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Soils and their Classification

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

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Need for Simple Classification u Usually soil on site has to be used. –Soils differ from other engineering materials in that one has little control over their properties u Extent and properties of the soil have to be determined u 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.

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Classification based on Particle Size u Particle size is used because it is related to mineralogy –e.g. very small particles usually contain clay minerals u Broad Classification –Coarse grained soils »sands, gravels - visible to naked eye

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Classification based on Particle Size u Particle size is used because it is related to mineralogy –e.g. very small particles usually contain clay minerals u Broad Classification –Coarse grained soils »sands, gravels - visible to naked eye –Fine grained soils »silts, clays, organic soils

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

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

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

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Grading curves WWell graded

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Grading curves WWell graded UUniform

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Grading curves WWell graded UUniform PPoorly graded

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Grading curves WWell graded UUniform PPoorly graded CWell graded with some clay

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Grading curves WWell graded UUniform PPoorly graded CWell graded with some clay FWell graded with an excess of fines

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Simple Classification u In general soils contain a wide range of particle sizes u Some means of describing the characteristics of soils with different proportions of sand/silt/clay is required.

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Simple Classification u In general soils contain a wide range of particle sizes u Some means of describing the characteristics of soils with different proportions of sand/silt/clay is required.

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Simple Classification u In general soils contain a wide range of particle sizes u Some means of describing the characteristics of soils with different proportions of sand/silt/clay is required. u Note the importance of clay - Finest 20% control behaviour

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Example: equal amounts sand/silt/clay

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

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Atterberg Limits u Particle size is not that useful for fine grained soils Figure 4 Moisture content versus volume relation during drying LL SL PL

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Atterberg Limits u Particle size is not that useful for fine grained soils Figure 4 Moisture content versus volume relation during drying u SL - Shrinkage Limit u PL - Plastic Limit u LL - Liquid limit LL SL PL

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Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit

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Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or I p

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Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or I p Liquidity Index = (m - PL)/I p = LI

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Classification Systems u Used to determine the suitability of different soils u Used to develop correlations with useful soil properties u Special Purpose (Local) Systems –e.g. PRA system of AAHSO »1. Well graded sand or gravel: may include fines »2. Sands and Gravels with excess fines »3. Fine sands »4. Low compressibility silts »5. High compressibility silts »6. Low to medium compressibility clays »7. High compressibility clays »8. Peat and organic soils

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Unified Soil Classification u Each soil is given a 2 letter classification (e.g. SW). The following procedure is used.

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

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

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

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

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

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

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

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Unified Soil Classification To determine W or P, calculate C u and C c D 90 = 3 mm x% of the soil has particles smaller than D x

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Unified Soil Classification To determine W or P, calculate C u and C c If prefix is G then suffix is W if C u > 4 and C c is between 1 and 3 otherwise use P If prefix is S then suffix is W if C u > 6 and C c is between 1 and 3 otherwise use P

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Unified Soil Classification Coarse grained soils To determine M or C use plasticity chart Below A-line use suffix M - Silt Above A-line use suffix C - Clay

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Unified Soil Classification –Fine grained soils (> 50% finer than 75 m) –Both letters determined from plasticity chart

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Example

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%fines (% finer than 75 m) = 11% - Dual symbols required

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

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Example Particle size fractions: Gravel 17% Sand 73% Silt and Clay 10%

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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 = 32 - 26 = 6

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Example

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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 = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix 2 = M

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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 = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix 2 = M Dual Symbols are SW-SM

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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 = 32 - 26 = 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

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