Soil classification as per Indian standards:

∴ According to IS Classification, re are 18 groups of soils:

8 groups of coarse-grained, 9 groups of fine-grained, and 1 of peat

Triangular classification of U.S. public road administration is a commonly used system for textural classification.

The electrical force responsible for inter-particle effect is proportional to specific surface.

Concept:

Coefficient of uniformity C_u:

• It is defined as ratio of D_60­ to D­₁₀.
• When C_u ≈ 1 uniformly graded soil.
• When C_u>4 well-graded gravel
• When C_u>6 well-graded sand.

Calculation:

Here, D₆₀ = 60% particles are finer than this size, D₁₀ = 10% particles are finer than this size.

∴ From table, D₆₀ = 600μ and D₁₀ = 500μ

$\therefore C_u=\frac{D_{60}}{D_{10}}=\frac{600}{500}=1.2$

Based on value of C_u, soil can be classified as uniformly graded soil.

Note:

1. Like ­C_u, re is one more shape factor called coefficient of curvature (C_C) which is given by

$C_c=\frac{{D_{30}}^2}{D_{60}{D}_{10}}$.

2. Range of C_c for well graded soil is 1 ≤ C_C ≤3 .

If more than 50 % of soil is retained on a NO.200 US sieve (0.075), it is designated as coarse grained soil.

Fine grained soil are subdivided into 3 types
i. Inorganic silts and very fine sands
ii. Inorganic clays
iii. Organic silts and clay and organic matter.

The purpose of soil classification is to arrange various types of soils in to groups according to ir engineering or agricultural properties.

As group index is a mean of rating value of soil as a sub-grade material. It can be used for finding performance and quality of soil.

If percentage of fines lies between 5 to 12 %, coarse grained soils are designated by dual symbols GW-GM or SP-SM.

As system is classified based on size, it is preferable to use word ?silt size? and ?clay size? in place of simply ?silt? or ?clay?.