ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY · This method is quite suitable for soft, fine grained soil. It cannot be used for stoney, gravelly soils. The method is practicable only

ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY

MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.

DEPARTMENT OF CIVIL ENGINEERING

Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – I 1

Geotechnical Engineering – I

B.E. FOURTH SEMESTER





UNIT – 2 Index Properties & Their Determination, Water content, specific gravity, sieve analysis, particle size

distribution curve, sedimentation analysis, Differential and free swell value, Consistency of soil,

Atterberge’s limits . Classification of Soil: Particle size classification, Textual classification, Unified

& I.S. classification system, field identification of Expansive soil, Swelling pressure.





2.1 INTRODUCTION

For a proper evaluation of the suitability of soil for use as foundation or construction

material, information about its properties, in addition to classification, is frequently necessary. Those

properties which help to assess the engineering behavior of a soil and which assist in determining its

classification accurately are termed “Index Properties”.

The tests required to determine index properties are in fact ‘classification tests’.

2.1.1 SOIL COLOUR

Colour of soil is one of the most obvious of its features. Soil colour may vary widely, ranging

from white through red to black. It mainly depends upon the mineral matter, quantity and nature of

organic matter and the amount of colouring oxides of iron and manganese, besides the degree of

oxidation.

2.1.2 PARTICLE SHAPE

Shape of individual soil grains is an important qualitative property. Individual particles are

frequently very irregular in shape, depending on the parent rock, the stage of weathering and the

agents of weathering. The particle shape of bulky grains may be described by terms such as

‘angular’, ‘sub-angular’, ‘sub-rounded’, ‘rounded’ and ‘well-rounded’. Silt particles rarely break

down to less that 2μ size (on μ = one micron = 0.001mm), because o t their mineralogical

composition.

Fig. 2.1 – Shapes of granular soil particles.

2.1.3 COHESIVE AND NON – COHESIVE SOILS

COHESIVE SOILS

1) Soils in which the adsorbed water and particle attraction act such that it deforms plastically at

varying water contents are known as cohesive soils or clays.

2) The cohesive property is due to presence of clay minerals in soils.

3) Clayey soils are cohesionless.





NON – COHESIVE SOILS

1) The soils composed of bulky grains are cohesionless regardless of the fineness of the particles.

2) The rock flour is cohesionless even when it has the particle size smaller than 2μ size.

3) Non-plastic silts and coarse-grained soils are cohesionless.

2.2 METHODS OF DETERMINING INSITU DENSITY

The following methods are generally used for the determination of mass density.

1) Core cutter method.

2) Sand replacement method.

3) Water balloon method.

4) Radiation method.

1) Core cutter method: -

It is a field method for determination of mass density. A core cutter consists of an open,

cylindrical barrel, with a hardened, sharp cutting edge as shown in fig.5. A dolly is placed over the

cutter and it is rammed into the soil. The dolly is to prevent burring of the edges of the cutter. The

cutter containing the soil is taken out of the ground. Any soil extruding above the edges of the cutter

is removed. The mass of the cutter filled with soil is taken. A representative sample is taken for water

content determination.

Fig.2.2Core cutter with dolly.

Bulk mass density =





Where,

M2 = mass of cutter, with soil,

M1 = mass of empty cutter.

V = volume of cutter.

This method is quite suitable for soft, fine grained soil. It cannot be used for stoney, gravelly

soils. The method is practicable only at the places where the surface of the soil is exposed and the

cutter can be easily driven.

2) Sand replacement method:-

Fig. 2.3 shows a sand pouring cylinder, which has a pouring cone at its base. The cylinder shown

is placed with its base at ground level. There is shutter between the cylinder and the cone. The

cylinder is first calibrated to determine the mass density of sand. For good results, the sand used

should be uniform, dry and clean, passing a 600 micron sieve and retained on a 300micron sieve.

(a) Calibration of apparatus:-

The cylinder is filled with sand and weighted. A calibrating container is then placed below the

cylinder and shutter is opened. The sand fills the calibrating container and cone. The shutter is

closed, and the mass of the cylinder is again taken. The mass of the sand in the container and the

cone is equal to the difference of the observations.

The pouring cylinder is again filled to the initial mass. The sand is allowed to run out of the

cylinder, equal to the volume of the calibrating container and the shutter is closed. The cylinder is

placed over a plain surface and the surface is opened. The sand runs out of the cylinder and fills the

cone. The shutter is closed when no further movement of the sand takes place. The cylinder is

removed and the sand filling the cone is collected and weighed (M2).

The mass density of the sand is determined as under:

Where, M1 = initial mass of the container with sand,

M2 = mass of sand in cone only,

M3 = mass of cylinder after pouring sand into the cone and the container.

Vc = Volume of the container.

(b) Measurement of Volume of Hole:-

A tray with a central hole is placed on the prepared ground surface which has been cleaned and

properly leveled. A hole about 100 mm diameter and 150 mm deep is excavated in the ground, using

the hole in the tray as a pattern. The soil removed is carefully collected and weighted.

The sand pouring cylinder is then placed over the excavated hole as shown in fig. 6. The shutter

is opened and the sand is filled in the cone and the hole. When the sand stops running out, the shutter





is closed. The cylinder is removed and weighed. The volume of the hole is determined from the mass

of sand filled in the hole and the unit mass density of sand.

Volume of hole =

Where, M1 = mass of cylinder and sand before pouring into the hole,

M2 = mass of sand in core only,

M4 = mass of cylinder after pouring sand into the hole,

ρs = mass density of sand, as found from calibration.

The bulk mass density of the in – situ soil is determined from the mass of soil excavated and

the volume of the hole.

The method is widely used for soils of various particle sizes, from fine – grained to coarse –

grained.

Fig. 2.3. Sand Replacement Method.

