Soils and their Classification Soils What are they
- Slides: 50
Soils and their Classification
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
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.
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
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
Procedure for grain size determination u Sieving - used for particles > 75 mm u Hydrometer test - used for smaller particles – Analysis based on Stoke’s Law, velocity proportional to diameter
Procedure for grain size determination u Sieving - used for particles > 75 mm u Hydrometer test - used for smaller particles – Analysis based on Stoke’s Law, velocity proportional to diameter Figure 1 Schematic diagram of hydrometer test
Procedure for grain size determination u Sieving - used for particles > 75 mm u Hydrometer test - used for smaller particles – Analysis based on Stoke’s Law, velocity proportional to diameter Figure 1 Schematic diagram of hydrometer test
Grading curves W Well graded
Grading curves W Well graded U Uniform
Grading curves W Well graded U Uniform P Poorly graded
Grading curves W Well graded U Uniform P Poorly graded C Well graded with some clay
Grading curves W Well graded U Uniform P Poorly graded C Well graded with some clay F Well graded with an excess of fines
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.
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.
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
Example: equal amounts sand/silt/clay
Example: equal amounts sand/silt/clay
Example: equal amounts sand/silt/clay
Atterberg Limits u Particle size is not that useful for fine grained soils
Atterberg Limits u Particle size is not that useful for fine grained soils SL PL LL Figure 4 Moisture content versus volume relation during drying
Atterberg Limits u Particle size is not that useful for fine grained soils SL PL LL Figure 4 Moisture content versus volume relation during drying u u u SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit
Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit
Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip
Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip Liquidity Index = (m - PL)/Ip = LI
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
Unified Soil Classification u Each soil is given a 2 letter classification (e. g. SW). The following procedure is used.
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 mm)
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 mm) » Prefix S if > 50% of coarse is Sand » Prefix G if > 50% of coarse is Gravel
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 mm) » Prefix S if > 50% of coarse is Sand » Prefix G if > 50% of coarse is Gravel » Suffix depends on %fines
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 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
Unified Soil Classification To determine if W or P, calculate Cu and Cc x% of the soil has particles smaller than Dx
Unified Soil Classification To determine W or P, calculate Cu and Cc x% of the soil has particles smaller than Dx
Unified Soil Classification To determine W or P, calculate Cu and Cc x% of the soil has particles smaller than Dx
Unified Soil Classification To determine W or P, calculate Cu and Cc x% of the soil has particles smaller than Dx D 90 = 3 mm
Unified Soil Classification To determine W or P, calculate Cu and Cc If prefix is G then suffix is W if Cu > 4 and Cc is between 1 and 3 otherwise use P If prefix is S then suffix is W if Cu > 6 and Cc is between 1 and 3 otherwise use P
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
Unified Soil Classification – Fine grained soils (> 50% finer than 75 mm) – Both letters determined from plasticity chart
Example
Example • %fines (% finer than 75 mm) = 11% - Dual symbols required
Example • %fines (% finer than 75 mm) = 11% - Dual symbols required • D 10 = 0. 06 mm, D 30 = 0. 25 mm, D 60 = 0. 75 mm
Example Particle size fractions: Gravel Sand 17% 73% Silt and Clay 10%
Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12. 5, Cc = 1. 38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6
Example
Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12. 5, Cc = 1. 38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix 2 = M
Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12. 5, Cc = 1. 38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix 2 = M Dual Symbols are SW-SM
Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12. 5, Cc = 1. 38 Suffix 1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 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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