In particle size analysis, soils are separated in different fractions depending on particle size. A quantitative expression of this analysis is done by determining proportions of different size of particles exist in the soil sample and its graphical representation is known as particle or grain size distribution curve. This analysis is done by two separate stages
Sieve analysis is suitable for coarse soil (having particle size >75µ). The sedimentation analysis is applicable for fine-grained soils (having particle soils<75µ). Soil sample may have soils of both sizes and both methods may be required for single sample.
When particle size is found smaller than 0.2µ, sedimentation analysis is not applicable and we should go for X-ray diffraction or electron microscope analysis. However such sophisticated analyses have limited application in geotechnical engineering.
We will discuss here about wet sieving. Most sieve operations are conducted for oven dry sample. But in some cases wet sieve analysis is required. At first we will learn about wet sieving.
In this method, lumps in soil sample are broken down to individual particles and required quantity of soil are left in oven to be dried. After drying, the sample is left in water and stirred to be soaked for one hour (at least). The slurry produced is sieved by 4.75 mm standard sieve under water jet to separate gravel fraction (retained on this sieve). This coarse fraction is dried in oven and dry sieving is conducted under coarse sieves.
When deflocculation is required Na-hexameta-phosphate is added at a rate of 2 gm/liter. The particles that pass through 4.75 mm standard sieve are sieve by 75µ sieve. Then the materials are washed in water until clear water is found. The retained materials in this sieve are collected and left in oven to de dried. The oven dried sample is sieved through standard sieves:
Now particles retain on each sieve are collected to weigh. It should keep in mind that materials found in pan=(Total mass) - (Sum of particles retained on all above sieve.
The percentage finer is found as usual. Now we will learn about cause of selection of wet sieving method-
In one word when soil contain significant amount of fine particles wet, sieving method is chosen the critical percentage is considered >5%. The conditions are:
-When sample found in suspension form and should not be dried
-When very fine particles remain in sample like powder (<45 µm) which have tendency to agglomerate. When dry sieving method is used they cause clogging of opening by closing meshes leaving further sieving impossible.