Lecture 4
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1. Soils are particulate materials that form from sedimentary or residual processes and contain a range of particle sizes from large particles like quartz down to very small clay particles. 2. Simple soil classification systems are needed for preliminary engineering design to determine properties like strength and stiffness from cheap, simple tests using core samples. 3. The most common classification systems are based on particle size using percentages of sand, silt, and clay, Atterberg limits for fine-grained soils, and systems like the Unified Soil Classification System which assigns a two-letter symbol based on grain size and plasticity.
Lecture 4
- 1. Soils and their Classification
- 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. Aragonite-rich soil x 2000
- 5. 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.
- 6. 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
- 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
- 8. Procedure for grain size determination Sieving - used for particles > 75 µm 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 µm 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 µm Hydrometer test - used for smaller particles – Analysis based on Stoke’s Law, velocity proportional to diameter Figure 1 Schematic diagram of hydrometer test
- 11. Grading curves 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer W Well graded
- 12. Grading curves 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer W Well graded U Uniform
- 13. Grading curves 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer W Well graded U Uniform P Poorly graded
- 14. Grading curves 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer W Well graded U Uniform P Poorly graded C Well graded with some clay
- 15. Grading curves 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer W Well graded U Uniform P Poorly graded C Well graded with some clay F Well graded with an excess of fines
- 16. 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.
- 17. 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. 1009080706050403020100 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) SandSizes(%) ClaySizes(%) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION, U. S. ENGINEER DEPT.
- 18. 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 1009080706050403020100 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) SandSizes(%) ClaySizes(%) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION, U. S. ENGINEER DEPT.
- 19. Example: equal amounts sand/silt/clay
- 20. Example: equal amounts sand/silt/clay
- 21. Example: equal amounts sand/silt/clay
- 22. Atterberg Limits Particle size is not that useful for fine grained soils
- 23. Atterberg Limits Particle size is not that useful for fine grained soils Figure 4 Moisture content versus volume relation during drying 0 10 20 30 40 0 20 40 60Moisture Content (%) Volume LLSL PL
- 24. 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 0 10 20 30 40 0 20 40 60Moisture Content (%) Volume LLSL PL
- 25. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit
- 26. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip
- 27. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip Liquidity Index = (m - PL)/Ip = LI
- 28. 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
- 29. Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The following procedure is used.
- 30. 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 µm)
- 31. 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 µm) » Prefix S if > 50% of coarse is Sand » Prefix G if > 50% of coarse is Gravel
- 32. 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 µm) » Prefix S if > 50% of coarse is Sand » Prefix G if > 50% of coarse is Gravel » Suffix depends on %fines
- 33. 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 µm) » 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
- 34. Unified Soil Classification To determine if W or P, calculate Cu and Cc C D D u = 60 10 C D D D c = × 30 2 60 10( ) x% of the soil has particles smaller than Dx
- 35. Unified Soil Classification To determine W or P, calculate Cu and Cc C D D u = 60 10 C D D D c = × 30 2 60 10( ) 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer x% of the soil has particles smaller than Dx
- 36. Unified Soil Classification To determine W or P, calculate Cu and Cc C D D u = 60 10 C D D D c = × 30 2 60 10( ) 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer x% of the soil has particles smaller than Dx
- 37. Unified Soil Classification To determine W or P, calculate Cu and Cc C D D u = 60 10 C D D D c = × 30 2 60 10( ) 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer D90 = 3 mm x% of the soil has particles smaller than Dx
- 38. 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 C D D u = 60 10 C D D D c = × 30 2 60 10( )
- 39. Unified Soil Classification 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60 Plasticityindex CH OH or MH CL OL ML or CL ML "A" line Comparing soils at equal liquid limit Toughness and dry strength increase with increasing plasticity index Plasticity chart for laboratory classification of fine grained soils 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
- 40. Unified Soil Classification – Fine grained soils (> 50% finer than 75 µm) – Both letters determined from plasticity chart 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60 Plasticityindex CH OH or MH CL OL ML or CL ML "A" line Comparing soils at equal liquid limit Toughness and dry strength increase with increasing plasticity index Plasticity chart for laboratory classification of fine grained soils
