Lecture 3 consistncy of soil

INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY

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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

Vol

ume

Water content

Semi-solid

Plastic LiquidSolid

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.

Vol

ume

Water content

Semi-solid

Plastic LiquidSolid

LLPLSL

PI

��

��

S = 100%


If we know the water content of our sample isrelative to the Atterberg limits, then we alreadyknow a great deal about the engineering responseof 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 soiland its strength, compressibility, andpermeability 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.


Wat

er c

onte

nt, (

%)

LL= 42 %

Number of Blows N

20 2530

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(butnotthattan3 �� "�&� �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 seconds Cone 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

Moi

stur

e C

onte

nt

100 20 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

Moi

stur

e C

onte

nt

100 20 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 Casagrandeplasticity 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 2µmA =

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 ( % )

Pla

sticity In

de

x

Plasticity Chart

Dr. Abdulmannan Orabi IUST

Plasticity Chart

57

LL

PI

Pla

stic

ity

Inde

x 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

cohesionlesssoil

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

Engineering

Transcript of Lecture 3 consistncy of soil