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GEOTECHNICAL ENGINEERING I (CE208)
MODULE II
INDEX PROPERTIES
The properties of soil which are not of primary interest to the geotechnical engineer but which are
indicative of the engineering properties are called index properties.
Simple properties which are required to determine the index properties are known as classification
tests.
The soils are classified and identified on the basis of index properties.
The main index properties of coarse grained soils are particle size and the relative density. For fine-
grained soils, the main index properties are Atterberg’s limits and the consistency.
The index properties of soils can be divided into two categories-
i) Soil grain properties – properties which are dependent on the individual grains of the soil and
independent of the manner of soil formation.
- Eg: size and shape of grains
ii) Soil aggregate properties – properties which are dependent on the soil mass as a whole.
- Influenced by soil stress history, mode of soil formation and the soil structure
GRAIN SHAPE
Engineering properties of soils, especially coarse-grained soils, depends upon the shape of particles.
- Though there is no universally accepted grain shape classification, the following general classification
can be considered:
i) Bulky grains – length, width and thickness are of same order of magnitude.
- Eg: sand and gravel soils
- Formed by physical disintegration of rocks. At this stage, bulky grains are angular.
During transportation, sharp edges may get worn out and the grains become rounded.
- In between the two extremes of angular and rounded shapes are sub-angular and sub-
rounded shapes.
ii) Flaky grains – one dimension of grain, namely thickness is very small compared to other two
lateral dimensions.
- Resembles a sheet of paper.
- Eg: clayey soils
iii) Needle- shaped grains – one dimension of the grain is fully developed and is much larger than
the other two. Eg: clay mineral “kaolinite”
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GRAIN-SIZE DISTRIBUTION
- It is an important soil grain property.
- It gives the percentage of various sizes of soil grains present in a given dry soil sample.
- Grain size analysis of coarse-grained soils is carried out by sieve analysis, whereas fine-grained soils
are analysed by sedimentation analysis.
SIEVE ANALYSIS
- For coarse grained soil (D>75 µ) (Gravel and sand)
- Sieves are wire screens having square openings.
- The sieves are designated by the size of square opening, in mm or microns (1 micron = 10 -6
m =
10 -3 mm)
- Sieves of various sizes ranging from 80 mm to 75 microns are available.
- Coarse sieve analysis – for the fraction of soil retained on 4.75 mm sieve ie. gravel fraction - use sieves
80,40,20,10,4.75 mm
- Fine sieve analysis- for the fraction of soil passing through 4.75 mm sieve ie. sand fraction - use sieves
2mm, 1 mm, 600µ, 425µ, 212 µ, 150µ and 75µ
- Sieves are stacked one over the other with decreasing size from top to the bottom.
- A receiver, pan, is placed at the bottom of the smallest sieve.
(a) Dry Sieve Analysis- suitable for cohesionless soi ls , with little ·or no fines.
1. The soil sample is taken in suitable quantity. The larger the particle size, the greater is the quantity
of soil required.
2. The soil should be oven-dry. It should not contain any lump.
3. The sample is sieved through a 4.75 mm IS sieve.
4. The portion retained on the sieve is the gravel fraction or plus 4. 75 mm material.
(b) Wet Sieve Analysis-
If the soil contains a substantial quantity (say, more than 5%) of fine particles, a wet sieve analysis is
required.
1. All lumps are broken into individual particles.
2. A representative soil sample in the required quantity is taken and dried in an oven.
3. The dried sample is taken in a tray and soaked with water.
5. The gravel fraction is sieved through the set of coarse
sieves manually or using a mechanical shaker. Hand
sieving is normally done.
6. The weight of soil retained on each sieve is obtained.
7. The minus 4.75 mm fraction is sieved through the set of
fine sieves using a mechanical shaker. The mass of soil
retained on each sieve and on pan is obtained to the
nearest 0.1 gm.
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4. 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.
5. 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.
6. 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.
7. 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 sum of the masses of material retained
on all sieves.