- 41. Give typical names: indicate ap- proximate percentages of sand and gravel: maximum size: angularity, surface condition, and hardness of the coarse grains: local or geological name and other pertinent descriptive information and symbol in parentheses. For undisturbed soils add infor- mation on stratification, degree of compactness, cementation, moisture conditions and drain- age characteristics. Example: Well graded gravels, gravel- sand mixtures, little or no fines Poorly graded gravels, gravel- sand mixtures, little or no fines Silty gravels, poorly graded gravel-sand-silt mixtures Clayey gravels, poorly graded gravel-sand-clay mixtures Well graded sands, gravelly sands, little or no fines Poorly graded sands, gravelly sands, little or no fines Silty sands, poorly graded sand-silt mixtures Clayey sands, poorly graded sand-clay mixtures GW GP GM GC SW SP SM SC Wide range of grain size and substantial amounts of all intermediate particle sizes Predominantly one size or a range of sizes with some intermediate sizes missing Non-plastic fines (for identification procedures see ML below) Plastic fines (for identification pro- cedures see CL below) Wide range in grain sizes and sub- stantial amounts of all intermediate particle sizes Predominantely one size or a range of sizes with some intermediate sizes missing Non-plastic fines (for identification pro- cedures, see ML below) Plastic fines (for identification pro- cedures, see CL below) ML CL,CI OL MH CH OH Pt Dry strength crushing character- istics None to slight Medium to high Slight to medium Slight to medium High to very high Medium to high Readily identified by colour, odour spongy feel and frequently by fibrous texture Dilatency (reaction to shaking) Quick to slow None to very slow Slow Slow to none None None to very high Toughness (consistency near plastic limit) None Medium Slight Slight to medium High Slight to medium Inorganic silts and very fine sands, rock flour, silty or clayey fine sands with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays Organic silts and organic silt- clays of low plasticity inorganic silts, micaceous or dictomaceous fine sandy or silty soils, elastic silts Inorganic clays of high plasticity, fat clays Organic clays of medium to high plasticity Peat and other highly organic soils Give typical name; indicate degree and character of plasticity, amount and maximum size of coarse grains: colour in wet con- dition, odour if any, local or geological name, and other pert- inent descriptive information, and symbol in parentheses For undisturbed soils add infor- mation on structure, stratif- ication, consistency and undis- turbed and remoulded states, moisture and drainage conditions Example Clayey silt, brown: slightly plastic: small percentage of fine sand: numerous vertical root holes: firm and dry in places; loess; (ML) Field identification procedures (Excluding particles larger than 75mm and basing fractions on estimated weights) Group symbols 1 Typical names Information required for describing soils Laboratory classification criteria C = Greater than 4 D D ----60 10 U C = Between 1 and 3 (D ) D x D ----------------------30 10 c 2 60 Not meeting all gradation requirements for GW Atterberg limits below "A" line or PI less than 4 Atterberg limits above "A" line with PI greater than 7 Above "A" line with PI between 4 and 7 are borderline cases requiring use of dual symbols Not meeting all gradation requirements for SW C = Greater than 6 D D ----60 10 U C = Between 1 and 3 (D ) D x D ----------------------30 10 c 2 60 Atterberg limits below "A" line or PI less than 4 Atterberg limits above "A" line with PI greater than 7 Above "A" line with PI between 4 and 7 are borderline cases requiring use of dual symbols Determinepercentagesofgravelandsandfromgrainsizecurve Usegrainsizecurveinidentifyingthefractionsasgivenunderfieldidentification Dependingonpercentagesoffines(fractionsmallerthan.075mm sievesize)coarsegrainedsoilsareclassifiedasfollows Lessthan5% Morethan12% 5%to12% GW,GP,SW,SP GM,GC,SM,SC Bordelinecaserequiringuseofdualsymbols The.075mmsievesizeisaboutthesmallestparticlevisibletothenakedeye Finegrainedsoils Morethanhalfofmaterialissmallerthan .075mmsievesize Coarsegrainedsoils Morethanhalfofmaterialislargerthan .075mmsievesize Siltsandclays liquidlimit greaterthan 50 Siltsandclays liquidlimit lessthan50 Sands Morethanhalfofcoarse fractionissmallerthan 2.36mm Gravels Morethanhalfofcoarse fractionislargerthan 2.36mm Sandswith fines (appreciable amountoffines) Cleansands (littleorno fines) Gravelswith fines (apreciable amountoffines) Cleangravels (littleorno fines) Identification procedure on fraction smaller than .425mm sieve size Highly organic soils Unified soil classification (including identification and description) Silty sand, gravelly; about 20% hard angular gravel particles 12.5mm maximum size; rounded and subangular sand grains coarse to fine, about 15% non- plastic lines with low dry strength; well compacted and moist in places; alluvial sand; (SM) 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60 Plasticityindex CH OH or MH OL ML or CL "A " line Comparing soils at equal liquid limit Toughness and dry strength increase with increasing plasticity index Plasticity chart for laboratory classification of fine grained soils CI CL-MLCL-ML
- 42. Example 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer
- 43. Example 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer • %fines (% finer than 75 µm) = 11% - Dual symbols required
- 44. Example 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer • %fines (% finer than 75 µm) = 11% - Dual symbols required • D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm
- 45. Example 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) %Finer Particle size fractions: Gravel 17% Sand 73% Silt and Clay 10%
- 46. 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
- 47. Example 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60Plasticityindex CH OH or MH CL OL ML or CL ML "A" line Comparing soils at equal liquid limit Toughness and dry strength increase with increasing plasticity index Plasticity chart for laboratory classification of fine grained soils
- 48. 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
- 49. 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
- 50. 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