2.3 INDEX PROPERTIES & THEIR DETERMINATION

Index properties include those properties of soils which are used in their identification and

classification. These include the determination of: (i) water content. (ii) Specific gravity, (iii) particle

size distribution, (iv) consistency limits, (v) in-situ density and (vi) density index.





(1) WATER CONTENT

The water content of a soil sample can be determined by the following methods:

(a) Oven dried method:- This is the most accurate method of determining the water content, and is

therefore, used in the laboratory.

A clean non-corrodible container is taken and its mass is found with its lid, on a balance

accurate to 0.01 g. A specimen of the moist soil is placed in the container and the lid is replaced. The

mass of the container and the contents is determined. With the lid removed, the container is then

placed in the oven for drying. After drying, the container is removed from the oven and allowed to

cool in desiccators. The lid is then replaced, and the mass of container and the dry soil is found. The

water content is calculated from the following expression:

Where,

M1 = mass of container with lid.

M2 = mass of container with lid and wet soil.

M3 = mass of container with lid and dry soil.

(2) SPECIFIC GRAVITY

The specific gravity of soil solids is determined by: (i) a 50 ml density bottle, or (ii) a 500 ml

flask, or (iii) a pycnometer. The density bottle method is the most accurate, and is suitable for all

types of soils. The flask or pycnometer is used only for coarse grained soils. The density bottle

method is the standard method used in the laboratory. The mass M1 of the empty, dry, bottle (or flask

or pycnometer) is first taken. A sample of oven – dried soil, cooled in a desiccator, is put in the

bottle, and the mass M2 is taken. The bottle is then filled with distilled water (or kerosene) gradually,

removing the entrapped air either by applying vacuum or by shaking the bottle. The mass M2 of the

bottle, soil and water (full up to the top) is taken. Finally, the bottle is emptied completely and

thoroughly washed, and clean water (or kerosene) is filled to the top, and the mass M4 is taken.

Based on these four observations, the specific gravity can be computed as follows.





(i) Bot. Empty (ii) Bot + Dry Soil (ii) Bot. + Soil + (iv) Bot. + water

Mass M1 M2 water mass M3 mass M4

Fig. 2.4. Specific gravity computation.

From Fig. 7 (i) and (ii), dry mass Md of the soil is

Mass of water in (iii) = M3 – M2

Mass of water in (iv) = M4 – M1

Hence, mass of water having the same volume as that of soil

solids is

=

Now,

=

=





(3) PARTICLE SIZE DISTRIBUTION CURVE

The particle size distribution curve, also known as a gradation curve, represents the

distribution of particles of different sizes in the soil mass. The percentage finer N than a given size is

plotted as ordinate (on natural scale) and the particle size as abscissa (on log scale). In fig. 9 (a), the

particle size decreases from left to right, whereas in fig. 9 (b), i.e., the particle size increases from left

to right, which is also the usual convention.

The semi – log plot for the particle size distribution, as shown in fig.9, has the following

advantages over natural plots.

(1) The soils of equal uniformity exhibit the same shape, irrespective of the actual particle size.

(2) As the range of the particle sizes is very large, for better representation, a log scale is

required.

(i) SIEVE ANALYSIS

The soil is sieved through a set of sieves are generally made of spun brass and phosphor

bronze (or stainless steel) sieve cloth. According to IS: 1498 – 1970, the sieves are designated by the

size of square opening, in mm or microns (1 micron = 10-6 = 10-3 mm). Sieves of various sizes

ranging from 80 mm to 75 microns are available. The diameter of the sieve is generally between 15

to 20cm. The nest of sieves consists of Nos 4 (4.75 mm), 8 (2.36 mm), 16 (1.18 mm) 30 (600 μm),

50 (300 μm), 100 (150 μm), and 200 (75 μm).

The sieve analysis is carried out by sieving a known dry mass of sample through the nest of sieves

placed one below the other so that the openings decrease in size from the top sieve downwards, with

a pan at the bottom of the stack as shown in Fig. 8. The whole nest of sieves is given a horizontal

shaking for about 10 minutes (if required, more) till the mass of soil remaining on each sieve reaches

a constant value (the shaking can be done by hand or using a mechanical shaker, if available). The

amount of shaking required depends on the shape and number of particles.

(a) Dry sieve Analysis :-

The soil sample is taken in suitable quantity. The larger the particle size, the greater is the

quantity of soil required. The soil should be oven – dry. It should not contain any lump. If necessary,

it should be pulverized. If the soil contains organic matter, it can be taken air – dry instead of oven

dry.

The sample is sieved through a 4.75 mm IS Sieve. The portion retained on the sieve is the

gravel fraction or plus 4.75 mm material. The gravel fraction is sieved through the set of coarse

sieves manually or using a mechanical shaker. Hand sieving is normally done. The weight of soil

retained on each sieve is obtained. The minus 4.75 mm fraction is sieved through the set of fine

sieves. The sample is placed in the top sieve and the set of sieves is kept on a mechanical shaker and

the machine is started. Normally, 10 minutes of shaking is sufficient for most soils. The mass of soil





retained each sieve and on pan is obtained to the nearest 0.1 gm. The mass of the retained soil is

checked against the original mass. Dry sieve analysis is suitable for cohesionless soils.

Fig.2.5. Stacking of Sieves

(b) Wet Sieve Analysis: -

If the soil contains a substantial quantity (say, more than 5 %) of fine particles, a wet sieve

analysis is required. All lumps are broken into individual particles. A representative soil sample in

the required quantity is taken, using a riffler, and dried in an open. The dried sample is taken in a tray

and soaked with water. The sample is stirred and left for a soaking period of at least one hour. The

slurry is then sieved through a 4.75 mm IS sieve, and washed with a jet of water. The material

retained on the sieve is the gravel fraction. It is dried in an oven, and sieved through set of coarse

sieves.

