Index Property of Soil

Those properties which help to access the engineering behaviour of soil and which assist in determining its classification accurately are termed Index properties. Index properties include indices that help in determining the engineering behaviour such as

a) Strength

(b) load-bearing capacity

(c) swelling and shrinkage

(d) settlement etc.

These properties may be relating to

1. Individual soil grain

2. Aggregate soil mass

• The properties of individual particles can be determined from a remoulded, disturbed sample. These depend upon the individual grains their mineralogical composition, size, and shape of grains and are independent of soil formation.

• The soil aggregate properties depend upon the mode of soil formation, soil history, and soil structure. These properties should be determined from undisturbed samples or preferably from in situ tests.

• Particularly useful in the case of coarse-grained soil. Soil grains are called bulky in the case of sand & gravel. It is called flaky grains in the case of sub-microscopic crystals of clay minerals. Clay mineral kaolinite is said to have needle-shaped grains.

• Bulky grains of sand and gravel are classified as angular, subangular, subrounded, rounded, well rounded. 

• The high angularity of soil grain leads to higher shearing strength.

Classification of bulky grain is done on the basis of sphericity where sphericity (S) =D_e/L 

Where,

D_e = dia equivalent Spherical Particles. L = length of particle

i.e. $$Volume(V) = {4 \over 3}\pi {\left( {{{{D_e}} \over 2}} \right)^3}$$

$$(V) = {\pi  \over 6}{\left( {{D_e}} \right)^3}$$

$${D_e} = {\left( {{{6V} \over \pi }} \right)^{1/3}}$$

• Grain Size Analysis is a method of separation of soils into different fractions based on particle size. It is also called Mechanical Analysis or Particle Size Analysis.