On the basis of the total weight of sample taken and the weight of soil retained on each sieve, the percentage of the total weight of soil
passing through each sieve (also termed as percent finer than) can be calculated as below:
% retained on a particular sieve = (weight of soil retained on that sieve/ total weight of soil taken) x 100
Cumulative % reatained = sum of % retained on all sieves of larger sizes and the % retained on that particular sieve.
Percentage finer than the sieve under reference = 100% - cumulative % retained
STOKE’S LAW
Soil particles finer than 75 µ size cannot be sieved. The particle size distribution of such
soils is determined by sedimentation analysis. The analysis is based on Stokes' law, which gives the
terminal velocity of a small sphere settling in a fluid of infinite extent. When a small sphere settles in a
fluid, its velocity first increases under the action of gravity, but the drag force comes into action, and
retards the velocity. After an initial adjustment period, steady conditions are attained and the velocity
becomes constant. The velocity attained is known as terminal velocity.
Terminal velocity, v = ((γs –γl) x D2)/ 18η
γs – unit weight of soil grains (g/cm3)
γl – unit weight of liquid (g/cm3)
η – viscosity of liquid (g-s/cm2)
D – Diameter of grain (cm)
γs = Gγw = G (γw = 1 g/cc)
v = ((G-1)/1800η) x D2 ; D in ‘mm’
at 20oC, the viscosity of water is approximately 0.01 poise. Taking an average value of specific gravity as
2.67, and substituting the values in the above equation,
v = 90.98 D2 ; D in ‘mm’ and v in cm/s
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SEDIMENTATION ANALYSIS
- Soil particles finer than 75 µ size cannot be sieved.
- The particle size distribution of such soils is determined by sedimentation analysis.
- The analysis is based on Stokes' law.
- Two methods; i) Hydrometer method
ii) Pipette method
Hydrometer analysis
- A hydrometer is an instrument used for the determination of the specific gravity of liquids.
- As specific gravity of the soil suspension depends upon the particle size, a hydrometer can
be used for particle size analysis.
- A special type of hydrometer with a long stem (neck) is used. The stem is marked from top to
bottom, generally in the range of 0.995 to 1.030.
- At t h e t i m e of commencement of sedimentation, t he specific gravity of suspension is uniform at
all depths.
- When the sedimentation takes place, the larger particles settle more deeper than the smaller ones. This
results in non-uniform specific gravity of the suspension at different depths. The lower layers of the
suspension have specific gravity greater than that of the upper layers.
- Casagrande has shown that the hydrometer measures the .specific gravity of suspension at
a point indicated by the centre of the immersed volume. If the volume of the stem is
neglected, the centre of the immersed volume of the hydrometer is the same as the centre of
the bulb.
- Thus, the hydrometer g i v e s the specific gravity of the suspension at the centre of the bulb.
(a) Calibration of hydrometer
- To determine the depth at which the specific gravity is measured, calibration of the hydrometer is
done.
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- The volume of the hydrometer, VH, is first determined by immersing it in a graduated cylinder
partly filled with water and noting down the volume due to the rise in water level.
- The depth of any layer A-A from the free surface B-B is the effective depth at which the specific
gravity measured by the hydrometer.
- As soon as the hydrometer is inserted in the jar, the layer of suspension which was at level A- A
rises to the level A' -A', and that at level B- B rise to the level B' -B'. The effective depth He is
given by
Where H = depth from the free surface B’- B' to the lowest mark on the stem,
h = height of bulb,
VH = volume of hydrometer,
A = cross-sectional area of jar.
- In Eqn it has been assumed that the rise in suspension level from A -A to A' -A' at the
centre of the bulb is equal to half the total rise due to the volume of the hydrometer.
Thus
- The markings on the hydrometer stem give the specific gravity of the suspension at the
centre of the bulb. The hydrometer readings are recorded after subtracting unity from the
value of the specific gravity and multiplying the remaining digit by 1000. Thus a specific
gravity of 1.015 is represented by a hydrometer reading Rh of (1.015 – 1.000) X 1000 = 15.
- The graduations on the right side of the stem directly give the reading Rh. As the effective depth
He depends upon the hydrometer reading Rh, a calibration chart can be obtained between the
hydrometer reading Rh and the effective depth He from the above Eq.
