SOIL MECHANICS LAB

1. DETERMINATION OF GRAIN
SIZE ANALYSIS
BY HYDROMETER TEST EXPERIMENT NO. 4
SOIL MECHANICS LABORATORY
CE PC 594

2. SEDIMENTATIONANALYSIS
• It is useful for determining the grain size distribution
of fine grained soils by using pipette test and
hydrometer test.
• It describes the process of fine particles falling
through a fluid, and is used to separate the particles
by size in space and time.
• This analysis is based on Stoke’s law.

3. STOKE’S LAW
• This law provides a governing equation based on the
hydrodynamics of a single spherical particle falling in
stationary fluid.
• It states that, When a spherical particle falls through
infinitely large medium of fluid, then it achieve
constant terminal velocity, also called as settling
velocity.

4. STOKE’S LAW
• The particle is driven downward by
gravitational forces and retarded by drag
forces and buoyant forces.
• A particle will accelerate downward in the
fluid until the summation of drag forces and
buoyant forces equal the gravitational force.
• Once these forces are in equilibrium the
particle will achieve terminal velocity (vS).
W
FB
FD
FD

5. STOKE’S LAW
• W = FB + FD
• Where, W = Weight of particle, FB = Buoyant
force, FD = Drag force)
• W= 𝛾𝑆 × 𝑉 = 𝛾𝑆 ×
4𝜋𝑟3
3
( Spherical particle)
• FB = 𝛾𝑤 × 𝑉 = 𝛾𝑤 ×
4𝜋𝑟3
3
; FB = 3𝜋𝜇𝑣𝑠𝐷
• Then, 𝑣𝑠 =
𝜌𝑠−𝜌𝑓 𝑔 𝐷2
18𝜇
=
𝐺𝑠−1 𝛾𝑤 𝐷2
18𝜇
(as 𝜌𝑓=1)
• D= Dia of particle, 𝜇 = dynamic viscosity of fluid
W
FB
FD
FD

6. ASSUMPTIONS OF STOKE’S LAW
• Particles must be spherical and smooth
• There is no interference between particles, no side
wall effects.
• All particles have to be of same quantity.
• Settling of particles should be discrete particle settling
• Diameter of soil grains should be in between 0.2 mm
and 0.0002 mm.

7. APPARATUS
• Hydrometer conforming IS:3104-1965
• Two measuring cylinders of 1000 ml
capacity
• Thermometers
• 75 micron sieve
• Balance accurate to 0.001 g
• Stop watch
• Wash bottles containing distilled water
• Reagent ( Sodium hexa metaphosphate and
sodium carbonate)

8. APPARATUS
HYDROMETER
Measuring
Cylinder

9. hydrometer
• It is a device which is used to measure density of soil
suspension in gm/ml or gm/cc at he chosen depth at
chosen time.
• It directly gives specific gravity of solid.
• Hydrometer have two parts, stem and bulb.
• Volume of hydrometer is almost equal to volume of
bulb.
• Reading on hydrometer increases in downward
direction.
• Rh = (𝜌𝑠-1) × 1000 = (Gs-1) × 1000
• Where, 𝜌𝑠is in gm/cc.
h
Stem
Bulb
H
Neck

10. hydrometer

11. CORRECTIONS OF HYDROMETER READING
• There is three types of corrections is necessary while
taking the reading Rh.
1. Meniscus correction (Cm)
2. Temperature Correction (CT)
3. Dispersion agent correction (CD)

12. MINISCUS CORRECTION (Cm)
• Reading in stem should be taken at the
lower level of meniscus.
• However due to presence of turbidity in
soil suspension, it is opaque in nature.
• As a result of which upper level of
meniscus is being read instead of lower
level.
• Since, the hydrometer reading increases
downwards, the meniscus correction
should be positive.

13. TEMPERATURE CORRECTION (CT)
• Calibration of hydrometer is done at 27°C.
• Hence if the temperature during the performance of test is more
than 27°C, it results in lower reading (lower density) is being
observed. Hence temperature correction is positive.
• Case A- If T>27°C, Rhcorrection=Rh +CT
• Case B- If T<27°C, Rhcorrection=Rh –CT
• The temperature shall be read with an, accuracy of at least of
0.5%.

14. DISPERSING AGENTCORRECTION (CD)
• Addition of dispersing agent in the soil suspension increases its
apparent density, which results in higher value of reading (lower
level). Hence, correction applied in negative.
COMPOSITE CORRECTION (C)
• C = Cm ± CT – CD
• Rhcorrected = Rh + C

15. TEST PROCEDURE
1. Determination of volume of the hydrometer bulb (Vh):
a) Pour about 800 ml of water in the 1000 ml measuring cylinder and note the
reading at the water level.
b) Immerse the hydrometer in water and note the water reading.
c) The difference between the two readings is recorded as the volume of the
Hydrometer bulb plus the volume of that part of the stem which is submerged.
d) For practical purpose the error due to the inclusion of this stem volume may be
neglected.
e) Alternatively, weigh the hydrometer to the nearest 0.1g. This mass in grams is
recorded as the volume of the hydrometer in ml. This includes the volume of
the bulb plus the volume of the stem.
CALIBRATION OF HYDROMETER

