AIM
To determine the particle size distribution of soil as per IS: 2720
(Part 4) - 1985.
APPARATUS
i) A set of fine IS Sieves of sizes - 2 mm, 600µm, 425µm,
212µm and 75µm
ii) A set of coarse IS Sieves of sizes - 20 mm, 10 mm and
4.75 mm
iii) Weighing balance, with an accuracy of 0.1% of the weight of
sample
iv) Oven
v) Mechanical shaker
vi) Mortar with rubber pestle
vii) Brushes
viii) Trays
PREPARATION OF SAMPLE
i) Soil sample, as received from the field, should be dried in air
or in the sun. In wet weather, the drying apparatus may be
used in which case the temperature of the sample should
not exceed 60 °C. The clod may be broken with wooden
mallet to hasten drying. Tree roots and pieces of bark
should be removed from the sample.
ii) The big clods may be broken with the help of wooden
mallet. Care should be taken not to break the individual soil
particles.
iii) A representative soil sample of required quantity as given
below is taken and dried in the oven at 105 to 120 °C.
PROCEDURE
i) The dried sample is taken in a tray, soaked in water and
mixed with either 2 g of sodium hexametaphosphate or 1 g of
sodium hydroxide and 1 g of sodium carbonate per litre of
water, which is added as a dispersive agent. The soaking of
soil is continued for 10 to 12 hrs.
ii) The sample is washed through 4.75 mm IS Sieve with water
till substantially clean water comes out. Retained sample on
4.75 mm IS Sieve should be oven-dried for 24 hrs. This dried
sample is sieved through 20 mm and 10 mm IS Sieves.
iii) The portion passing through 4.75 mm IS Sieve should be
oven-dried for 24 hrs. This oven-dried material is riffled and
about 200 g taken.
iv) This sample of about 200 g is washed through 75µm IS Sieve
with half litre distilled water, till substantially clear water
comes out.
v) The material retained on 75µm IS Sieve is collected and
dried in oven at a temperature of 105 to 120 °C for 24 hrs. The
dried soil sample is sieved through 2 mm, 600µm, 425µm
and 212µm IS Sieves. Soil retained on each sieve is
weighed.
vi) If the soil passing 75µm is 10% or more, hydrometer method
is used to analyse soil particle size.
HYDROMETER ANALYSIS
i) Particles passed through 75µm IS Sieve alongwith water are
collected and put into a 1000 ml jar for hydrometer analysis.
More water, if required, is added to make the soil water
suspension just 1000ml. The suspension in the jar is
vigorously shaken horizontally by keeping the jar in-between
the palms of the two hands. The jar is put on the table.
ii) A graduated hydrometer is carefully inserted into the
suspension with minimum disturbance.
iii) At different time intervals, the density of the suspension at
the centre of gravity of the hydrometer is noted by seeing
the depth of sinking of the stem. The temperature of the
suspension is noted for each recording of the hydrometer
reading.
iv) Hydrometer readings are taken at a time interval of
0.5 minute, 1.0 minute, 2.0 minutes, 4.0 minutes, 15.0
minutes, 45.0 minutes, 90.0 minutes, 3hrs., 6hrs., 24hrs.
and 48hrs.
v) By using the nomogram given in IS: 2720 (Part 4) - 1985,
the diameter of the particles for different hydrometer readings
is found out.
REPORTING OF RESULTS
After completing mechanical analysis and hydrometer analysis,
the results are plotted on a semi-log graph with particle size as
abscissa (log scale) and the percentage smaller than the
specified diameter as ordinate.