The material passing through 4.75 mm sieve is sieved through a 75 μ sieve. The material is

washed until the wash water becomes clear. The material retained on the 75 μ sieve is collected and

dried in an oven. It is then sieved through the set of fine sieves of the size 2 mm, 1 mm, 600 μ, 425 μ,

212 μ, 150 μ, and 75 μ. The material retained on each sieve is collected and weighed. The material

that would have been retained on pan is equal to the total mass of soil minus the masses of material

retained on all sieves.





(ii) SEDIMENTATION ANALYSIS

In the wet mechanical analysis, or sedimentation analysis, the soil fraction, finer than 75

micron size is kept in suspension in a liquid (usually, water) medium. The analysis is based on

Stroke’s law according to which the velocity at which grains settle out of suspension, all other

factors being equal, is dependent upon the shape, weight and size of the grain. However, in the usual

analysis it is assumed that the soil particles are spherical and have the same specific gravity (the

average specific gravity). With this assumption, the coarser particles settle more quickly than the

finer ones. If ν is the terminal velocity of sinking of a spherical particle, it is given by,

or

where,

r = radius of the spherical particle (m)

D = diameter of the spherical particle (m)

ν = terminal velocity (m/sec)

s = unit weight of particles (kN/m3)

w = unit weight of water / liquid (kN/m3)

viscosity of water / liquid (kN – s / m2) =

The sedimentation analysis is done either with the help of a hydrometer or a pipette.

Stoke’s Law:-

The velocity, at which grains settle out of suspension, all other factors being equal, is

dependent upon the shape, weight and size of the grain. Hence larger the particle the settling velocity

is greater and vice versa.

The Stoke’s law states as,

Where,

V = Settling velocity cm/sec.

D = diameter of particle (mm)

g = Acceleration due to gravity.

G = Specific gravity.

η = Viscosity of water. (Poise).





Limitations:-

1) Soil particles are spherical.

2) Particles settle independent of other particles and the neighbouring particles do not have any

effect on its velocity of settlement.

3) The walls of jar, in which the suspension is kept, also do not effect the settlement.

4) Law remains valid for the range of 0.0002 mm to 0.2 mm in diameter.

Grading of Soils :-

The distribution of particles of different sizes in a soil mass is called grading. The grading of

soils can be determined from the particle size distribution curves. Fig.2.6 Shows the particle size

distribution curves of different soils.

Fig. 2.6. Particle size curve.

Fig. 2.7. Gradation of soils.





A curve with a hump, such as curve A, represents the soil in which some of the intermediate

size particles are missing. Such a soils is called gap – graded or skip – graded.

A flat S – curve, such as curve B, represents a soil which contains the particles of different

sizes in good proportion. Such a soil is called a well – graded (or uniformly graded) soil.

A steep curve, like C, indicates a soil containing the particles of almost the same size. Such

soils are known as uniform soils.

The particle size distribution curve also reveals whether a soil is coarse – grained or fine –

grained. In general, a curve situated higher up and to the left (curve D) indicates a relatively fine –

grained soil, whereas a curve situated to the right (curve E) indicates a coarse – grained soil.

The uniformity of a soil is expressed qualitatively by a term known as uniformity coefficient,

Cu, given by

Where,

D60 = particle size such that 60 % of the soil is finer than this size, and

D10 = particle size such that 10 % of the soil is finer than this size.

D10 size is also known as the effective size.

The general shape of the particle size distribution curve is described by another coefficient

known as the coefficient of curvature (Cc) or the coefficient of gradation (Cg).

Where D30 is the particle size corresponding to 30% finer.

For a well – graded soil, the value of the coefficient of curvature lies between 1 and 3.

(a) PIPETTE METHOD

In this method, 500 ml of soil suspension is required. All the quantities required for 1000 ml

of suspension are halved to get a 500 ml of suspension. The suspension is taken in a sedimentation

tube. Fig.11 shows a 10 ml capacity pipette used for extraction of the sample. The pipette is fitted

with a suction inlet.





Fig. 2.8. Pipette analysis.

(b) HYDROMETER METHOD

A hydrometer is an instrument used for the determination of the specific gravity of liquids.

As the specific gravity of the soil suspension depends upon the particle size, a hydrometer can be

used for the particle size analysis.

Fig. 2.9. Hydrometer analysis.

Calibration is done and chart is prepared before using the Hydrometer. 50 gms. of soil finer than 75μ

is taken and mixed in 1000 ml water in a sedimentation jar. To have proper dispersion of soil,

dispersing agent is added to the water. The jar is shaken vigorously and stop watch is started





immediately,=. Hydrometer is slowly inserted in the jar and readings are taken at 0.5, 1, 2, 4, 8, 15,

30 minutes ans 1, 2, 4 upto 24 hrs.

2.4 DIFFERENTIAL AND FREE SWELL VALUE

This is probably the simplest and the most useful method for identification and classification

of expansive soils. It has been widely accepted that the free swell test accompanied with Atterberg

limit tests can provide satisfactory tool for qualitative understanding of the expansivity of soil.

Holtz and Gibbs (1956) originally proposed this test in which 10cc of dry soil powder (425µ

fraction in soil) is poured gradually in 100cc distilled water contained in graduated glass cylinder.

The volume of soil after 24 hours of free swelling in water is observed and the free swell value

(FSV) is calculated as

The free swell value for bentonites (highly swelling soil, derived from chemical

decomposition of volcanic ash & weathering in arid regions & which is used extensively in civil

construction as drilling mud) is of the order of 1200 to 2000% .Illites and kaolinites have FSV of 30-

80% and 20-50 % respectively. However , the method suffers drawback, that the mass of soil in 10cc

soil volume may differ in various trials depending on the pulverization of soil samples and the

method of pouring, thus giving no consistency in the test results.