16. TEST PROCEDURE
CALIBRATION OF HYDROMETER

17. TEST PROCEDURE
2. In order to find the area of cross-section (A) of the measuring
cylinder in which the hydrometer is be used, measure the distance, in
cm, between two graduations of the cylinder. The cross-sectional area
(A) is then equal to the volume included between the two graduations
divided by the distance between them.
3. Measure the distance (h) from the neck to the bottom of the bulb, and
record it as the height of the bulb. The distance from the neck of the
bulb to the nearest calibration mark (H) shall be measured and recorded.
4. With the help of an accurate scale, measure the height between the neck
of the hydrometer to each of the other major calibration marks (Rh).
CALIBRATION OF HYDROMETER

18. TEST PROCEDURE
1. Calibrating the hydrometer with the measuring cylinder for determining
effective depth (He) corresponding to hydrometer reading.
2. According to IS 2720 recommendation, 33 gm of sodium hexa meta phosphate
(deflocculating agent) and 7 gm of sodium carbonate are mixed in 1 liter of
distilled water for the preparation of dispersing agent.
3. Filling the measuring cylinder up to 1000 ml with 900 ml of distilled water
including 100 ml of dispersing agent solution.
4. Then hydrometer is inserted and the top meniscus and bottom meniscus reading
on the stem is noted down. The difference between the top and the bottom
meniscus readings give the meniscus correction (Cm), which is positive.
SEDIMENTATION TEST BY HYDROMETER

19. TEST PROCEDURE
5. 50 gm oven dried soil sample passing 75 micron sieve is taken in a dispersion
cup and 100 ml of dispersing agent solution is added.
6. Then the mixture is stirred for about 15 minutes using a high speed stirrer.
7. The soil suspension is transferred completely to the measuring cylinder.
8. The measuring cylinder is filled by adding distilled water to increase the
volume up to the 1000 ml mark.
9. The measuring cylinder is then shake using palm of one hand on open end of
the cylinder, turning the cylinder up side down and back for a number of times.
10. After shaking, placed the cylinder on a levelled surface and a stopwatch is
started simultaneously.
SEDIMENTATION TEST BY HYDROMETER

20. TEST PROCEDURE
11. Immediately the hydrometer is inserted carefully and the top meniscus reading
at ¼, ½, 1 and 2 minute intervals.
12. The hydrometer is removed and floated it in another cylinder containing
distilled water. Then it is rinsed by a twisting motion to remove any soil
particles that may have adhered on it.
13. For any subsequent reading, the hydrometer is re- inserted just before reading.
14. Further readings are taken after periods of 4, 8, 15 and 30 minutes and again
after 1, 2 and 4 hours.
15. After the end of 4 hours, readings are taken twice within 24 hours and the final
reading is recorded at the end of 24 hours.
SEDIMENTATION TEST BY HYDROMETER

21. TEST PROCEDURE
16. The temperature of the suspension once during the first 15 minutes is recorded
and then after every reading is taken.
17. Hydrometer readings shall also be taken in pure distilled water at the
corresponding temperatures and the temperature correction (CT) calculated as
the difference between this reading and the reading corresponding to the density
of water at the calibration temperatures.
SEDIMENTATION TEST BY HYDROMETER

22. GENERAL INFORMATION
We will use such information for hydrometer test
• Vh = Volume of bulb in ml = 90 cc
• A = Cross sectional area of the cylinder = 29.85 cm2
• Cm = Meniscus Correction = 0.0005
• Cd = Dispersion agent correction = 0.0035

23. CALCULATION
1. The effective depth He corresponding to any hydrometer reading Rh is
given by, He = H +
1
2
(h-
𝑉ℎ
𝐴
)
2. Hydrometer reading (Rh) is determined at different time intervals t (in
minutes). Plot graph between H and He. Plot another graph between Rh
and He. µ value should be take from chart for each reading.
3. Determination of D; D = K ×
𝐻𝑒
𝑡
; Where, K =
30×𝜇
𝑔 (𝐺𝑆−𝐺𝑤)
Where, He = Effective depth in cm, t = time in minutes, 𝜇 = Viscosity in
poise, g = 981 cm/sec2, Gs = Specific gravity of soil, Gw = Specific gravity
of water= 1 under normal temperature conditions

24. CALCULATION
4. Calculation of corrected hydrometer reading
Rc = { ( Rh + Cm ± CT - CD ) -1}× 1000
5. Determination of N using formula
Percentage finer = N(%) =
𝐺𝑆
𝐺𝑆−1
×
𝑅𝐶
𝑀𝑆
× 100 %
where, Ms = Mass of dry soil in gm
6. The diameter of the particle in suspension at any time is calculated using
equation by taking the value of He from the calibration curve for reading
Rh.
7. Plot the graph between dia of particle (in mm) in log scale and % finer N

25. Table for CT andµ

26. CALCULATIONtable

27. CALCULATIONtable

28. CALCULATIONtable

29. GRAINSIZE DISTRIBUTION CURVE

30. THANK
YOU