- An accurate scale is used to determine the height h and the depth H to various graduations. Fig
below shows a typical calibration chart.
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Fig. Calibration Chart
- As the sedimentation progresses, the specific gravity of the suspension decreases and the hydrometer goes deeper, and the effective depth increases. The hydrometer reading Rh, of course, decreases.
(b) Test Procedure - 50 g of dry soil passing through 75µ is taken.
- Add 100 ml water to make a paste.
- Add 100 ml deflocculating agent (sodium oxalate) and mix thoroughly.
- Add some water and stir the mixture in the mechanical stirrer for about 10 minutes.
- After stirring, the suspension is washed into a 1000 ml jar and water is added to it to bring
the level to 1000 ml mark.
- The suspension i s mixed thoroughly by placing a b u n g (or t he palm of a hand) on the open
end of the jar upside down and back a few times.
- The jar is then placed on a table, and a stop watch is started.
- The hydrometer is inserted in the suspension and the first reading is taken after ½
minute of the commencement of the sedimentation.
- Further readings are taken after one minute, two minutes and four minutes of the
commencement of the sedimentation.
- The hydrometer is then removed from the jar and rinsed with distilled water and floated
in a comparison cylinder containing distilled water with the dispersing agent added to the same
concentration as in the soil suspension.
- Further readings are taken after 8, 15 and 30 minutes and 1, 2, 4, 8 and 24 hours reckoned
from beginning of sedimentation.
- For each of these readings, the hydrometer is inserted about 20 seconds before the reading.
The hydrometer is taken out after the reading and floated in the comparison cylinder.
Fig. Downward movement of hydrometer
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(c) Corrections of Hydrometer Reading
The hydrometer readings are corrected as under:
i) Meniscus correction - Since the suspension is opaque, the observations are taken at the
top of meniscus. The meniscus correction is equal to the rending between the top of the
meniscus and the level of the suspension. As t h e marking on the stem increases downward, the
correction is positive.
The meniscus correction (C,,,) is determined from the readings at the top and bottom of
meniscus in comparison cylinder. The meniscus correction is constant for a hydrometer.
If Rh.' is the hydrometer reading of the suspension at a particular time, the corrected reading is given by
The corrected hydrometer reading (Rh) is required for determining the effective depth from the
calibration chart.
ii) Temperature Correction - The hydrometer is generally calibrated at 27°C. If the temperature of
suspension is different from 27°C, a temperature correction (Ct) is required for the hydrometer
reading. If temperature is more than 27°C, the suspension is lighter, and the actual reading will be
less than the corrected reading. The temperature correction is positive. On the other hand, if the
temperature is less than 27°C, the temperature correction is negative.
The temperature correction is obtained from the charts supplied by the manufacturer.
iii) Dispersion agent Correction- Addition of the dispersing agent to the soil specimen causes
increase in the specific gravity of the suspension. Therefore, the dispersing agent correction
is always negative. The dispersing agent correction (Cd) can be determined by noting the
hydrometer reading in clear water and again in the same water after adding the dispersing agent.
Thus the corrected reading R can be obtained from the observed reading Rh’ as under.
Percentage finer can be determined by
Diameter of particle can be determined by
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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).
- The semi-log plot for the particle size distribution 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.
GRADING OF SOILS
- The distribution of particles of different sizes in a soil mass is called grading.
- Grading of soils can be determined from _the particle size distribution curves.
- Fig shows the particle distribution curves of different soils.
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- A curve with a hump, such as curve A, represents the soil in which some of
the intermediate particles are missing. Such a soil 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.
- 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 larger the numerical value of cu, the more is the range of particles.
- Soils with a value of Cu less than 2 are uniform soils.
- Sands with a value of Cu of 6 or more are well-graded.
- Gravels with a value of Cu of 4 or more are well-graded.
-
- 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 (C8).
where D3O is the particle size corresponding to 30% finer.
- Cc should be 1 to 3 for a well-graded soil.