According to Holtz & Gibbs , soil with FSV ≥ 50% may show considerable volume change

under light loads when wetted and hence should be viewed with caution. Soil having FSV≤ 50% are

not normally expected to present any problem even under light surcharges.

2.5 CONSISTENCY OF SOILS

Consistency means the relative ease with which soil can be deformed. This term is mostly

used for fine grained soil for which the consistency is to a large extent to water content. Consistency

denotes the degree of firmness of the soil which may be termed as soft, firm, stiff, or hard .Fine

grained soil may be mixed with water to form a plastic paste which can be moulded into any form

by pressure. The addition of water reduces the cohesion making the soil still easier to mould. Further

addition of water reduces the cohesion until the material no large retains its shape under its own

weight, but flows as a liquid. Enough water may be added until the soil grains are dispersed in a

suspension. If water is evaporated from such a soil suspension, the soil passes through various stages

or states of consistency. In 1911, Swedish agriculturist Atterberg divided the entire range from liquid

to solid state into four stages





• the liquid state

• the plastic state

• semi-solid state

• the solid state.

He set arbitrary limits, knowns as consistency limits or Atterberg limits, for these divisions in

terms of water content. Thus , the consistency limits are the in terms of water content. Thus, the

consistency limits are the water contents at which the soil mass passes from one state to the next.

Fig.3.0 Variation of volume of soil mass with variation of water content.

The Atterberg limits which are most useful for engineering purpose are: liquid limits, plastic

limits and shrinkage limit. These limits are expressed as percent water content.





Liquid limits(wL):-

Liquid limit is the water content corresponding to the arbitrary limits between liquid and

plastic state of consistency of a soil. It is defined as the minimum water content at which the soil is

still in the liquid state, but has a small shearing strength against flowing which can be measured by

standard available means. With reference to the standard liquid limit device, it is defined as the

minimum water content at which a part of soil cut by a groove of standard dimensions, will flow

together for a distance of 12mm(half inch) under an impact of 25 blows in the device.

Plastic limit(wp):-

Plastic limit is the water content corresponding to an arbitrary limit between the plastic and

the semi-solid states of consistency of a soil. It is defined as the minimum water content at which a

soil will just begin to crumble when rolled into a thread approximately 3mm in diameter.

Shrinkage limit(ws):- Shrinkage limit is defined as the maximum water content at which a reduction

in water content will not cause a decrease in the volume of a soil mass. It is lowest water content at

which a soil can still be completely saturated.

Plastic index(Ip):-

The range of consistency within which a soil exhibits plastic properties is called plastic range

and is indicated by plasticity index is defined as the numerical, difference between the liquid limit

and the plastic limit of a soil:

Ip = wL –wP

In the case of sandy soil, plastic limit should be determined first. When plastic limit cannot be

determined. The plasticity index is reported as NP (non-plastic). When the plastic limit is equal to or

greater than the liquid limit, the plasticity index is reported as zero.

Plasticity:-

Plasticity is defined as that property of a soil which allows it to be deformed rapidly, without

rupture, without elastic rebound, and without volume change. According to Goldschmidt theory, the

plasticity is due to the presence of thin scale like particles which carry on their surface electro-

magnetic charges. Water molecules are bi-polar and orient themselves like tiny magnets in the

magnetic field next to the surface of the soil particles. Water becomes highly viscous near the

particle, but as the distance increases, the viscosity of water decreases until at some distance ordinary

water exists. When enough water is present the particles are separated by molasses-like water which

allows particles to slip past each other to new positions without any tendency to return to their

former positions, without changes in volume of voids, and without impairing the cohesion. The

correctness of theory for the cause of plasticity is evidenced by the fact that clay does not become

plastic when mixed with a liquid of non-polarizing molecules like kerosene.





Consistency Index(Ic):-

The consistency index or the relative consistency is defined as the ratio of the liquid limit

minus the natural water content to the plasticity index of a soil:

Ic= wL – w

IP

Where w is the natural water content of the soil

Consistency index is useful in the study of the field behavior of saturated fine grained soils. Thus ,

if the consistency index of a soil is equal to unity, it is at the plastic limit. Similarly, a soil with Ic

equal to zero is at its liquid limit. If Ic exceeds unity, the soil is in a semi-solid state and will be stiff.

A negative consistency index indicates that the soil has natural water content greater than the liquid

limit and hence behaves just like liquid.

Liquidity index (IL):-

The liquidity index or water –plasticity ratio is the ratio expressed as a percentage of the

natural water content of a soil minus its plastic limit, to its plasticity index:

IL = w - wp

IP

Where w is the natural water content of the soil.

2.5.1 DETERMINATION OF LIQUID AND PLASTIC LIMITS

The liquid limit is the water content at which the soil changes from the liquid state to the

plastic state. At the liquid limit, the clay is practically like a liquid, but possesses a small shearing

strength. The shearing strength at that stage is the smallest value that can be measured in the

laboratory. The liquid limit of soil depends upon the clay mineral present. The strong the surface

charge and the thinner the particle, the greater will be the amount of adsorbed water and, therefore

the higher will be the liquid limit.

The liquid limit is determined in the laboratory either by Casagrande’s apparatus or by cone

penetration method. The device used in Casagrande’s method consists of a brass cup which drops

through a height of 1cm on a hard base when operated by the handle. The device is operated by

turning the handle which raises the cup and lets it drop on the rubber base. The height of drop is

adjusted with the help of adjusting screws.

