Classification of Soils

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Piers Geoffrey Booker
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1 Classification of Soils

2 Soils - What are they? Particulate materials - Sedimentary origins (usually) - Residual Wide range of particle sizes - larger particles: quartz, feldspar - very small particles: clay minerals Voids between particles

3 Need for Simple Classification Usually soil on site has to be used. Soils differ from other engineering materials in that one has little control over their properties Extent and properties of the soil have to be determined Cheap and simple tests are required to give an indication of engineering properties, e.g. stiffness, strength, for preliminary design The classification must use core samples obtained from the ground. This information is often supplemented by in-situ tests such as cone penetration tests.

4 What is Classification It is a system of labelling soils, which would convey the same meaning universally. Objective is to sort the soils into groups based on similarity in engineering behaviour. Most commonly used properties to use in classification are grain-size distribution and plasticity.

5 Methods of Classification 1. Field Identification 2. Laboratory Identification (sieve analysis, plasticity properties) A. Unified Soil Classification System (USCS) B. Indian Standard Soil Classification System (ISSCS)

6 Field Identification Visual examination (for coarse fraction) Dilatancy (reaction to shaking) Toughness (consistency near plastic limit) Dry strength (crushing resistance) Organic content and colour Other identification tests 6

7 Classification based on Particle Size Particle size is used because it is related to mineralogy e.g. very small particles usually contain clay minerals Broad Classification Coarse grained soils» sands, gravels - visible to naked eye Fine grained soils» silts, clays, organic soils Textural or grain-size classifications are inadequate primarily because plasticity characteristics consistency limits and indices do not find any place in these classifications

8 Procedure for grain size determination Sieving - used for particles > 75 mm Hydrometer test - used for smaller particles Analysis based on Stoke s Law, velocity proportional to diameter

9 Procedure for grain size determination Sieving - used for particles > 75 mm Hydrometer test - used for smaller particles Analysis based on Stoke s Law, velocity proportional to diameter Figure 1 Schematic diagram of hydrometer test

10 Procedure for grain size determination Sieving - used for particles > 75 mm Hydrometer test - used for smaller particles Analysis based on Stoke s Law, velocity proportional to diameter Figure 1 Schematic diagram of hydrometer test

11 % Finer Grading curves Particle size (mm) W Well graded

12 % Finer Grading curves Particle size (mm) W U Well graded Uniform

13 % Finer Grading curves Particle size (mm) W U P Well graded Uniform Poorly graded

14 % Finer Grading curves Particle size (mm) W U P C Well graded Uniform Poorly graded Well graded with some clay

15 % Finer Grading curves Particle size (mm) W U P C F Well graded Uniform Poorly graded Well graded with some clay Well graded with an excess of fines

16 Atterberg Limits Particle size is not that useful for fine grained soils

17 Volume Atterberg Limits Particle size is not that useful for fine grained soils SL PL LL 0 50 Moisture 100 Content (%) Figure 4 Moisture content versus volume relation during drying

18 Volume Atterberg Limits Particle size is not that useful for fine grained soils SL PL LL 0 50 Moisture 100 Content (%) Figure 4 Moisture content versus volume relation during drying SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit

19 Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Moisture content mass of water massof solids

20 Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Moisture content mass of water massof solids Plasticity Index = LL - PL = PI or I p

21 Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Moisture content mass of water massof solids Plasticity Index = LL - PL = PI or I p Liquidity Index = (m - PL)/I p = LI

22 Unified Soil Classification Classification system is based on those characteristics of the soil which indicate how it will behave as a construction material.

23 Unified Soil Classification System (USCS) Table. Prefix and Suffix of USCS Soil type Prefix Subgroup Suffix Gravel G Well graded W Sand S Poorly graded P Silt M Silty M Clay C Clayey C Organic O w L < 50% L Peat Pt w L > 50% H

24 Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The following procedure is used. Coarse grained (>50% larger than 75 mm)

25 Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The following procedure is used. Coarse grained (>50% larger than 75 mm)» Prefix S if > 50% of coarse is Sand» Prefix G if > 50% of coarse is Gravel

26 Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The following procedure is used. Coarse grained (>50% larger than 75 mm)» Prefix S if > 50% of coarse is Sand» Prefix G if > 50% of coarse is Gravel» Suffix depends on %fines

27 Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The following procedure is used. Coarse grained (>50% larger than 75 mm)» Prefix S if > 50% of coarse is Sand» Prefix G if > 50% of coarse is Gravel» Suffix depends on %fines» if %fines < 5% suffix is either W or P» if %fines > 12% suffix is either M or C» if 5% < %fines < 12% Dual symbols are used Soil given in lab

28 Coarse grained soils are classified on basis of GSD and fine grained soils on basis of their plasticity characteristics. Four major groups coarse grained, fine grained, organic soils and peat. Coarse grained soils more than 50% retained on 75μ sieve - Gravel (G) if more than 50% retained on 4.75mm, else Sand (S) - well graded (W) or poorly graded (P) based on C u and C c Fine grained soils more than 50% passing 75μ sieve - silt (M) and clay (C) based on LL and PI - low (L) or high (H) plasticity if LL is less than or greater than 50% Boundary soils soils possessing characteristics of more than one group - assigned dual symbols, e.g. GW-GC (well graded gravel with clay fines)

29 Unified Soil Classification To determine if W or P, calculate C u and C c C C c D D u 60 D ( D D ) x% of the soil has particles smaller than D x

30 % Finer Unified Soil Classification To determine if W or P, calculate C u and C c 100 C C c D D u 60 D ( D D ) x% of the soil has particles smaller than D x Particle size (mm)