RELATIVE DENSITY
The relative density is defined as
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where emax = maximum void ratio of the soil in the loosest condition.
emin = minimum void ratio of the soil in the densest condition. e = void ratio in the natural state
The relative density of a soil indicates how it would behave under loads. If the deposit is dense, it can
take heavy loads with very little settlements. Depending upon the relative density, the soils are
generally divided into 5 categories.
DETERMINATION OF RELATIVE DENSITY
Figs (a), (b) and (c) show the soil in the densest, natural and loosest states. As it is difficult to
measure the void ratio directly. Eq. 3.20 cannot be used. However, it is convenient to express the void ratio in
terms of dry density (ρd),
Representing the dry density in the loosest, densest and natural conditions as ρm i n , ρm a x and ρ d , Eq becomes
This Eq is used to determine the relative density of an in-situ deposit.
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CONSISTENCY OF CLAYS
- The consistency of a fine-grained soil is the physical state in which it exists.
- It is used to denote the degree of firmness of a soil.
- Consistency of a soil is indicated by such terms as soft, firm or hard.
- The physical properties of clays are considerably influenced by the amount of water present in them.
- Depending upon the water content, the following four stages or states of consistency are used to describe
the consistency of a clay soil:
i) Liquid state
ii) Plastic state
iii) Semi-solid state
iv) Solid state
ATTERBERG LIMITS
- Also called consistency limits.
- Consistency limits are very important index properties of fine-grained soils.
- The water content at which the soil change from one state to other is known as consistency limits or
Atterberg limits.
- The consistency limits are
i) Liquid limit
ii) Plastic limit
iii) Shrinkage limit
- A soil containing high water content is in a liquid state. It offers no shearing resistance and
can flow like liquids.
- As the water content is reduced the soil becomes stiffer and starts developing resistance to shear
deformation. At some particular water content, the soil becomes plastic. The water content at which the
soil changes from the liquid state to the plastic state is known as liquid limit (LL, Wl)
- As the water content is reduced, plasticity of the soil decreases. Ultimately, the soil passes from the
plastic state to the semi-solid state. The water content at which the soil changes from the plastic state
to the semi-solid state is known as plastic limit (PL, Wp).
- Upto the semi-solid state, the soil remains fully saturated and any reduction in the volume of water will
result an almost equal reduction in the volume of the soil mass.
- A further reduction in the water content brings about a state when with a decrease in moisture, the
volume of the soil mass does not decrease any further but remains the same; the sample changes from
the semi-solid to the solid state.
- The water content at which the soil changes from the semi-solid state to the solid state is knoryn as the
shrinkage limit (SL, Ws).
- Below the shrinkage limit, the sample begins to dry up at the surface and the soil is no longer fully
saturated.
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i) Determination of 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 1 cm on a hard base when operated by the handle.
- About 120 gm of an air-dried sample passing through 425 µ IS 'sieve is taken in a dish and mixed
with distilled water to form a uniform paste.
- A portion of this paste is placed in the cup of the liquid limit device, and the surface is
smoothened and levelled with a spatula to a maximum depth of 1 cm.
- A grooving tool is used to cut a groove in the pat of soil placed in the cup. IS 2720 Part V recommends
two types of grooving tools: (1) Casagrande tool (2) ASTM tool. The Casagrande tool cuts a groove of
width 2 mm at the bottom, 11 mm at the top and 8 mm deep. The ASTM tool cuts a groove of width 2
mm at the bottom, 13.6 mm at the top and 10 mm deep. The Casagrande tool is recommended for
normal fine grained soils, whereas the ASTM tool is recommended for sandy, fine grained soils.
- After the soil pat has been cut by a proper grooving tool, the handle is turned at a rate of 2 revolutions
per second until the two parts of the soil sample come into contact at the bottom of the groove along a
distance of 13 mm.
- About 15 gm of soil near the closed groove is taken for water content determination.
- Repeat the procedure 4 to 5 times with different water content.
- Plot a graph between number of blows, N on a log scale and water content, w on natural scale. It will be
seen that the semi-logarithmic plot is a straight line called the flow curve.
