Classification of Soil
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This document discusses different systems for classifying soils, including particle size, textural, Highway Research Board (HRB), Unified Soil Classification System (USCS), and Indian Standard Classification System (ISCS). The USCS system divides soils into four main groups - coarse grained, fine grained, organic, and peat. Each soil is designated a two letter symbol based on grain type and plasticity. The ISCS system is similar but subdivides fine grained soils into low, intermediate, and high plasticity groups. Classification involves analyzing particle size distribution and plasticity characteristics on a chart to determine the appropriate group.
Classification of Soil
- 1. Classification of Soils 1 by Pranamesh Chakraborty
- 3. Purpose Arrange various types of soils into groups according to their engineering or various other characteristics. Soil possessing similar characteristics can be placed in the same group. 3Pranamesh Chakraborty
- 4. Soil Classification Systems 1. Particle Size Classification 2. Textural Classification 3. Highway Research Board (HRB) Classification 4. Unified Soil Classification System (USCS) 5. Indian Standard Classification System (ISCS) 4Pranamesh Chakraborty
- 5. Particle Size Classification 5Pranamesh Chakraborty
- 6. Soils arranged according to grain sizes. Terms such as gravel, sand, silt and clay are used to indicate grain sizes. These terms are used only as designation of particle sizes. They do not signify naturally occurring soil types. Naturally occurring soil are mixture of particles of different sizes. Introduction 6Pranamesh Chakraborty
- 7. IS Classification of Grain Size 7Pranamesh Chakraborty
- 9. Soil classification of composite soils exclusively based on Particle Size Distribution. Most popular is the Triangular Classification of U.S. Public Roads Administration. Classification based on % of sand, silt and clay size. More suitable for describing coarse grained soil. Clay soil properties are less dependent on particle size distribution. Introduction 9Pranamesh Chakraborty
- 10. Textural Classification Chart 10Pranamesh Chakraborty
- 11. Example 30% sand 30% silt 40% clay Soil Type Clay 11Pranamesh Chakraborty
- 12. Highway Research Board (HRB) Classification 12Pranamesh Chakraborty
- 13. Based on both particle-size composition as well as plasticity characteristics. Mostly used for pavement construction. 7 primary groups (A-1, A-2,…., A-7). A-1 subdivided into 2 subgroups. A-2 subdivided into 4 subgroups. Introduction 13Pranamesh Chakraborty
- 14. Used to describe performance of soil when used for pavement construction. Not used to place a soil in a particular group. It means rating the value of soil as a subgrade material within its own group. Higher the value of GI, poorer is the quality of material. Group Index (GI) 14Pranamesh Chakraborty
- 15. Group Index of a soil depends on: 1. Amount of material passing 75 micron IS sieve. 2. Liquid Limit. 3. Plastic Limit. Group Index (GI) determination 15Pranamesh Chakraborty
- 16. HRB Classification Table For A-7-5, IP<= wL-30 For A-7-6, IP > wL-30 16Pranamesh Chakraborty
- 17. HRB Classification Procedure Proceed from left to right on the chart. Correct group will be found by process of elimination. The first group from left into which the test data will fit will be the correct classification. Group Index should be rounded off to nearest integer and placed in parenthesis, such as, A-2-2 (6). 17Pranamesh Chakraborty
- 18. Example 56% passes 75 micron sieve. Plastic Limit (wP) =23% Liquid Limit (wL)=36% Plasticity Index (IP) = 36-23=13% 18Pranamesh Chakraborty
- 19. Example Passing 75 micron>35% IP < 40% wL>10% A-6 wL=36% IP =13% 19Pranamesh Chakraborty
- 20. Example Group Index Calculation a= 56-35=21 b=56-15=41, Take 40 56% passes 75 micron sieve. Plastic Limit (wP) =23% Liquid Limit (wL)=36% Plasticity Index (IP) =13% c=0 d=13-10=3 GI=5.4=5 A-6 (5) 20Pranamesh Chakraborty
