Index properties of soil are indicative of the engineering properties which are not of primary interest to the geotechnical engineer. From practical point of view, index properties of soil is a first come first serve property which needs to be find out to estimate the behaviour of soil i.e. before conducting major tests like compaction, consolidation, CBR etc. index properties of soil needs to be found out.
The main index properties of coarse grained soils are – particle size and relative density.
Index Properties of Fine Grained Soils :
1. Shrinkage Limit (Ws) : It is the maximum water content at which there will be no decrease in volume of soil mass.
2. Plastic Limit (Wp): It is the water content below which the soil stops behaving as a plastic material. It begins to crumble when rolled into a thread of soil of 3mm diameter.
3. Liquid Limit (Wl): It is the water content at which soil changes from liquid state to plastic state. At the liquid limit, the soil behaves like liquid but possesses a small shear strength, which can be measured in the laboratory. The liquid limit is determined by Casagrande’s Apparatus or Cone Penetration Method.
4. Plasticity Index : It is the range of water content over which the soil remains in plastic state.
5. Liquidity Index : It indicates the nearness of its water content to its liquid limit. When the soil is at liquid limit, its liquidity index is 100% and behaves as liquid whereas, when soil is at plastic limit, its liquidity index is zero.
6. Consistency Index : It indicates the firmness of soil i.e. it shows the nearness of water content of soil to its plastic limit. The soil with consistency index of zero is at the liquid limit i.e. extremely soft and negligible shear strength. When the soil is at water content equal to the plastic limit has a consistency index of 100%, indicating soil is relatively firm. Soil having consistency index greater than 100% shows that the soil is relatively strong i.e. in semi-solid state.
7. Flow Index : It is defines as the rate at which a soil mass losses its shear strength with an increase in water content. It is a slope of flow curve obtained between number of blows and water content.
8. Toughness Index : It is the measure of shear strength of soil at the plastic limit. It can also be defined as the ratio of plasticity index to the flow index.
9. Shrinkage Index : It is the numerical difference between liquid limit and shrinkage limit.
Index Properties of Coarse Grained Soils :
1. Particle Size Analysis : It is the method of separation of soils into different fractions based on the particle size. It expresses quantitatively the proportions by mass of various sizes of particles present in a soil. It is represented graphically on particle size distribution curve.
Particle Size Analysis are performed in two stages :
i) Sieve Analysis : It is done for coarse grained soils for particle size greater than 75 micron.
ii) Sedimentation Analysis : It is done for fine grained soils for particle size less than 75 micron. The analysis is based on Stoke’s Law, which gives the terminal velocity of a small sphere settling in a fluid of infinite extent.
|Grain Size Analysis|
2. Relative Density : It indicates how the soil would behave under loads. It gives a more clear idea of the denseness than does the void ratio. If the deposit of soil is dense, it can take heavy loads with very little settlements.