Soils and their Classification

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

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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.

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 notes a,b,c

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 notes a,b,c

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

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

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

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

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

Simple Classification In general soils contain a wide range of particle sizes Some means of describing the characteristics of soils with different proportions of sand/silt/clay is required. 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 Particle size is not that useful for fine grained soils

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

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

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 Used to determine the suitability of different soils Used to develop correlations with useful soil properties 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 Each soil is given a 2 letter classification (e.g. SW). The following procedure is used.

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)

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

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

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

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 D90 = 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

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

Example %fines (% finer than 75 mm) = 11% - Dual symbols required D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

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

Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12.5, Cc = 1.38 Suffix1 = 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 Suffix1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix2 = M

Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12.5, Cc = 1.38 Suffix1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix2 = M Dual Symbols are SW-SM

Of the coarse fraction about 80% is sand, hence Prefix is S Cu = 12.5, Cc = 1.38 Suffix1 = W From Atterberg Tests LL = 32, PL = 26 Ip = 32 - 26 = 6 From Plasticity Chart point lies below A-line Suffix2 = M Dual Symbols are SW-SM To complete the classification the Symbols should be accompanied by a description