The soil properties on which their classification and identification are based are known as index properties. The index properties which are used are:
- Grain Size Distribution
- Consistency Limits
- Plasticity Index
Grain Size Distribution (Sieve Analysis)
The sieves are arranged, keeping the finest below and the coarser above it. A definite quantity of soil is dried in an electric oven (for 24 hrs at 105°C) and put in the top sieve. The cover is placed over the top sieve and a pan below the lowest sieve. Then the sieves are placed in the sieve shaker and are shaken for few minutes. The weight of soil thus retained on each sieve is determined and is converted into a percentage.
Consistency of Soil
Consistency of soil is the physical state of soil with respect to moisture content present at that time. By consistency means the relative ease with which soil can be deformed. Consistency is related to the fine grained soil.
Consistency Limits or Atterberg Limits
The moisture content at which the soil change from one state to another State is called consistency limits or Atterberg limits There are four different states of soil:
- Solid State
- Semi Solid State
- Plastic State
- Liquid State
When water is added to the dry soil mass its change from solid state to liquid state passing through semi solid and plastic state. Atterberg identify some other limits which are most useful for engineering purpose.