- 21. Unified Soil Classification System USCS 21Pranamesh Chakraborty
- 22. Origin of USCS First developed by Professor A. Casagrande (1948) for the purpose of airfield construction during World War II. Afterwards, it was modified to enable the system to be applicable to dams, foundations, and other construction . 22Pranamesh Chakraborty
- 24. Classification Groups •The soil is classified into 15 groups. •Each group is designated a symbol consisting of two capital letters. •The first letter is based on main soil type. •The second letter is based on gradation and plasticity . 24Pranamesh Chakraborty
- 25. Classification Group Symbols Main Soil Type Prefix Subgroup Suffix Classification Group symbols Gravel G Well-graded Poorly-graded Silty Clayey W P M C GW GP GM GC Sand S Well-graded Poorly-graded Silty Clayey W P M C SW SP SM SC Silt M LL < 50% LL > 50% L H ML MH Clay C LL < 50% LL > 50% L H CL CH Organic O LL < 50% LL > 50% L H OL OH Peat Pt Pt 25Pranamesh Chakraborty
- 26. Unified Soil Classification System (Contd.) Coarse-grained soils: Fine-grained soils: NO.200 0.075 mm Well or Poor Graded based on Cu and Cc 50 % NO. 4 4.75 mm 50% wL >50 wL <50 Gravel (G) Sand (S) Silt (M) Clay (C) Based on wL and IP (Plasticity Chart) For G, use W if Cu > 4 and 1<Cc <3 For S, use W if Cu > 6 and 1<Cc <3 26Pranamesh Chakraborty
- 27. Plasticity Chart Below A-line, use M (Silt) or O (Organic) Above A-line, use C - Clay High Plasticity use H - wL >50 Low Plasticity use L - wL <50 27 Silt (M) Clay (C) Organic (O) Pranamesh Chakraborty
- 28. Plasticity Chart “The soil’s liquid limit (wL) after oven drying is less than 75 % of its liquid limit before oven drying.” If the above statement is true, then it is Organic Soil (OL or OH). Otherwise, it is Inorganic Soil (ML or MH) 28Pranamesh Chakraborty
- 29. Plasticity Chart When IP and wL are in the hatched portion of the plasticity chart, the soil is given dual symbol (CL-ML). Soil possessing properties of more than one group are termed as boundary soil and designated by dual group symbol. 29Pranamesh Chakraborty
- 30. USCS at a glance (Santamarina et al., 2001) 30Pranamesh Chakraborty
- 31. Group Symbols and Group Names Group Symbol Typical Name GW Well-graded gravels. GP Poorly-graded gravels. GM Silty gravels. GC Clayey gravels SW Well-graded sands. SP Poorly-graded sands. SM Silty sands. SC Clayey sands. 31Pranamesh Chakraborty
- 32. Group Symbol Typical Name CL Inorganic clays of low plasticity. ML Inorganic silts with slight plasticity. OL Organic soil of low plasticity. CH Inorganic clays of high plasticity. MH Inorganic silts with high plasticity. OH Organic soil of high plasticity. Pt Peat. Group Symbols and Group Names 32Pranamesh Chakraborty
- 33. 33 Example Passing No.200 sieve 30% Passing No.4 sieve 70 % LL= 33 PI= 12 PI= 0.73(LL-20), A-line PI=0.73(33-20)=9.49 Soil Type= SC Clayey sand (Santamarina et al., 2001) Passing No.200 sieve 30 % Passing No.4 sieve 70 % LL= 33 PI= 12 Highly Pranamesh Chakraborty
- 34. Indian Standard Classification System (ISCS) IS:1498-1970 34Pranamesh Chakraborty
- 35. Introduction Based on USCS system Modifications are: Fine grained soils subdivided into 3 groups (low, intermediate and high plasticity) as against two groups(low and high) in USCS. wL <35=L, 35<wL <5o=I, wL >50=H Thus, soil classified into 18 groups as against 15 in USCS. 35Pranamesh Chakraborty
- 36. Classification Group Symbols Main Soil Type Prefix Subgroup Suffix Classification Group symbols Gravel G Well-graded Poorly-graded Silty Clayey W P M C GW GP GM GC Sand S Well-graded Poorly-graded Silty Clayey W P M C SW SP SM SC Silt M LL < 35% 35<LL<>50 LL > 50% L I H ML MI MH Clay C LL < 35% 35<LL<>50 LL > 50% L I H CL CI CH Organic O LL < 35% 35<LL<>50 LL > 50% L I H OL OI OH Peat Pt Pt 36Pranamesh Chakraborty
- 37. Plasticity Chart High Plasticity use H - wL >50 Intermediate Plasticity use I – 35< wL >50 Low Plasticity use L - wL <35 37 Below A-line, use M (Silt) or O (Organic) Above A-line, use C - Clay Pranamesh Chakraborty