Particle Distribution for Soil Test

For classification of soil for engineering purposes, we oath to know the distribution of the grain sizes in any given soil mass especially the one obtained from the construction site or burrow pits. Particle size distribution test, also known as sieve analysis test is a method used to determine the grain (granular) size distribution of soil samples.

The sieves are normally made of woven wires with square openings and steel body frames. It has different numbers which respect to the opening sizes. BS Sieve Aperture and ASTM Sieve Aperture sizes are mostly the same especially from 4.75 mm to 63 μm, and slightly different from 75 mm to 6.3 mm.

The Objective and Scope of Test

The sieve analysis (grain size analysis) is widely used in the classification of soils. The data obtained from grain size distribution curves is used in the design of filters for earth dams and to determine the suitability of soil for road/highway construction, embankment fill of dam, airport runway/taxiway, etc. The information that we obtained from sieving test could be used to predict soil water movement although permeability tests are more generally used.The objective of this test is to determines the relative proportions of different granular sizes as they are passing through certain sieve sizes. Thus, the percentage of sand, gravel, silt and clay can be obtained from the sieve analysis test.

The Technical Standards
The sieve analysis of soil test is accordance to ASTM D-422 (American Society for Testing and Materials) or BS 1377: Part 2 1990 (British Standards) as both are the most widely used technical standards in construction. The dry sieving of soil is the simplest and cheapest method among others.

The Required Apparatus

As per figure above:

• Stack of Sieve Aperture sizes (including the cover and pan)
• Electronic Balance (decimal reading to 0.01 g)
• Rubber pestle, mortar (for crushing the soil if lumped), and brush
• Mechanical sieve vibrator (shaker)
• Oven Dry (thermostatically controlled temperature)

The Test Procedure / Method
Here is how we going to do it:

1. Take out the dried soil samples from the oven dry and weighs about 500 g (normal amount used for any soil samples the greatest particle size of which is 4.75 mm).
2. The dried soil particles should be first crush (in lumped) using the rubber pestle and mortar.
3. Determine the mass of sample accurately and label as W_total (in g).
4. Then prepare a stack of sieve aperture sizes with larger opening sizes of sieve at the top (having lower number) and down to the last one with smaller opening sizes (having higher number). Not forgetting the sieve pan underneath and cover on top.
5. Weigh all sieves and the pan separately if necessary (mostly neglected).
6. Pour the soil slowly into the stack of sieves from the top and place the cover, put the stack onto the sieve shaker (vibrator), tighten the clamps, adjust the time within 5 to 10 minutes and turn it on…shake it baby…
7. When times out, take it out and measure the mass of each sieve aperture + retained soil inside, from the top sieve until the pan. This procedure should be done carefully…one by one…
8. Record down the weight in the result sheet and ready for calculation and plotting of graph.

The Calculations
Both calculation methods taken into account with an example of test sheet result:-

Calculation following BS 1377: Part 2 1990:

Calculation following ASTM D-422:

The Results Documentation
Draw graph of log sieve size vs % finer. The graph is known as grading curve. Corresponding to 10%, 30% and 60% finer, obtain diameters from graph these are D10, D30, D60, using these obtain Cc and Cu which further represent how well the soil is graded i.e whether the soil is well-graded, gap-graded or poorly graded.

The Graphs
Graph for BS 1377: Part 2 1990:

Graph for ASTM D-422:

US Standard Sieve Sizes

Referring to the graph;
Uniformity Coefficient, C_u = D₆₀ / D₁₀ = 0.9 / 0.16 = 5.625

Coefficient of Gradation, C_c = (D₃₀)² / (D₆₀ x D₁₀) = (0.37)² / (0.9 x 0.16) = 0.95

Things to Remember

A few thing remember during the sieve analysis testing:

• Make sure the sieve aperture in dry condition and properly cleaned from any particles by poke them out using brush before commencing the test.
• Make sure to double check the stack of sieve aperture sizes arrangement in order before shaking begins.
• Make sure the balance have an adequate battery for a long run (if lots of soil sample to be test).
• The sieve shaker should be in good condition as well for a long run.
• The oven-dry and the balance calibration certificate still valid (haven’t due yet) for an accurate results.
• Do not shake the soil sample with the shaker for too long as the finer particles could easily lost. For more accurate results especially doing some research or independent lab test, manual approach is recommended.

The sieve analysis of soil test above is applicable not only to soil samples but can be tested upon aggregate, cement, and sand samples. The procedure would be the same as well as the calculation method and the graph plotting. The particle distribution test would eventually allows the grading of soil particles. Happy testing…