- The liquid limit is determined by reading the water content corresponding to 25 blows on the flow curve.
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One-point Method
- Attempts have been made to simplify the trial and error procedure of the determination of liquid limit
described above. One such is the ‘One-point method’ which aims at determining the liquid limit with
just one reading of the number of the blows and the corresponding moisture content.
- The water content wN of the soil of the accepted trial shall be calculated. The liquid limit wL of the soil
shall be calculated by the following relationship.
Where N = number of drops required to close the groove at the moisture content wN. Preliminary work
indicates that x = 0.092 for soils with liquid limit less than 50% and x = 0.120 for soils with liquid limit
more than 50%.
ii) Determination of Plastic Limit
- It 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,
1. Sieve the given sample of soil through 425 micron IS Sieve.
2. Take 50 g of soil sample and mix it with water till the soil becomes plastic enough to be easily moulded
with fingers.
3. Prepare the ball of uniform diameter of the above sample.
4. Roll it on glass plate with just sufficient finger pressure.
5. Continue the rolling operation till the thread is of 3 mm diameter. Compare the diameter of thread at
intervals with the given rod.
6. Again press the soil and roll it.
7. Continue the above process till the threads show sign of crumble, thus making the soil unable for further
rolling.
- The water content at which the soil can be rolled into a thread of 3 mm in diameter without crumbling is
known as the plastic limit.
- The plastic limit is taken as the average of three values.
iii) Determination of Shrinkage Limit
- Fig. (a) shows the block diagram of a soil sample when it is fully saturated and has the water 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 ovendried
Fig. stages for derivation of shrinkage limit
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- The total volume V3 in Fig.(c) is the same as total volume V2 in Fig. (b). The three figures indicate, respectively, stage I, II and III.
where w1 represents the water content in stage I.
- For determination of the shrinkage limit in the laboratory,
i) about 50 gm of soil passing a 425 µ sieve is taken and mixed with distilled water to
make a creamy paste.
ii) The water content (w1) of the soil is kept greater than the liquid limit.
iii) circular shrinkage dish, made of porcelain or stainless steel and having a diameter 30
to 40 mm and a height of 15mm, is taken. The shrinkage dish has a flat bottom and has
its internal comers well rounded.
iv) The capacity of the shrinkage dish is first determined by filling it with mercury. The
capacity of the shrinkage dish in ml is equal to the mass of mercury in gm divided by
the specific gravity of mercury.
v) T h e m a s s of empty shrinkage dish is obtained accurately.
vi) The soil sample is placed in the shrinkage dish, one-third its capacity. The dish is
tapped on a firm surface to ensure that no air is entrapped. More soil and the tapping
continued till the dish is completely filled with soil. The excess soil is removed by
striking off the top surface with a straight edge.
vii) The mass of the shrinkage dish with soil is taken to obtain the mass (M1) of the soil.
viii) The volume of the soil V is equal to the capacity of the dish.
ix) The soil in the shrinkage dish is allowed to dry in air until the colour of the soil pat
turns light. It is then dried in an oven.
x) The mass of the shrinkage dish with dry soil is taken to obtain the mass of dry soil
Ms.
xi) For the determination of the volume of the dry pat, a glass cup, about 50 mm diameter
and 25 mm height, is taken and placed in a large dish.
xii) 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.
xiii) The cup full of mercury is transferred to another large dish.
xiv) The dry pat of the soil is removed from the shrinkage dish, and placed on the surface
of the mercury in the cup merged into it by pressing it with the glass plate having
prongs.
xv) The volume of the mercury is determined from its mass and specific gravity.
xvi) The volume of the dry pat Vd is equal to the volume of the meu displaced.
1
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xvii) The shrinkage limit of the soil is determined using the Eq, from the measured values of V1, V2,
M1 and Ms.
Fig. determination of volume of dry pat
SHRINKAGE PARAMETERS
i) Shrinkage Index - It is defined as the difference between the liquid and shrinkage limits of a soil.
Is = wL – wS
ii) Shrinkage Ratio – It is defined as the ratio of a given volume change expressed as a percentage of dry
vorume,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,
Another expression for shrinkage ratio (SR) can be found, by expressing the water content
iii) Volumetric Shrinkage – It 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 to the
shrinkage limit.