31 % Finer Unified Soil Classification To determine W or P, calculate C u and C c 100 C C c D D u 60 D ( D D ) x% of the soil has particles smaller than D x Particle size (mm)

32 % Finer Unified Soil Classification To determine W or P, calculate C u and C c 100 C C c D D u 60 D ( D D ) x% of the soil has particles smaller than D x Particle size (mm) D 90 = 3 mm

33 Unified Soil Classification To determine W or P, calculate C u and C c C D D u C c D 2 30 ( D D ) For Gravels, if prefix is G then suffix is W if C u > 4 and C c is between 1 and 3 otherwise use P For Sands, if prefix is S then suffix is W if C u > 6 and C c is between 1 and 3 otherwise use P

34 Plasticity index Unified Soil Classification Coarse grained soils To determine M or C use plasticity chart Plasticity index Comparing Comparing soils at soils equal liquid at equal limit liquid limit Toughness Toughness and dryand strength dryincrease strength increase with with increasing increasing plasticity plasticity index index CH CH "A" line 20 OH 20 OH CL CL or or CL CL OL OL MH ML or MH 0 ML ML or 0 ML Liquid limit Liquid limit Plasticity Plasticity chart chart for laboratory for laboratory classification classification of fine grainedof soils fine grained soils Below A-line use suffix M - Silt Above A-line use suffix C - Clay "A" line

35 Plasticity index Plasticity index Unified Soil Classification Fine grained soils (> 50% finer than 75 mm) Both letters determined from plasticity chart 60 Comparing 60 soils at equal liquid limit 50 Toughness Comparing andsoils dryat strength equal liquid increase limit with 50 increasing Toughness andplasticity dry strengthindex increase 40 with increasing plasticity index 40 CH 30 CH OH 20 OH CL or 10 CL CL OL MH ML or or 10 CL OL 0 ML MH ML or ML Liquid 50 limit Liquid limit Plasticity chart for laboratory classification Plasticity chartof fine grained soils for laboratory classification of fine grained soils "A" line "A" line

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Indian Standard Soil Classification System IS: 1498-1970 Table.

38 Indian Standard Soil Classification System IS: Table. Prefix and Suffix of ISSCS Soil type Prefix Subgroup Suffix Gravel G Well graded W Sand S Poorly graded P Silt M Silty M Clay C Clayey C Organic O w L < 35% L 35% < w L < 50% I Peat Pt w L > 50% H

39 Soils shall be broadly divided into three divisions 1. Coarse-grained Soils: More than 50% of the total material by weight is larger than 75-μ IS Sieve size. Subdivisions (a) Gravels: More than 50% of coarse fraction (+ 75 μ) is larger than 4.75 mm IS Sieve size. (Symbol G) (b) Sands: More than 50% of Coarse fraction (+ 75 μ) is smaller than 4.75 mm IS Sieve size. (Symbol S) 3

40 2. Fine-grained Soils: More than 50% of the total material by weight is smaller than 75-μ IS Sieve size. Subdivisions (a)silts (M) and clays (C) of low compressibility : Liquid limit less than 35% (L). (Symbol- ML or CL) (b) Silts and clays of medium compressibility : Liquid limit greater than 35% and less than 50% (I). (Symbol- MI or CI) (c) Silts and clays of high compressibility: Liquid limit greater than 50 (H). (Symbol- MH or CH)

41 3. Highly Organic Soils and Other Miscellaneous Soil Materials: These soils contain large percentages of fibrous organic matter, such as peat, and particles of decomposed vegetation. OL, OI or OH depending on liquid limit Subdivisions Boundary soils (Border line cases) Soils possessing characteristics of more than one group, i.e., having 5-12% fines - Assigned dual symbols, e.g. GW-GC (well graded gravel with clay fines) - First part of dual symbol indicates gradation, while the second indicates the nature of fines 4

42 *Note: This dual symbol is when %fines > 12% When %fines is between 5 and 12%, dual symbols are like GW-GC, GW-GM, etc.

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45 % Finer Example Particle size (mm)

46 % Finer Example Particle size (mm) %fines (% finer than 75 mm) = 11% - Dual symbols required

47 % Finer Example Particle size (mm) %fines (% finer than 75 mm) = 11% - Dual symbols required D 10 = 0.06 mm, D 30 = 0.25 mm, D 60 = 0.75 mm

48 % Finer Example Particle size (mm) Particle size fractions: Gravel 5% Sand 84% Silt and Clay 11%

49 Of the coarse fraction about 85% is sand, hence Prefix is S C u = 12.5, C c = 1.38

50 Of the coarse fraction about 80% is sand, hence Prefix is S C u = 12.5, C c = 1.38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 I p = = 6

51 Of the coarse fraction about 80% is sand, hence Prefix is S C u = 12.5, C c = 1.38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 I p = = 6 From Plasticity Chart point lies below A-line Suffix 2 = M

52 Of the coarse fraction about 80% is sand, hence Prefix is S C u = 12.5, C c = 1.38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 I p = = 6 From Plasticity Chart point lies below A-line Suffix 2 = M Dual Symbols are SW-SM To complete the classification the Symbols should be accompanied by a description

53 Chapter Over

Thi_ Qar University College of Engineering/Civil Engineering Department. Highway Lectures. Fourth Class. Part #2: - Subgrade Soil

Thi_ Qar University College of Engineering/Civil Engineering Department Highway Lectures Fourth Class Part #2: - Subgrade Soil Lecture #2 Soil Classification DAS, Chapter 4, Engineering Classification