About 120gm of an air-dried sample passing through 425µ IS sieve is taken in dish and

mixed with distilled water to from a uniform paste. A portion of this paste is placed in the cup of the

liquid limit device, and the surface is smoothened and a leveled with a spatula to a maximum depth

of 1 cm. A groove is cut through the sample along the symmetrical axis of the cup, preferably in one

stroke , using a standard grooving tol. IS:2720-Part V recommends two types of grooving tools:





Casagrande tool 2) ASTM tool. (American society of testing material). Casagrande tool cuts a

groove of width 2mm at the bottom, 13.6mm at the top and 10mm deep. The Casagrande too; is

recommended for normal fine –grained soils, whereas the ASTM tool is recommended for sandy,

fine grained soil, in which the Casagrande tool tends to tear the soil in the groove.

After the soil pat has been cut by a proper grooving tool, the handle is turned at a rate of 2

revolution per second until the two parts of the soil sample come into contact at the bottom of the

groove along a distance of 12mm. The groove should close by a flow of the soil, and not by slippage

between the soil and the cup. When the groove closes by a flow, it indicates the failure of slopes

formed on the two sided of the groove.

The soil in the cup is again mixed, and the test is repeated until two consecutive tests give the

same number of blow. About 15gm of soil near the closed groove is taken for water content

determination.

The soil in the cup is transferred to the dish containing the soil pate and mixed thoroughly

after adding more water. The soil sample is again taken in the cup of the liquid limit device and the

test is repeated. The liquid limit is the water content at which the soil is sufficiently fluid to flow

when the device is given 25 blows. As it is difficult to get exactly 25 blows for the sample to flow,

the test is conducted at different water contents so as to get blows in the range of 10 to 40. A plot is

made between the water content as ordinate and the number of blows on log scale as abscissa. The

plot is approximately a straight line. The plot is known as flow curve. The liquid limit is obtained,

from the plot corresponding to 25 blows. The liquid limit is expressed as the nearest whole number.





Fig.3.1 Liquid limit apparatus.

The rapping in the liquid limit device cause small shearing stresses on the sample. The liquid

limit is arbitrarily taken as the water content when the soil has shear strength just sufficient to

withstand the shearing stresses induced in 25 blows. The shear strength of the soil at liquid limit is

about 2.7 kN/m2.

Fig.3.2 Flow curve.





2.5.2 PLASTIC LIMIT

Plastic limit is the water content below which the soil stops behaving as a plastic material. It

begins to crumble when rolled into a thread of soil of 3mm diameter. At this water content , the soil

loses its plasticity and passes to a semi-solid state.

For determination of the plastic limit of a soil, it is air dried and sieve through a 425µ IS

sieve. About 30gm of soil is taken in an evaporating dish. It is mixed thoroughly with distilled water

till it become plastic and can be easily mould with fingers.

About 10gm of the plastic soil mass is taken in one hand and a ball is formed. The ball is

rolled with finger on a glass pate to form a soil thread of uniform diameter. The rate of rolling is kept

about 80 to 90 strokes per minute. If the diameter of thread becomes smaller than 3mm, without

crack formation, it shows that the water content is more than the plastic limit. The soil is kneaded

further. This results in the reduction of the water content, as some water is evaporated due to the heat

of the hand. The soil is re-rolled and the procedure repeated till the thread crumbles. The water

content at which the soil can be rolled into a thread of approximately 3mm in diameter without

crumbling is known as the plastic limit.

The test is repeated taking a fresh sample each time. The plastic limit is taken as the average

of three values. The plastic limit is reported to the nearest whole number.

The shear strength at the plastic limit is about 100 times that at the liquid limit.

Fig.3.3 Determination of Plastic limit.

2.5.3 SHRINKAGE LIMIT

Shrinkage limit is the smallest water content at which the soil is saturated. It is also defined as

the maximum water content at which a reduction of water content will not cause a decrease in the

volume of the soil mass. In other words, at this water content, the shrinkage ceases. An expression

for the shrinkage limit can be obtained as given below

Fig (a) shows the block diagram of a soil sample when it is fully saturated and has the water

content greater than the expected shrinkage limit. Fig (b) shows the sample at shrinkage limit. Fig (c)

depicts the condition when the soil sample has been oven dried. The volume V3 in fig (c) is the same

as the total volume V2 in fig (b). The three fig indicate the stage I, II, III.





Fig.3.4 Stages of derivation of Shrinkage limit.

Let Ms be the mass of solids.

Mass of water in stage I =M1-Ms

Loss of mass of water from stage I to stage II =(V1-V2) ρw

Mass of water in stage II = (M1-Ms)-(V1-V2) ρw

From definition, shrinkage limit = water content in stage II

Or

Where w1 represent the water content in stage I

For determination of the shrinkage limit in the laboratory, about 50gm of soil passing at 425µ

sieve is taken and mixed with distilled water a creamy paste. The water content (w1) of the soil is

kept greater than the liquid limit.

A circular shrinkage dish, made of porcelain or stainless steel and having a diameter 30 to

40mm and a height of 15mm, is taken. The shrinkage dish has a flat bottom and has its internal

corners well rounded. The capacity of the shrinkage dish is first determined by filling it with

mercury. The shrinkage dish is placed in large porcelain evaporating dish and filled with mercury.

Excess mercury is removed by pressing a plain glass plate firmly over the top of the shrinkage dish.

The mass of mercury is the shrinkage dish is obtained by transferring the mercury into a mercury

weighing dish. The capacity of the shrinkage dish in ml is equal to the mass of mercury in gm

divided by the specific gravity of mercury.

The inside surface of the empty shrinkage dish is coated with a thin layer of Vaseline or

silicon grease. The mass of empty shrinkage dish is obtained accurately. The soil sample is placed in

the shrinkage dish, about one-third its capacity. The dish is tapped on a firm surface to ensure that no

air is entrapped. More soil is added and the tapping continued till the dish is completely filled with

soil. The excess soil is removed by striking off top surface with a straight edge. The mass of the





shrinkage dish with soil is taken to obtain the mass (M1) of the soil. The volume of the soil V1 is

equal to the capacity of the dish.

