GRAIN SIZE ANALYSIS.( IS : 2720 – PART – 4 )

GRAIN SIZE ANALYSIS.

( IS : 2720 – PART – 4 )

INTRODUCTION:

Most of the methods for soil identification and classification are based on certain physical properties of the soils. The commonly used properties for the classification are the grain size distribution, liquid limit and plasticity index. These properties have also been used in empirical design methods for flexible pavements, and in deciding the suitability of sub grade soils.

The soils received from the field are divided into two parts: one, the fraction retained on 2mm sieve and the other passing 2mm sieve. The sieve analysis also may be carried out separately for these two fractions. The fraction retained on 2mm sieve may be subjected to dry sieving using bigger sieves and that passing 2mm sieve may be subjected to wet sieving; however if this fraction consists of single grained soil with negligible fines passing 0.075mm size, dry sieving may be carried out.

Apparatus:

Various apparatus include set of standard sieves of different sieve sizes, balance, rubber covered pestle and mortar, oven, riffle and sieves shaker.

Procedure:

(a) For the fraction retained on 2.0mm sieve: Sufficient quantity of the dry soil retained on 2.0mm sieve is weighed out. The quantity of sample taken may be increased when the maximum size of particles is higher. The sample is separated into various fractions by sieving through the set of sieves of sizes 100, 63, 20, 6, 4.75 and 2 mm IS sieves. Additional sieve size may also be introduced if necessary. After initial sieving, the material retained on each sieve is collected, the lumps broken down using mortar and rubber covered pestle and is re-sieved thus the soil fraction retained on each sieve is carefully collected and weighed.

(b) For the fraction passing 2.0mm sieve and retained on 0.075mm sieve: Dry sieving may be done in the case of soils which are cohesion less, single grained and without lumps. Rifling or quartering method takes the required quantity of soil sample, dried in oven at 105⁰ to 110⁰C and is subjected to dry sieve analysis using a set of sieves with sieve openings 2.0, 0.6, 0.425, 0.15 and 0.075 mm, pan and lid, additional sieves may be used or any of the sieves removed, depending upon the requirement of the test. The material retained on each sieve and on the pan are separately collected and weighed.

Wet sieving may be adopted in the case of clayey or cohesive soils. Required quantity of sample taken by riffling is weighed. The sample is spread in a tray or bucket and covered with water. In case of soils having fractions that are likely to flocculate a dispersing agent like sodium hexametaphosphate (2.0g) or sodium hydroxide (1.0g) and sodium carbonate (1.0g) per liter of water may be added to the water. The mix is stirred and left for soaking. The soaked soil specimen is placed over the set of sieves of sizes with the finest sieve and pan at the bottom and washed thoroughly. Washing is continued till the water passing each sieve is substantially clean. The fraction of each sieve is emptied carefully without loss of material in separate trays, oven dried at 105⁰ to 110⁰C and each fraction weighed separately.

Calculations:

The weight of dry soil fractions retained on each sieve is calculated as a percentage of the total dry weight of the sample taken.

Results:

The results are plotted on a semi-logarithmic graph with the grain size or sieve size on the X-axis in log. scale and the percentage finer of each sieve on the Y-axis in ordinary scale. The smooth curve joining the points thus obtained is known as the particle size distribution curve or diagram.

Uniformity coefficient of soil, C_u = D₆₀ / D₁₀

Coefficient of curvature, C_c = (D₃₀)² / (D₁₀ X D₆₀)

Where, D60, D30 and D10 are particle sizes corresponding to 60, 30 and 10 percent finer.