CLASSIFICATION AND IDENTIFICATION OF SOILS

Classification and identification of soils in the ongoing system of cutting in earth work is very important issue and matter for which various systems of classifying soils for civil engineering purposes have been devised and the use of different systems has led to much confusion. The course grained soils are usually classified mainly on the basis of their particles size distribution and the fine grained soils on the basis of their plasticity characteristics. The two most widely used systems of soil classification are useful for the public roads administration under civil engineering works. Particle-size analysis or the Mechanical analysis of soils is the process of separation of a soil into several fractions of different grain size. As such soils are divided into various size groups as, gravels, sands, silts and clays for the particles size analysis which is carried out by combining sieving and sedimentation methods. Particle sizes for gravels and sands are determined by the sieve analysis. The gravel fraction is removed by sieving on the 2.36 mm sieve. The samples of soil which pass the 2.36 mm sieve are dried and then shaken through a series of sieves ranging from course to fine, and the amount (percentage of sample dry weight) retained on each sieve is weighed and recorded. For the sedimentation test of the finer fractions than 75 micron sieve, the soil is shaken up in a test tube full of water and allowed to settle. The coarser particles soon settle at the bottom and the proportions of finer material can be gauged from the thickness of the succeeding layers and the turbidity of the water. Such particles size is found by observing the rate at which the grains will settle through a liquid, as particles of different size have different settling velocities. This is done with a hydrometer or a pipette. It consists of determining the variation in density of the suspension with time. As for as the field classification of soils is concerned Particles size with 90 parcent of parts greater than 200 mm is considered Boulders and 80 mm to 200 mm Cobbles and 2.36 to 75 mm pebbles and 2 to 60 mm Gravels and sand, course from 0.6 mm to 2 mm sand medium 0.2 to 0.6 and sand fine 0.06 to 0.2 , Silt course 0.02 to 0.06 silt medium 0.006 you. 02 silt fine 0.002 you. 006 and the Clay 0.002 mm.