The soil in the shrinkage dish is allowed to dry in air until the colour of the soil pat turn light.

It is then dried in a oven them as of the shrinkage dish with dry soil is taken to obtain the mass of dry

soil Ms.

Fig.3.4 Determination of volume of dry pat.

For determination of the volume of the dry pat, a glass cup, about 50mm diameter and 25mm height,

is taken and placed in a large dish. The cup is filled with mercury. The excess mercury is removed by

pressing a glass plate with three prongs firmly over the top of the cup. Any mercury adhering on the

side of the cup is wiped off, and the cup full of mercury is transferred to another large dish.

The dry pat of the soil is removed from the shrinkage dish, and placed on the surface of the

mercury and submerged into it by pressing it with the glass plate having prongs.The mercury

displaced by the soil pat is transferred to a mercury weighing dish and weighed. The volume of the

mercury is determined from its mass and specific gravity. The volume of the dry pat Vd is equal to

the volume of the mercury displaced. Of course, the volume V2 in stage II is also equal to Vd.

The shrinkage limit of the soil is determined, using above equation from the measured values

of V1, V2, M1, & Ms.

2.5.4 SHRINKAGE PARAMETERS

The following parameters related with shrinkage limit are frequently used in soil engineering.

1) Shrinkage Index- the shrinkage index(Is) is the numerical difference between the plastic

limit(wp) and the shrinkage limit(ws)

2) Shrinkage Ratio- the shrinkage ratio (SR) is defined as the ratio of a given volume change

expressed as a percentage of dry volume, to the corresponding change in water content





Where V1 = Volume of soil mass at water content w1

V2= Volume of soil mass at water content w2

Vd= volume of dry soil mass

When the volume V2 is at the shrinkage limit,

3) Volumetric Shrinkage - The volumetric shrinkage (VS), or volumetric change, is defined as the

change in volume expressed as a percentage of the dry volume when the water content is reduced

from a given value of the shrinkage limit. Thus

4) Linear shrinkage - Linear shrinkage (LS) is defined as the change in length divided by the initial

length when the water content is reduced to the shrinkage limit. It is expressed as a percentage

and reported to the nearest whole number

The linear shrinkage can be determined in a laboratory (IS: 2720-Part XX). A soil sample

about 150 gm in mass and passing through a 425µ sieve is taken in a dish. It is mixed with distilled

water to form a smooth paste at water content greater than the liquid limit. The sample is placed is

placed in a brass mould,140mm long and with a semi- circular section of 25mm diameter.

The sample is allowed to dry slowly first in air and then in oven. The sample is cooled and its

length measured. The linear shrinkage is calculated using the following equation

2.5.5 PLASTICITY, LIQUIDITY AND CONSISTENCY INDEXES

1) Plasticity Index-plasticity index (Ip or PI) is the range of water content over which the soil

remain in the plastic state. It is equal to the difference between the liquid (wl) and the plastic limit

(wp). thus





When either wl or wp cannot be determined, the soil is non-plastic (NP). When the plastic limit is

greater than the liquid limit, the plasticity index is reported as zero.

2) Liquidity Index- Liquidity index (Il or LI) is defined as

where w= water content of the soil in natural condition

the liquidity index of a soil indicates the nearness of its water content to its liquid limit. When the

soil is at its liquid limit, its liquidity index is 100% and it behaves as a liquid. When the soil is at the

plastic limit, its liquidity index is zero. Negative values of the liquidity index indicate a water content

smaller than the plastic limit. The soil is then in a hard state.

The liquidity index is also known as Water-plasticity ratio

3) Consistency Index:-Consistency index (Ic, CI) is defined as

Where w = water content of the soil in natural condition.

The consistency index indicates the consistency (firmness) of a soil. It shows the nearness of the

water content of the soil to its plastic limit. A soil with a consistency index of zero is at the liquid

limit. It is extremely soft and has negligible shear strength. On the other hand, a soil at a water

content equal to the plastic limit has a consistency index of 100%, indicating that the soil is relatively

firm. A consistency index of greater than 100% shows that the soil is relatively strong, as it is the

semi-solid state. A negative value of consistency index is also possible, which indicates that the

water content is greater than the liquid limit

.

4) Flow Index:- Flow index (If) is the slope of the flow curve obtained between the number of

blows and the water content of determination of the liquid limit. Thus

Where N1= number of blows required at water content of w1.

N2= number of blows required at water content of w2.

The above equation can be written in the general form

The flow index can be determined from the flow curve from any two points. For convenience, the

number of blows N3 and N1 are taken corresponding to one log cycle i.e. N3/N1 =10 in that case





The flow index is the rate at which a soil mass loses its shear strength with an increase in water

content. Fig shows the curves of two soils (1) and (2). The soil (2) with a greater value of flow index

has a steeper slope and possesses lower shear strength as compared to soil (1) with a flatter slope. In

order to decrease the water content by the same amount, the soil with a steeper slope takes a smaller

number of blows, therefore has lower shear strength.

5) Toughness index:- toughness index (It) of a soil is defined as the ratio of the plasticity index (Ip)

to the flow index (If).

Thus

Toughness index of a soil is a measure of the shearing strength of the soil at the plastic limit. This

can be proved as under:

Let us assume that the flow curve is a straight line between the liquid limit and the plastic limit. As

the shearing resistance of the soil is directly proportional to the number of blows in Casagrande’s

device,

and

where

Nl = number of blows at the liquid limit when the shear strength is Sl

Np = number of blows at the plastic limit when the shear strength is Sp

k =constant.





