21:14:00
OBJECTIVE
To introduce the
students to the method of making a mechanical grain size analysis of a soil and
presenting the resulting data.
THEORY
Grain size analysis carries much importance in
determination of engineering properties of soil e.g. suitability criteria of soils
(for road, airfield, levee, dam and foundation material), soil water movement,
susceptibility to frost action etc.
The grain size analysis is the attempt to determine
the relative proportions of the different grain sizes which make up soil mass. For
this, sample should be statically representative of the soil mass.
By carrying out mechanical analysis, particle sizes
and their relative distribution can be done for the particle greater than 0.075
mm. The mechanical analysis is carried out by stacking the sieves, one on top
of the other, pouring a known weight of soil into the top sieve on the stack,
and shaking the sieve in a certain manner to allow the soil to fall down
through the stack.
The
stack of sieves is known as nest of sieves. The nest is arranged with the
largest screen openings (smallest sieve number) on top, progressing to the
sieve with the smallest screen opening (largest sieve number) on the bottom of
the nest. A lid is placed on the top of the nest and pan is placed below the
bottom sieve to catch any soil that passes through the smallest opening. The
number or the sizes of the sieves used in the nest depends on the type of the
soil and the distribution of the particle sizes. Generally sieve No. 4, 10, 40,
100, 200 are used for classifying the soil.
APPARATUS
1.
A
set of sieves
2.
Mechanical
soil pulverizer
3.
Weighing
balance (Least count = 0.01 grams)
4.
Mechanical
sieve shaker
PROCEDURE
1. Obtain 500 grams of soil sample
which has already been pulverized by placing it on sieve No. 200 and then oven
dried.
2. Arrange a nest of sieves including
sieves No. 4, 10, 40, 50, 100, 200, pan.
3. Place the set of sieves in the
mechanical sieve shaker and sieve it for 5 to 10 minutes. Note that if the
entire set of sieves does not fit into the shaker perform a hand shaking
operation until the top few sieves can be removed from the stack and then place
the remainder of the stack in the mechanical shaker.
4. Remove the nest of sieves from the
shaker and obtained the weight of the material retained on each sieve. Sum
these weights and compare with the actual weight taken. A loss of more than 2
percent by the weight of the residual material is considered unsatisfactory and
the test should be replaced.
5. Compute the percent retained on
each sieve by dividing the weight retained on each sieve by the original sample
weight.
6. Compute the percent passing by
starting with 100 percent and subtracting the cumulative percent retained for
that sieve.
OBSERVATIONS AND CALCULATIONS
Weight
of sample = __________ grams
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Sieve
No.
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Diameter
(mm)
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Weight
of Soil Retained (grams)
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Percentage
WeightRetained
(%)
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Cumulative
Percent Retained
(%)
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Percent
Passing
(%)
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D10
= D30
= D60
=
Cu = (D60) / (D10)
Cc = (D30)2 / (D10) x (D60)
PRECAUTIONS
1. Particles that appear to be stuck
in the sieve screen should never be forced on through the mesh. There are two
reasons for not doing this.
a) The particles would have passed the
screen on their own had they been smaller than the mesh opening. Forcing these
particles through the screen to be retained on the next size would distort the
grain size results.
b) Secondly forcing the particles
through the mesh can damage the screen and necessitate its replacement.
Particles
caught in a screen should be removed by brushing with the proper sieve brush.
Brushing should be done from the underside of the screen in order that the
particles can be brushed out of the screen in the direction from which it
entered in the screen opening. Stubborn (obstinate) particles that cannot be
removed by rushing should be left in place.
2. Lumps of soils must have broken
down into their individual particles in order for the grain size analysis to be
valid. This is accomplished in two ways. The first is to break up lumps with a
rubber-tipped pestle in ceramic mortar. It has been found that the
rubber-tipped pestles will not grind or crush the individual particles while a
ceramic or metal-tipped pestle will.The second is to wet-sieve the soil.
Washing the particles that are retained on the No.200 sieve with water and this
will accomplish two things.
a) It separates those small lumps that
might not have been broken up with the rubber tipped pestle into individual
particles.
b) It washes the “Dust size” particles
and through the No.200 sieve.
3.
A
10 minute shaking period is suggested in procedure. A large sample is requires
longer shaking than a sample. Similarly a sample comprising primarily of fine
grained material will require a longer shaking period than a coarse grained
sample of equal weight.
REFERENCE:
ASTM
D422
Standard Test Method for Particle-Size Analysis of
Soils
COMMENTS:
