content
increases.
Plasticity Chart

Three limits are defined based on the water
content that causes a change of state.
The plasticity index defines the range of
water content for which the soil behaves like
a plastic material.
The liquidity index gives a measure of
strength.
The soil strength is lowest at the liquid state
and highest at the solid state.
Plasticity Chart

Lecture 3 consistncy of soil

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