2.6 CLASSIFICATION OF SOILS

The purpose of soil classification is to arrange various types of soils into groups according to

their engineering or agricultural properties and various other characteristics. From engineering point

of view, the classification may be done with the objective of finding the suitability of the soil for

construction of dams, highways of foundations, etc. For general engineering purposes, soils may be

classified by the following systems:-

i) Particle size classification.

ii) Textural classification,

iii) Highway Research Board (HRB) classification,

iv) Unified soil classification and IS classification system.

I) Particle Size Classification

The size of individual particles has an important influence on the behavior of soils. It is not

surprising that the first classification of soil was based on the particle size. It is a general practice to

classify the soils into four broad groups, namely gravel, sand, silt and clay size. While classifying the

fine grained soils on the basics of particle size, it is a good practice to write silt size and clay size and

not just silt and clay. In general usage, the term silt and clay are used to denote the soils that exhibit

plasticity and cohesion over a wide range of water content. The soil with clay-size particle may not

exhibits the properties associated with clays.

Any system of classification based only on particle size may be misleading for fine-grained soils.

The behaviors of such soils depend on the plasticity characteristics and not on the particle size.

However classification based on particle size is of immense value in the case of coarse-grained soils,

since the behavior of such soils depends mainly on the particle size.

Some of the classification system based on particle size alone is discussed below.

1) MIT System - MIT system of classification of soil was developed by Prof.G. Gilboy at

Massachusetts Institute of Technology in USA. In this system , the soil is divided into four group

i) Gravel, particle size greater than 2mm.

ii) Sand, particle size between 0.06mm to 2.0mm

iii) Silt size, particle size between 0.002mm to 0.06mm

iv) Clay size, particle size smaller than 0.002mm(2µ)

Boundaries between different types of soil correspond to limit when important changes occurs in the

soil properties. The particles less than 2µ size are generally colloidal fraction and behave as clay. The

soils with particle size smaller than 2µ are classified as clay size.

The naked eye can see the particle size of about 0.06mm and larger. The soils with particle size

smaller than 0.06mm but larger than 2µ are classified as silt-size. Important changes in the behavior

of soil occur if particle size is larger than 0.06mm when it behaves as cohesionless soil.





The boundary between gravel and sand is arbitrarily kept as 2mm.This is about the size of lead in the

pencil.

The soils in sand and silt-size range are further subdivided into three categories: coarse©, medium

(M) and fine (F), as shown in fig. it may be noted that MIT system uses only two integers 2 and 6

and is easy to remember

2) International Classification system:- The International Classification System was proposed for

general use at the International Soils Congress held as Washington in 1927. This classification

system was known as the Swedish classification system before it was adopted as International

system however, the system was not adopted by the United States.

In this system, in addition to sand, silt and clay a term mo has been used for soil particle in the

size range between sand and silt.

3) U.S. Bureau of Soil Classification:- this is one of the earliest classification system developed in

1895 by U.S. Bureau of Soil. In this system, the soil below the size 0.005 mm are classified as

clay size in contrast to 0.002mm size in other system. The soils between 0.005mm and 0.05mm

size are classified as silt size. Sandy soils between the size 0.05mm and 1.0mm are subdivided

into four categories as very fine , medium and coarse sands. Fine gravels are in the size range of

1.0 to 2.0mm.

Fig.3.5 Classification systems.





II) TEXTUAL CLASSIFICATION

Soils occurring in nature are composed of different percentage of sand, silt and clay size particles.

Soil classification of composite soils exclusively based on the particle size distribution is known as

textural classification. Probably the best known of this textural classification is the triangular

classification of U.S. public roads administration. The classification is based on the percentage of

sand, silt and clay sizes making up the soil. Such a classification is more suitable for describing

coarse grained soils rather than clay soils whose properties are less dependent on the particle size

distribution.

Fig.3.6 Textural classification chart

To use the chart, for the given percentages of the three constituents forming a soil, lines are

drawn parallel to the three sides of the equilateral triangle, as shown by arrows in the ’key’ .for

example, if a soil is composed of 30 % sand, 30 % silt sizes and 40% clay sizes, the three lines so

drawn intersect at the point P situated in the sector designated as ‘clay’. Such a soil will be termed as

‘clay’.





The textural classification system is useful for classifying soils consisting of different

constituents. The system assumes that the soil does not contain particles larger than 2.0mm size.

However, if the soil contain a certain percentage of soil particles larger than 2.0mm, a correction is

required in which the sum of the percentages of sand, silt and clay is increased to 100% .

In this system, the term loam is used to describe a mixture of sand, silt and clay particles in

various proportions. The term loam originated in agricultural engineering where the suitability of a

soil is judged for crops. The term is not used in soil engineering. In order to eliminate the term loam,

the term loam is replaced by soil engineering terms such as silty clay.

III) UNIFIED & I.S. CLASSIFICATION SYSTEM

The unified classification is based on the Airfield Classification system that was developed by A.

Casagrande. The system is based on both grain size and plasticity properties of the soil, and is

therefore applicable to any use. The unified Soil Classification (USC) system was adopted jointly by

the Corps of Engineers, U.S. Army, and the U.S. Bureau of reclamation in 1952. A description of the

modified USC system and as adopted by the IS (IS: 1498-1970: Classification and identification of

soils for general engineering purpose) is given below.

Divisions. Soils are broadly divided into three divisions:-

1. Coarse grained soils:-In these soils, more than half the total material by mass is large than 75

micron IS sieve size.

2. Fine grained soils: - In these soils, more than half the material by mass is smaller than 75micron

IS sieve size.

3. Highly organic soils and other miscellaneous soil materials:- These soils contain large % of

fibrous organic matter such as peat, and the particle of decomposed vegetation. In addition, certain

soils containing shells, concretions, cinkers and other non-soil materials in sufficient quantities are

also grouped in this division.

i) Coarse grained soils:- Coarse grained soils are further divided into two sub-divisions:

a) Gravels (G):- In these soils, more than half the coarse fraction(+75 microns) is larger than 4.75

mm IS sieve size. This sub-division includes gravel and gravelly soil, and is designated by symbol

G.

b) Sands (S):- In these soils more than half the coarse fraction (+75 microns) is smaller than

4.75mm IS sieve size. This sub- division includes sands and sandy soils.

