This document discusses methods for classifying soils based on particle size analysis. It describes separating soils into gravel, sand, silt and clay fractions based on particle diameter size ranges. It presents equations for calculating uniformity coefficient (Cu) and curvature coefficient (Cc) to characterize soil gradation. It also summarizes the process of hydrometer analysis for determining soil particle size distribution and provides the Stokes' law equation for calculating particle settling velocity in suspension. Key criteria are outlined for classifying gravels and sands as well as fine-grained soils based on liquid limit, plasticity index and other properties in accordance with standardized soil classification systems.

1. UNIT1
CHAPTER 3

2.  For classifying the soil, we can do particle
size analysis

4. Gravel ... 80 mm to 4.75 mm
Sand ... 4.75 mm to 0.075 mm
Silt ... 0.075 mm to 0.002 mm
Clay ... Less than 0.002 mm

10.  The uniformity of a soil is expressed
qualitatively by a term known as uniformity
co efficient.
Cu =
𝑫𝟔𝟎
𝑫𝟏𝟎
 Where D60 is the particle size such that, 60% of
soil is finer than this size.
 D10 is the particle size such that, 10% f soil is
finer than this size. It is called as effective size.

11.  The general shape of the particle size
distribution curve is defined by CC.
CC =
D30
2
𝐷𝟔𝟎
𝑋 𝐷10
 Where, D30 is the particle size such that, 30% f
soil is finer than this size.
 For a well graded soil, Cc is 1 to 3 range.
 It may be noted that the gap grading of the soil
cannot be detected by Cu only. The value of Cc
is also required to detect it.

12.  Soil gradation is a classification of a coarse-
grained soil that ranks the soil based on the
different particle sizes contained in the soil.
 it is an indicator of other engineering properties
such as compressibility, shear strength, and
hydraulic conductivity.
 A poorly graded soil will have better drainage
than a well graded soil.
 Poorly graded soils are more susceptible to soil
liquefaction than well graded soils.
 When a fill material is being selected for a
project such as a highway embankment or
earthen dam, the soil gradation is considered. A
well graded soil is able to be compacted more
than a poorly graded soil.

13.  The following criteria are in accordance with the
Unified Soil Classification System:
 For a gravel to be classified as well graded, the
following criteria must be met:
Cu > 4 & 1 < Cc < 3
 If both of these criteria are not met, the gravel is
classified as poorly graded or GP. If both of these
criteria are met, the gravel is classified as well graded
or GW.
 For a sand to be classified as well graded, the
following criteria must be met:
 Cu ≥ 6 & 1 < Cc < 3
 If both of these criteria are not met, the sand is
classified as poorly graded or SP. If both of these
criteria are met, the sand is classified as well graded
or SW.

14. The soil particles less than 75–μ size can be further
analysed for the distribution of the various grain-
sizes of the order of silt and clay.
 The soil fraction is kept in suspension in a liquid
medium, usually water. The particles descend at
velocities, related to their sizes.
 The analysis is based on ‘Stokes Law’ for what is
known as the ‘terminal velocity’ of a sphere
falling through an infinite liquid medium.

15.  If a single sphere is allowed to fall in an
infinite liquid medium without
interference, its velocity first increases
under the influence of gravity, but soon
attains a constant value. This constant
velocity, which is maintained indefinitely
unless the boundary conditions change, is
known as the ‘terminal velocity’.
 The principle is obvious; coarser particles
tend to settle faster than finer ones.

16.  By Stokes’ law, the terminal velocity of the
spherical particle is given by
v =
1
18
[(γs – γL)/μL] × D2
γs = unit weight of the material of falling sphere g/cm3,
γL = unit weight of the liquid medium in g/cm3,
μL = viscosity of the liquid medium in g sec/cm2,
D = diameter of the spherical particle in cm,
V =the terminal velocity, is obtained in cm/s .

17.  If the particle falls through H cm in t minutes
in water.
Here,
G = grain specific gravity of the soil particles,
γw = unit weight of water in kN/m3
μw = viscosity of water in N-sec/m2
H = fall in cm, and t = time in min.

19.  Hydrometer is a device which is used to
measure the specific gravity of liquids.
 However, for a soil suspension, the particles
start settling down right from the start, and
hence the unit weight of the suspension
varies from top to bottom.

20.  Let W be weight of fine soil fraction mixed in
water V be the volume of suspension
 The volume of the suspension is 1000 ml in this
case. The sedimentation jar is shaken vigorously
and is then kept vertical over a firm base and
stopwatch is started simultaneously.
 The hydrometer is slowly inserted in the jar and
readings taken at elapsed times 30 s, 1 min and
2 min. The hydrometer is then taken out.
 Further readings are taken at elapsed times of 4
min,8 min, 15 min, 30 min, 1 h, 2 h, 4 h, etc.,
by inserting the hydrometer about 20 seconds
prior to the desired instant.

21.  Let W be weight, V be the volume ,N be
percentage finer, D be the particle size
 Thus for each hydrometer reading, Rh, we
obtain a set of values for D and N, fixing one
point on the grain-size distribution curve.

25. IS: 1498-1970 describes the Indian Standard on
Classification and Identification of Soils for general
engineering purposes
Soils shall be broadly divided into three
divisions
1. Coarse-grained Soils: More than 50% of the total
material by weight is larger than 75-μ IS Sieve size.
2. Fine-grained Soils: More than 50% of the total
material by weight is smaller than 75-μ IS Sieve size.
3. Highly Organic Soils and Other Miscellaneous
Soil Materials: These soils contain large
percentages of fibrous organic matter, such as peat,
and particles of decomposed vegetation.

26. Gravels: More than 50% of coarse
fraction (+ 75 μ) is larger than 4.75
mm IS Sieve size.
 Sands: More than 50% of Coarse
fraction (+ 75 μ) is smaller than 4.75
mm IS Sieve size.

27.  Silts and clays of low compressibility : Liquid
limit less than 35%(L).
 Silts and clays of medium compressibility:
Liquid limit greater than 35% and less than 50%
(I).
 Silts and clays of high compressibility: Liquid
limit greater than 50(H).
The coarse-grained soils shall be further sub-
divided into eight basic soil groups, and the fine-
grained soils into nine basic soil groups; highly
organic soils and other miscellaneous soil materials
shall be placed in one group.

28.  A line is the line drawn in graph between P.I
and L.L Where P.I = 0.73(L.L – 20)

29.  Atterberg limits are useful in classifying the
soil and also putting the soil in a correct
group that is basically done by soil
classification systems.
 These used directly in specifications for
controlling soil for use in the fill and used for
predicting the activity of clay or frost
susceptibility.
 So greater the liquid limit we will understand
that greater is the compressibility of the soil.

30.  Liquidity and consistency indices are the
good indicators of the consistency soil. If you
want to understand about whether the soil is
soft or stiff or medium stiff or very stiff or
hard.
 Activity is an important index property which
is being used for determining the swelling
potential of a given soil that is whether the
soil has got the swell susceptibility or not.