PARTICLE SIZE ANALYSES (MECHANICAL ANALYSIS) FOR COARSE GRAIN SOILS

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

Sieve No.	Diameter (mm)	Weight of Soil Retained (grams)	Percentage WeightRetained (%)	Cumulative Percent Retained (%)	Percent Passing (%)

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

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