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iv) Linear Shrinkage- It is defined as the change in length divided by the initial length when the water
content is reduced to the shrinkage limit.
ATTERBERG INDEX
i) Plasticity Index (PI)- It is the range of water content over which the soil remains in the plastic state. It
is equal to the difference between the liquid (wL) and the plastic limit (wP).
- It indicates degree of plasticity
- Greater the P.I, greater is the plasticity
ii) Liquidity Index (LI or IL) – It is defined as
where w is the water content of soil in natural condition
- Liquidity index of a soil indicates its nearness of its water content to liquid limit
- When soil is at LL, liquidity index is 100% and behaves as liquid
- When soil is at PL, liquidity index is 0
iii) Consistency index (CI or IC) or Relative consistency – It is defined as
- Indicates consistency of soil
- Consistency index of a soil indicates its nearness of its water content to the plastic limit.
- If it is zero, soil is at LL
- If it is 100%, soil is at PL
iv) Flow index, IF – It is the slope of the flow curve obtained between the number of blows and the water
content in Casagrande's method of determination of the liquid limit.
Where N1 is the number of blows required at water content, w1
N2 is the number of blows required at water content, w2
v) Toughness Index, (TI or IT) – It is defined as the ratio of plasticity index to the flow index.
APPLICATIONS OF CONSISTENCY LIMITS
- It is an index property of fine-grained soils
- LL and PL depend upon the type and amount of clay in a soil.
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- PI mainly depends on the amount of clay. It is a measure of amount of clay in a soil.
- The PI of a soil is a measure of fineness of particles.
- Study of PI in combination with LL gives information about the type of clay
- LL of a soil is an indicator of the compressibility of a soil. Compressibility of a soil increases with an
increase in LL
- SI is directly proportional to the percentage of clay-size fraction present in the soil. It can be used as an
indicator for the amount of clay.
- TI is a measure of the shearing strength of the soil at its plastic limit.
SOIL CLASSIFICATION
- Soil classification is the arrangement of soils into different groups such that the soils in a particular
group have similar behaviour.
- If the classification of a soil has been done according to some standard classification system, its
properties and behaviour can be estimated based on the experience gained from similar soils elsewhere.
- A classification system thus provides a common language between engineers dealing with soils.
INDIAN STANDARD CLASSIFICATION OF SOILS
- Indian Standard Classification (ISC) system adopted by Bureau of Indian Standards is in many respects
similar to the Unified Soil Classification (USC) system.
- However, there is one basic difference in the classification of fine-grained soils. The fine- grained soils
in ISC system are subdivided into three categories of low, medium and high compressibility instead of
two categories of low and high compressibility
- Soils are divided into three broad divisions:
(1) Coarse-grained soils, when 50% or more of the total material by weight is retained on 75 micron sieve.
(2) Fine-grained soils, when more than 5O% of the total material passes 75 micron IS sieve.
(3) If the soil is highly organic and contains a large percentage of organic matter and particles
decomposed vegetation, it is kept in a separate category marked as peat (Pt).
- ISC system classifies the soils into 18 groups: 8 groups of coarse- grained, 9 groups of fine-grained
and one of peat.
- Basic soil components are given in Table.
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1. Coarse-grained soils- they are subdivided into gravel and sand. The soil is termed gravel (G) when more than
50% of coarse fraction is retained on 4.75mm IS sieve, and termed sand (S) if more than 50% of the coarse
fraction is smaller than 4.75 mm ISD sieve.
2. Fine-grained soils- they are further divided into three subdivisions, depending upon the values of the liquid
limit:
(a) Silts and clays of low compressibility- LL less than 35 (represented by symbol L)
(b) Silts and clays of medium compressibility – LL greater than 35 but less than 50 (represented by symbol I)
(c) Silts and clays of high compressibility – LL greater than 50 (represented by symbol H)
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Classification of coarse-grained soils (ISC system)
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Classification of fine-grained soils (ISC system)
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WATER CONTENT DETERMINATION
The following methods will be adopted:
i) Oven- drying method – mostly commonly adopted and simplest laboratory method.