Each of the above sub-divisions are further sub-divided into four groups depending upon grading

and inclusion of other materials:

W: Well graded, clean

C: Well graded with excellent clay binder

P: Poorly graded fairly clean





M: Containing Fine materials not covered in other groups.

This symbol used in combination designate the type of coarse grained soils. For example, GC

means clayey gravel.

ii) Fine grained soils:- Fine grained soils are further divided into three sub-divisions:

a) Inorganic silts and very fine sands: M

b) Inorganic clays: C

c) Organic silts and clays and organic matter : O

The fine grained soils are further divided into the following groups on the basic of the following

arbitrarily selected values of liquid limit which is a good index of compressibility:

i) Silts and clay of medium compressibility , having a liquid limit greater than 35 , and

represented by symbol I

ii) Silts and clays of medium compressibility, having a liquid limit greater than 35 and less than

50, and represented by symbol I

iii) Silts and clays of high compressibility, having a liquid limit greater than 50, and represented

by a symbol H.

Combination of these symbols indicates the type of fine grained soil. For example, ML means

inorganic silt with low to medium compressibility. Laboratory classification of fine grained soil is

done with the help of plasticity chart the A-line, dividing inorganic clay from silt and organic soil

has the following equation

Fig.3.7 Plasticity chart (IS soil classification system).





Majority of Indian Black cotton soils lie along a band above the A –line. The plot of some of the

black cotton soils is also found to lie below the A-line. Care should be taken in classifying such soils.

Some other inorganic clays, such as kaoline, behave as inorganic silts and usually lie below A-line

and should be classified as such (ML,MI, MH) although they are clays from mineralogical stand

point.

2.7 FIELD IDENTIFICATION OF EXPANSIVE SOIL, SWELLING PRESSURE.

2.7.1 EXPANSIVE SOILS

Various ways by which expansive soils can be identified are summarized below

a) Field observation: appearance of large scale shrinkage cracks at ground surface in summer and

formation of cracks of different patterns in light structure (building, walls) that are improperly

designed and constructed usually give the first indication and possibility of the existence of

swelling soil stratum.

b) Detection of montmorillonite clay mineral in soil:-

Swelling in soil is due to presence of montmorillonite clay mineral and hence if the type and

amount of clay mineral in a given soil is determined identification and classification of expansive

soil is precisely done. The differential thermal analysis (DTA), X-ray diffraction method &

electron micrography are the available technique for detection of clay mineral in soil. However

the equipment and instrumentation to be used in these methods are too sophisticated and costly

for their general use in civil engineering practice. Availability of such facilities is also rare.

2.7.2 GENERAL CHARACTERISTICS OF SWELLING SOILS

Swelling soils, which are clayey soils, are also called expansive soils. When these soils are

partially saturated, they increase in volume with the addition of water. They shrink greatly on

drying and develop cracks on the surface. These soils possess a high plasticity index. Black

cotton soils found in many parts of India belong to this category. Their colour varies from dark

grey to black. It is easy to recognize these soils in the field during either dry or wet seasons.

Shrinkage cracks are visible on the ground surface during dry seasons. The maximum width of

these cracks may be up to 20 mm or more and they travel deep into the ground. A lump of dry

black cotton soil requires a hammer to break. During rainy seasons, these soils become very

sticky and very difficult to traverse.

Expansive soil is mainly composed of sedimentary rocks such as shell, clay stone, lime stone,

marl rich in magnesium. Extreme weathering of these parent rocks will trigger the formation of

clay minerals that promotes the formation of expansive soil.

Studies have shown that this kind of expansive soil are often formed in the semi-arid region

where there are enough water available to weather the parent rock but not quite enough to wash

out the cations formed in that environment . The depths of these soils in some regions may be up





to 6 m or more. Normally the water table is met at great depths in these regions. As such the soils

become wet only during rainy seasons and are dry or partially saturated during the dry seasons.

In regions which have well-defined, alternately wet and dry seasons, these soils swell and shrink

in regular cycles. Since moisture change in the soils bring about severe movements of the mass,

any structure built on such soils experiences recurring cracking and progressive damage. If one

measures the water content of the expansive soils with respect to depth during dry and wet

seasons, the variation is similar to the one shown in Fig. 3.8.

Fig.3.8 Moisture content variation with depth below ground surface (Chen, 1988).

2.7.3 SWELLING PRESSURE

The swelling pressure (Ps) is defined as the pressure required to be applied over a

swelling over a swelling soil specimen to prevent its expansion when it is in contact with water. The

swelling pressure test is conducted in an oedometer. The test requires continuous adjustment of the

pressure on the soil specimen. Thus, the volume of the specimen remains equal to its initial volume

throughout the test. Details of the test are given in IS : 2720 (Part XII) – 1977.

The swelling pressure of a soil does not have a unique value because it depends upon a

number of factors such a initial moisture content, initial dry density, method of compaction, the

height of the specimen, the surcharge pressure, and others. Moreover, the actual swelling pressure of

a soil in the field is usually much less that measured in the laboratory.

If the swelling pressure of a soil is less than 30 kN/m2, it indicates the expansivenessis

low and the conventional design of shallow foundations can be adopted. For higher values of the

swelling pressure, special design for expansive soils, is to be used. Some types of clays, such as

bentonite, may have the swelling pressure of even 2000 kN/m2.

Documents

ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY · This method is quite suitable for soft, fine grained soil. It cannot be used for stoney, gravelly soils. The method is practicable only