Index Properties of Soil

Index Properties:

• These are the properties useful for classification and identification of soils.
• Also give rough estimation of Engineering properties of soils.
• Example: Particle size distribution and relative density for coarse grained soils and consistency limits for fine grained soils.

Engineering Properties:

• These are the properties useful for engineering applications.
• Examples: Permeability, Compressibility and shear strength.

Shape of Particles: Engineering properties of coarse grained soil depends on shape of particles

Particle size distribution:

• Sieve analysis is used for coarse grained soils.
• Wet sieve analysis is suitable if soil contains fines.
• Sedimentation analysis or wet analysis is used for fine grained soils, finer than 75 micron , size.
• Sedimentation analysis is based on Stoke’s law.
• Stoke’s law is valid only if the size of the particle is between 0.2mm and O.2 micron.
• Pipette method is the standard sedimentation method used in laboratory. It is an accurate method.
• Dispersion solution containing 33g of the sodium hexametaphosphate and 7g. of sodium carbonate in distilled water to make one litre of solution. 
• The process of removal organic matter and calcium compounds is known as pre treatment.
• The hydrometer method of sedimentation analysis differs from the pipette analysis in the method of taking observations only.

>>Approximate version of Stroke’s Law, vs= 900 d2

Where

• V velocity, in mm/sec
• d = Diameter of particle, in mm.

Corrections to hydrometer reading:

• Meniscus correction Cm is always positive.
• Dispersing agent correction Cd is always negative.
• Temperature correction is positive when greater than 27°C and negative when less than 27°C.
• The hydrometer readings increase on the downward direction towards the hydrometer bulb.
• Reading is taken corresponding to the upper level of the meniscus.

Particle size distribution curve :

Particle size distribution curve : It gives an idea about type of soil and gradation of soil.

• Well graded soil: Soil contains good representation of particles of all sizes.
• Gap graded soil: Excess of certain particles and deficiency of others. cr
• Uniformly grades soil: Most of the particles are of the same size.
• For coarse grained soils sizes D10, D30 and D60 are important.

Effective size (or) effective diameter (D10): Expressed in ‘mm’ It means, only 10% of particles are finer than this size.

Coefficient of uniformity(Cu): D60 / D10

• It is a measure of particle size range. It is always greater than or equal to 1.
• A soil is called uniformly graded if Cu lies between 1 and 2.
• For a well graded gravel, C must be greater than 4.
• Where as for a well graded sand, Cu must be greater than 6.

Coefficient of curvature (Cc): 

• It represents shape of particle size curve.
• For well grades soils, C lies between 1 and 3.
• For uniformly graded soils, C = 1.

Relative density (or) Density index (ID)):

• Most important for coarse grained soils.
• Indicates relative compactness of soils.

• emax = Max.void ratio (in loosest state)
• emin = Min. void ratio (in densest state)
• e = natural void ratio
• Range in %age  0  ≤ ID  ≤ 100.

Limits of consistency (Atterberg'S  limits) :

• Mostly used for fine grained soils.
• Atterberg divided the entire range from liquid to solid state into four stages.

1. Liquid state
2. Plastic state
3. Semisolid state
4. Solid state.

• Atterberg limits are the water contents at which soil mass passes from one state to the next.

Liquid Limit:

• It is the minimum water content at which the soil is still in the liquid state, but has a small shearing strength against flowing.
• It is the minimum water content at which part of a soil cut by a groove of standard dimensions will flow together for a distance of 12mm under an impact of 25 blows in the device.

Apparatus: Casa grand’s apparatus is used to determine liquid limit.

Flow Curve:

• Flow curve gives an idea of shear strength variation with water content of the soil.
• Soil passing through 424 micron IS sieve is used for the test.
• The drop of the cup on hard base is 1 cm.
• The shear strength õf the soil at liquid limit is about 2.7 kN/m2

Plastic Limit (Wp): It is the minimum water content at which the soil can be rolled into a thread of 3 mm in diameter without crumbling. It is determined by Plastic limit test or ‘Thread Test’.

Shrinkage Limit (Ws): It is the moisture content after which further loss of moisture does not cause a decrease in the volume of the soil.

Shrinkage limit is lowest water content at which a soil can still be completely  saturated. At shrinkage limit the shrinkage ceases.

Shrinkage Ratio : Volume change expressed in terms of percentage of dry volume to the corresponding change in moisture content.

>>Note: Shrinkage ratio of a soil is equal to the mass specific gravity of its dry state. (Gm.)