- This method basically consists of drying a weighed moist sample of a soil in an oven at a
controlled temperature (105oC – 110
oC) for a period of 24 hours after which the dry weight of
sample is taken.
- The observations are:
Weight of an empty container = W1
Weight of container + wet soil = W2
Weight of container + dry soil = W3
- The calculations are as follows:
Weight of dry soil = W3 – W1
Weight of water in the soil = W2 – W3
ii) Pycnometer method - This method may be used when the specific gravity of solids is known.
- This is a relatively quick method and is considered suitable for coarse-grained soils only.
- A pycnometer is a glass jar of about 1 litre capacity and fitted with a brass conical cap. The cap
has a small hole of 6 mm diameter at its apex.
- The following are the steps involved:
(i) The weight of the empty pycnometer with its cap and washer is found (W1).
(ii) The wet soil sample is placed in the pycnometer (upto about 1/4 to 1/3 of the volume) and its
weight is obtained (W2).
(iii) The pycnometer is gradually filled with water, stirring and mixing thoroughly with a glass
rod, such that water comes flush with the hole in the conical cap. The pycnometer is dried on the
outside with a cloth and its weight is obtained (W3).
(iv) The pycnometer is emptied and cleaned thoroughly; it is filled with water upto the hole in
the conical cap, and its weight is obtained (W4).
Weight of water in the soil = (W2-W1) – Ws
If from W3, weight of solids Ws could be removed and replaced by the weight of an equivalent
volume of water, the weight W4 would be obtained.
W4 = W3 – Ws + (Ws/ G.γw) x γw Ws = (W3-W4) (G/(G-1))
The water content of the soil sample may be calculated as follows:
W = Ww/Ws
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iii) Sand bath method – Field method. Rapid, but not very accurate.
- A sand bath is a large, open vessel containing sand filled to a depth of 3 cm or more.
- In this method, wet soil is put in a container and dried by placing it on a sand bath.
- Sand bath is heated over a kerosene stove.
- Weight of wet soil and dried soil is noted
- Water content is determined by using equation used in the oven-drying method.
iv) Rapid moisture meter method – it is a portable equipment
- This method is on the principle that calcium carbide, introduced in the weighed quantity of
sample reacts with water in the sample and releases acetylene gas.
- Water content of soil sample is determined indirectly from the pressure of acetylene gas
formed.
v) Alcohol method – Soil is mixed with methylated spirit (alcohol).
- Wet weight is noted.
- Alcohol is then ignited.
- After completely burned, dry soil is weighed.
IN-SITU UNIT WEIGHT DETERMINATION
Two important methods for the determination of the in-situ unit weight are being given:
(i) Sand-replacement method - The principle of the sand replacement method consists in obtaining the volume
of the soil excavated by filling in the hole in-situ from which it is excavated, with sand, previously calibrated
for its unit weight, and thereafter determining the weight of the sand required to fill the hole.
The procedure consists of calibration of the cylinder and later, the measurement of the
unit weight of the soil.
(a) Calibration of the Cylinder and Sand: This consists in obtaining the weight of sand required to fill the
pouring cone of the cylinder and the bulk unit weight of the sand.
- Uniformly graded, dry, clean sand is used. The cylinder is filled with sand almost to be top and the
weight of the cylinder with the sand is taken (W1).
- The sand is run out of the cylinder into the conical portion by pulling out the shutter. When no further
sand runs out, the shutter is closed. The weight of the cylinder with the remaining sand is found (W2).
- The cylinder is placed centrally above the calibrating container such that the bottom of the conical
portion coincides with the top of the container. There sand is allowed to run into the container as well as
the conical portion until both are filled, as indicated by the fact that no further sand runs out; then the
shutter is closed.
- The weight of the cylinder with the remaining sand is found (W3).
- The weight of the sand filling the calibrating container (Wcc) may be found by deducting the weight of
sand filling the conical portion (Wc) from the weight of sand filling this and the container (W2 – W3).
