Why sieve analysis of aggregates is done?
Soil usually consists of particles of different sizes. These particle sizes and their distribution in the soil affects the engineering properties of the soil. Therefore, to find out the particle size distribution of a soil, Mechanical analysis, or Grain size analysis or Particle size, sieve analysis of aggregates is done.
The result of the mechanical analysis is used broadly in soil classification, soil stabilization, soil compaction, filter design for Earth dams and many others.
Methods of analysis of aggregates
This method consists of two parts, Sieve analysis for gravel and sand (Coarse-grained soil) and sedimentation analysis or wet analysis for silt and clay (Fine-grained soil) by using Stokes’ law.
Today we are going to talk about Sieve Analysis.
Sieve analysis is a method of determining the particle size distribution of coarse-grained soils (particle size greater than 75 microns).
Introduction of sieve analysis of fine and coarse aggregates
In this method, the soil is sieved through a set of sieves. Sieves are generally made up of spun brass and stainless steel.
As mentioned above, the sieve analysis is done for coarse-grained soils. The coarse-grained soils can further be divided into gravel (size > 4.75 mm) and sand (size from 75µ – 4.75 mm), where µ represents micron.
A set of sieves, consisting of the sieves of size 4.75mm, 3.35mm, 2mm, 1mm, 600 µ, 300 µ,150 µ, 75 µ, and the pan is used for sand fraction whereas a set of sieves, consisting of size 100mm, 63mm, 20mm, 10mm, 4.75mm, and the pan is used for gravel fraction.
The selection of the number of sieves is done to obtain a good particle size distribution curve. The sieves are stacked one over other, with decreasing size from top to bottom.
A pan is placed at the bottom of the smallest sieve which has no opening and the sieve with the largest opening is kept at the top and covered with the help of a lid.
Objective of Sieve analysis of aggregates
Particle size distribution helps to classify the soils. As particle size determines how fast or slow water or other fluids moves through the soil, particle size distribution assists engineering and agriculture purpose. It also helps to maintain the quality of concrete produced.
How to perform Sieve analysis of aggregates?
- IS sieves (4.75mm, 3.35mm, 2mm, 1mm, 600 µ, 300 µ,150 µ, 75 µ, and pan)
- Weighing machine
- Metal trays
- Mechanical sieve shaker
- Thermostatically controlled oven
- A sample of soil is collected from the site.
- The soil sample is then dried in the oven.
- The quantity of soil samples for the test depends upon the maximum size of material present in substantial quantities. For example, if a particle of size 75mm is present then 60 kg of the sample should be taken and if the particle of size 4.75 mm is present then 500 gm of the sample should be taken.
- Based on the percentage of silt and clay particles present in the soil sample, the analysis should be done by either dry sieving or wet sieving method.
Dry Sieve Analysis
- 500 gm of a dry soil sample is taken.
- It is then kept in IS sieves arranged in the order as shown in Table 1.
- Mass of each sieve is weighed and noted before placing the soil sample in them.
- The set of sieves were then placed in the mechanical sieve shaker and sieved for 10 minutes. The time is recorded.
- After 10 minutes the weight of soil retained in each sieve is recorded. The soil sample retained in 4.75 mm sieve represents the gravel fraction.
- The retained soil mass is checked whether it matches with the original mass or not.
- Finally, % finer than the sieve sizes are calculated and used in the analysis.
Sample table for calculation
Mass of soil sample taken = 500 gm
|S.N||Sieve size||Mass of sieve (gm)||Mass of soil and sieve (gm)||Mass of soil retained (gm)||Cumulative mass of soil retained (gm) Say A||Mass of soil passing (gm) Say B B = 500-A||% finer = (B/500)*100%|
|9||Pan||Must be zero|
Now, the semi-log graph is plotted with % finer in Y-axis and sieve size in X-axis as shown below:
- Now, the Coefficient of uniformity (Cu) and Coefficient of curvature (Cc) is determined with the help of the semi-log graph and formula listed below:
- At last, the soil is classified as per the following rules:
- If Cu < 2: uniform graded soil
- If Cu > 4 and Cc is in between 1 and 3: well-graded gravel else poorly graded or gap graded gravel
- If Cu > 6 and Cc is in between 1 and 3: well-graded sand else poorly graded sand.
- The sum of the retained mass should be equal to the original soil sample. If it is not obtained, then the fraction retained on different sieves should be adjusted proportionately.
- The electric connection of mechanical sieve shaker should be checked before starting the test.
- The sieves should be cleaned properly so that no particles are stuck in them.
I hope this post remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Sushmita Niraula