SOIL MECHANICS INTRODUCTION WITH ADVANTAGES AND WHY SOIL MECHANICS STUDY IS SO IMPORTANT
- 1. Problems in Geotechnical Engineering Natural Slope Tunnel Embank- ment Fill Dam Bridge Foundation Underground Excavation Reclama- tion Housing Site Building Foundation
- 6. Soils and their Classification ■ The behaviour of a structure depends upon the properties of the soil materials on which the structure rests. The properties of the soil materials depend upon the properties of the rocks from which they are derived. A brief discussion of the parent rocks is, therefore, quite essential in order ■ to understand the properties of soil materials.
- 7. UNIQUE NATURE OF SOIL AND ROCK MATERIALS ■ Geotechnical engineering is highly empirical and is perhaps much more of an “art” than the other disciplines within civil engineering because of the basic nature of soil and rock materials ■ These materials are often highly variable, even within a distance of a few millimetres. ■ Soils are heterogeneous rather than homogeneous materials. I.e. Properties may vary widely from point to point within a soil mass ■ They are non linear materials; their stress-strain curves are not straight lines ■ Soils are non-conservative materials i.e. they have a fantastic memory as they remember almost everything that ever happened to them, and this fact strongly affects engineering behaviour ■ Instead of being isotropic, soils are typically anisotropic, i.e. their properties are not the same in all directions
- 8. Soils - What are they? igneous, sedimentary and metamorphic. soil is any uncemented or weakly cemented accumulation of mineral particles formed by the weathering of rocks, the void space between the particles containing water and/or air. Weak cementation can be due to carbonates or oxides precipitated between the particles or due to organic matter. ■ Particulate materials -Sedimentary origins (usually). If the products are transported and deposited in a different location they constitute a transported soil, the agents of transportation being gravity, wind, water and glaciers - Residual. If the products of weathering remain at their original location they constitute a residual soil ■ Wide range of particle sizes - larger particles: quartz, feldspar - very small particles: clay minerals ■ Voids between particles
- 10. Aragonite-rich soil x 2000
- 12. 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.
- 13. SOIL PROFILE ■ Topsoil A layer of organic soil, usually not more than 500 mm thick, in which humus (highly organic partly decomposed vegetable matter) is often found. ■ Subsoil The portion of the Earth’s crust affected by current weathering, and lying between the topsoil and the unweathered soil below. ■ Hardpan In humid climates humic acid can be formed by rainwater causing decomposition of humus. This acid leaches out iron and alumina oxides down into the lower layers where they act as cementation agents to form a hard, rock-like material. Hardpan is difficult to excavate and, as it does not soften when wet.
- 16. 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
- 17. 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
- 19. An example of particle Sizes
- 25. Procedure for grain size determination ■ Sieving - used for particles > 75 mm Sieve analysis Particle size (mm) Total Sieves Plate Mass (g)
- 26. 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
- 27. 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
- 28. 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
- 29. 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
- 30. 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
- 31. 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
- 32. 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
- 33. 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
- 36. 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.
- 37. 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. 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) S a n d S i z e s ( % ) C l a y S i z e s ( % ) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION, U. S. ENGINEER DEPT.
- 38. 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 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) S a n d S i z e s ( % ) C l a y S i z e s ( % ) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION, U. S. ENGINEER DEPT.
- 39. Example: equal amounts sand/silt/clay 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) S a n d S i z e s ( % ) C l a y S i z e s ( % ) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION,
- 40. Example: equal amounts sand/silt/clay 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) S a n d S i z e s ( % ) C l a y S i z e s ( % ) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION,
- 41. Example: equal amounts sand/silt/clay 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 Silt Sizes (%) S a n d S i z e s ( % ) C l a y S i z e s ( % ) Sand Silty Sand Sandy Silt Clay-Sand Clay-Silt Sandy Clay Silty Clay Clay LOWER MISSISSIPPI VALLEY DIVISION,
- 42. Atterberg Limits ■ Particle size is not that useful for fine grained soils
- 43. 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 60 Moisture Content (%) Volume LL SL PL Liquid (soup) Plastic (butter) Semi-solid (cheese) Solid (cookie) Volume of soil
- 44. 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 60 Moisture Content (%) Volume LL SL PL Liquid (soup) Plastic (butter) Semi-solid (cheese) Solid (cookie) Volume of soil
- 45. 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 50 0 50 100 Moisture Content (%) Vol. of Soil LL SL PL
- 46. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Moisture content massof water massof solids
- 47. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip Moisture content massof water massof solids
- 48. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit Plasticity Index = LL - PL = PI or Ip Liquidity Index = (m - PL)/Ip = LI Moisture content massof water massof solids
- 52. 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
- 53. Unified Soil Classification ■ Each soil is given a 2 letter classification (e.g. SW). The following procedure is used.
- 54. 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)
- 55. 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
- 56. 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
- 57. 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
- 58. 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
- 59. 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
- 60. 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
- 61. 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
- 62. 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 C D D u 60 10 C D D D c 30 2 60 10 ( )
- 63. 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 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60 Plasticity index 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
- 64. Unified Soil Classification – Fine grained soils (> 50% finer than 75 mm) – 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 Plasticity index 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
- 65. 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 Determine percentages of gravel and sand from grain size curve Use grain size curve in identifying the fractions as given under field identification Depending on percentages of fines (fraction smaller than .075mm sieve size) coarse grained soils are classified as follows Less than 5% More than 12% 5% to 12% GW, GP, SW, SP GM, GC, SM, SC Bordeline case requiring use of dual symbols The .075mm sieve size is about the smallest particle visible to the naked eye Fine grained soils More than half of material is smaller than .075mm sieve size Coarse grained soils More than half of material is larger than .075mm sieve size Silts and clays liquid limit greater than 50 Silts and clays liquid limit less than 50 Sands More than half of coarse fraction is smaller than 2.36mm Gravels More than half of coarse fraction is larger than 2.36mm Sands with fines (appreciable amount of fines) Clean sands (little or no fines) Gravels with fines (apreciable amount of fines) Clean gravels (little or no 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 Plasticity index 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-ML CL-ML
- 66. Example 0.0001 0.001 0.01 0.1 1 10 100 0 20 40 60 80 100 Particle size (mm) % Finer
- 67. 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 mm) = 11% - Dual symbols required
- 68. 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 mm) = 11% - Dual symbols required • D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm
- 69. 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%
- 70. 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
- 71. Example 0 10 20 30 40 50 60 70 80 90 100 Liquid limit 0 10 20 30 40 50 60 Plasticity index 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
- 72. 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
- 73. 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
- 74. 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