Background and Basis of Classification:
The Geotechnical Engineers/Agencies had evolved many soil classification systems, over the world. The soil classification system developed by Casegrande was subsequently modified and named as 'Unified Classification' system. In 1959, Bureau of Indian Standards adopted the Unified classification system as a standard, which was revised in 1970. According to BIS classification system, soils are primarily classified based on dominant particle sizes and its plasticity characteristics. Soil particles mainly consist of following four size fractions.
1 Broad classification of soils
1. Coarse-grained soils, with average grain-size greater than 0.075 mm, e.g., gravels and sands.
2. Fine-grained soils, with average grain-size less than 0.075 mm, e.g., silts and clays. These exhibit different properties and behavior but certain general conclusions are possible
even with this categorization. For example, fine-grained soils exhibit the property of 'cohesion'-bonding caused by inter-molecular attraction while coarse-grained soils do not;
thus, the former may be said to be cohesive and the latter non-cohesive or cohesion less.
Gravel : 80 - 4.75 mm
Sand : 4.75mm - 0.075mm (75 micron)
Silt : 75 - 2 micron
Clay : less than 2 micron
Particle size distribution of a soil is determined by a combination of sieving and sedimentation analysis as per procedure detailed in IS: 2720 (Part 4) - 1985 and its plasticity characteristics are determined by Liquid Limit and Plastic Limit as per procedure detailed in IS:2720 (Part 5) -1985.
2 Symbols used in Soil Classification:
Symbols and other soil properties used for soil classification are given below. Brief procedure for Classification of soils has been explained in tabular form and Flow Chart. Plasticity Chart required for classification of fine grained soils has also been given.
Primary Letter Secondary Letter
G : Gravel W : well-graded
S : Sand P : poorly graded
M : Silt M : with non-plastic fines
C : Clay C : with plastic fines
P: Peat I : medium plasticity
H : high plasticity
3 Other soil parameters required for soil classification:
Cu : Coefficient of Uniformity = D60 / D10 .
Cc : Coefficient of Curvature = (D30)2 / (D60 * D10) .
D60, D30 & D10 are particle sizes, below which 60,30 and 10 percent soil particles by weight are finer than these sizes.
Plasticity Index, PI = Liquid Limit (LL) - Plastic Limit ( PL).
Coarse-grained soils: Soils having fines ( particles of size less than 75 micron) < 50%.
Fine grained soils: Soils having fines more than 50%.
4 Brief Procedure for soil classification:
Conduct Sieve analysis and Hydrometer analysis on soil sample and plot particle size gradation curve and determine Cu and Cc.
Conduct liquid limit and plastic limit test on soil samples as per procedure given fig
Based on above soil parameters, classification should be done as per procedure explained in the following table/Flow Chart. The classification should be done in conjunction with the Plasticity Chart given below.
Broad categorization of soil type
The classified soil types shall be grouped in four broad categories for the purpose of planning of works (Blanket thickness requirement as indicated in bracket is for new constructions. Planning for rehabilitation, if any, is to be done in consultation with RDSO.)
a)Soils type A (not needing blanket):
Rocky beds except those, which are very susceptible to weathering e.g. rocks consisting of shales and other soft rocks, which become muddy after coming into contact with water.
Well graded Gravel (GW) Well graded Sand (SW)
Soils conforming to specifications of blanket material. b)Soils type B (needing 45cm thick blanket):
Poorly graded Gravel (GP) having Uniformity Coefficient more than 2. Poorly grade Sand (SP) having Uniformity Coefficient more than 2. Silty Gravel (GM)
Silty Gravel - Clayey Gravel (GM - GC). c)Soils type C (needing 60cm thick blanket): Clayey Gravel (GC)
Silty Sand (SM)
Clayey Sand (SC)
Clayey Silty sand (SM-SC)
Note: The thickness of blanket on above type of soils shall be increased to 1m, if the plasticity index exceeds 7.
d)Soils type D (needing 1m thick blanket): Silt with low plasticity (ML)
Silty clay of low plasticity (ML-CL) Clay of low plasticity (CL)
Silt of medium plasticity (MI) Clay of medium plasticity (CI)
Rocks which are very susceptible to weathering
Soils having fines passing 75 micron sieve between 5 & 12%, i.e. for soils with dual symbol e.g., GP-GC, SW-SM, etc., thickness of blanket should be provided as per soil of second symbol (of dual symbol)