If we take a soil sample and analyze its each grain, then we will notice that different kinds of soil grains of different shapes and sizes make up the soil.
After examining few more such soil samples we may come to know that soils rarely consist entirely of single sized grains, rather they have different sized grains ranging from smaller to larger.
Different sized grains impart different kinds of properties to the soil. So as a civil engineer we are interested in knowing the composition of soil so that we can better understand the soil on which we are about to lay our foundation of a big structure.
We are interested in knowing that in the soil, how much of bigger particles and how much of smaller particles are present.
We also try to determine the size of the grains which make up the soil.
We evaluate mean size and size range of the grains present in the soil sample.
The laboratory techniques and methods we apply to determine these sizes are collectively called Particle size analysis. It is also called Grain Size Analysis and Mechanical Analysis.
By Grain Size Analysis methods, we separate soil into different fractions based on their particles sizes.
Bigger sized grains are called coarse grains and smaller sized grains are called fine grains.
So how we distinguish between them?? Which one is bigger and which one is smaller?
Two grains could be bigger and smaller relative to each other and both still can be coarse grains. Similarly two grains could be bigger and smaller relative to each other and but still they both can be part of category fine grains.
Well, it has been established that soil particles having size greater than 0.075 mm or 75 micron are considered as coarse grains and particles having size smaller than 75 micron are considered as fine grains.
Grain size analysis of soil is done in two stages.
Sieve Analysis is carried out for coarse grained soils and is done using different sized sieves.
Whereas fine grained soils grain analysis is done by Sedimentation Analysis, which is done by Hydrometer Method or Pipette Method.
But in general soil mass contains both coarse and fine grained constituents, so for grain size distribution a combined analysis of sieve analysis and sedimentation analysis is carried out.