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- Since the volume of the cylindrical calibrating container (Vcc) is known precisely from its dimensions,
the unit weight of the sand may be obtained by dividing the weight Wcc, by the volume Vcc.
- The observations and calculations relating to this calibration part of the work will be as
follows:
Initial weight of cylinder + sand = W1
Weight of cylinder + said, after running sand into the conical portion = W2
∴ Weight of sand occupying conical portion, Wc = (W1 – W2)
Weight of cylinder + sand, after running sand into the conical portion and calibrating container = W3
∴ Weight of sand occupying conical portion and calibrating container = (W2 – W3)
∴ Weight of sand filling the calibrating container,
Wcc = (W2 – W3) – Wc
= (W2 – W3) – (W1 – W2)
= (2W2 – W1 – W3)
Volume of the calibrating container = Vcc
∴ Unit weight of the sand:
(b) Measurement of Unit Weight of the Soil: The site at which the in-situ unit weight is to be determined is
cleaned and levelled.
- A test hole, about 10 cm diameter and for about the depth of the calibrating container (15 cm), is made
at the site, the excavated soil is collected and its weight is found (W).
- The sand pouring cylinder is filled with sand to about 3/4 capacity and is placed over the hole, after
having determined its initial weight with sand (W4), and the sand is allowed to run into it.
- The shutter is closed when not further movement of sand takes place.
- The weight of the cylinder and remaining sand is found (W5).
- The weight of the sand occupying the test hole and the conical portion will be equal to (W4 – W5).
- The weight of the sand occupying the test hole, Ws, will be obtained by deducting the weight of the sand
occupying the conical portion, Wc, from this value.
- The volume of the test hole, V, is then got by dividing the weight, Ws, by the unit weight of the sand.
- The in-situ unit weight of the soil, γ, is then obtained by dividing the weight of the soil, W, by its
volume, V.
- If the moisture content, w, is also determined, the dry unit weight of the soil, γd, is obtained as γ/(1+w).
(ii) Core-cutter method - The apparatus consists of a mild steel-cutting ring with a dolly to fit its top and a
metal rammer.
- The core-cutter is 10 cm in diameter and 12.5 cm in length. The dolly is 2.5 cm
long. The bottom 1 cm of the ring is sharpened into a cutting edge.
- The empty weight (W1) of the corecutter is found.
- The core-cutter with the dolly is rammed into the soil with the aid of a 14-cm
diameter metal rammer.
- The ramming is stopped when the top of the dolly reaches almost the surface of
the soil.
- The soil around the cutter is excavated to remove the cutter and dolly full of
soil, from the ground.
- The dolly is also removed later, and the soil is carefully trimmed level with the
top and bottom of the core-cutter.
- The weight of the core-cutter and the soil is found (W2).
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- The weight of the soil in the core-cutter, W, is then got as (W2 – W1). The volume of this soil is the same
as that of the internal volume of the cutter, V, which is known.
- The in-situ unit weight of the soil, γ, is given by W/V. If the moisture content, w, is also found, the dry-
unit weight, γd, may be found as γd = γ/(1 + w).
SPECIFIC GRAVITY OF SOLIDS DETERMINATION
- Pycnometer is used for the determination of specific gravity of solids.
- First, the weight of the empty pycnometer is determined (W1) in the dry condition.
- Then the sample of oven-dried soil, cooled in the desiccator, is placed in the pycnometer and its weight
with the soil is determined (W2).
- The remaining volume of the pycnometer is then gradually filled with distilled water. The entrapped air
should be removed either by gentle heating and vigorous shaking or by applying vacuum.
- The weight of the pycnometer, soil and water is obtained (W3) carefully.
- Lastly, the bottle is emptied, thoroughly cleaned and filled with distilled water, and its weight taken
(W4).
- From the readings, the wt of solids Ws = W2 – W1,
Wt of water = W3 – W2,
Wt of distilled water = W4 – W1
∴ Weight of water having the same volume as that of soil solids = (W4 – W1) – (W3 – W2).
- By definition, and by Archimedes’ principle,
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