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Soil Mechanics - I

Soil Mechanics - I

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Soil Mechanics - I

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  1. Soil Mechanics - I Lecture # 3,4 Chapter # 1. Introduction to Soil Mechanics (Part 2) Prepared by: EngrMamoonKareem Department of Civil Engineering Swedish College Of Engg & Tech Wah Cantt.

  2. Chapter Outlines • Introduction to Soil Mechanics • Weathering of Rocks • Soil and its Types • Physical Properties of Soil

  3. Physical Properties of Soil Color Soil Structure Particle Shape and Size Specific Gravity Soil Phases Porosity Void Ratio Moisture Content Degree of Saturation Air Content Consistency Limit Particle Size Distribution Relative Density

  4. 1. Color • Significance: Identification Purposes • Colour depends upon: • Type of soil mineral • Organic content • Amount of coloring oxides • Degree of oxidation • Examples: • Black color Manganese Compound • Green or Blue Ferrous Compounds • Red, Brown or Yellow Iron • Grey Organic matter

  5. 2. Soil Structure • Soil Structure is defined as the grouping orarrangementof soil particles with respect to one another. • Factors that affect the structure are: • Shape and Size • Mineralogical Composition • Nature and Composition of Water

  6. 2. Soil Structure • Structures in Cohesionless Soil • Single Grained • Soil particles are in stable position • The shape and size distribution of soil particles and their relative positions influence the denseness of packing. • Irregularity in the particle shapes generally yields an increase in the void ratio • Honeycombed • Relatively small sand and silt form small arches with chains of particles. • They can carry an ordinary static load because of large inter-particle spaces.

  7. 2. Soil Structure • Structures in Cohesive Soil • Flocculent Structure: • The clay minerals are extremely flaky in shape and have a large surface area-to-mass ratio. • Flocculated structure is developed when the edge of one clay particle is attracted to the flat face of another • Dispersed Structure: • Develops when the edges and faces of the clay particles have similar electrical charge • Also develops as a result of remolding by the transportation process (man-made earth fills )

  8. 2. Soil Structure

  9. 3. Particle Shape and Size • Different shapes:

  10. 3. Particle Shape and Size • Nomenclature of material (soil type) and range of sizes

  11. 4. Specific Gravity • The ratio of the unit weight of a substance, to the unit weight of water at 4oC • How many times a substance (or material) is heavier than water

  12. 4. Specific Gravity • Significance: • Used for determination and calculation of many other soil properties ,as • Particle size analysis by hydrometer test • Porosity and void ratio • Unit weight • Critical hydraulic gradient • Degree of saturation or zero air void

  13. 4. Specific Gravity • Specific Gravity of some Minerals and Soil types

  14. 5. Soil Phases • Any homogeneous part of a soil mass different from other parts in the mass and clearly separated from them is called a phase. • Fundamental phases: • Solid phase, • Liquid phase • Gaseous or vapour phase. • Ice phase (in cold regions)

  15. Schematic diagram indicating different soil phases

  16. 6. Porosity • The ratio of volume of all the voids “Vv” to the total volume of the soil mass “V” is known as the porosity. Where V = Vs + Vv V = Total volume of soil mass Vs= Volume of solid particles of soil Vv= Volume of voids, which may be filled with air or water or both Porosity falls in the range of 0 n  100

  17. How to calculate Porosity?

  18. 7. Void Ratio • The ratio of volume of voids present in a soil mass to the volume of solid particles. • It is denoted by “e”. • The void ratio is expressed as a number and the limiting values can be within the range.

  19. How to calculate Void Ratio?

  20. 8. Air Content • The ratio of the volume of air present in the voids to the total volume of a soil mass. Since; Vv = Va + Vw • Air content or Air Void Ratio fall within the range of

  21. 9. Degree of Saturation • The condition when voids are partially filled with water is expressed by the degree of saturation or relative moisture content. It is the ratio of actual volume of water in voids “Vw” to the total volume of voids “Vv”. Ww – is the weight of water actually present in the voids. Wv – is wt of water that can fill all the voids. m – actual moisture content. msat – moisture content when all voids are totally filled with water. The range of “S” 0  S  100.

  22. 10. Moisture Content • The amount of water present in the voids of a soil in its natural state. • The common range of moisture content for most soil is 20-40 percent. • Oven dried soil has zero percent moisture and the soils which appear dry (i.e., air dried soil) often have 2 to 4 percent moisture content. • The range of water content is:

  23. Different forms of moisture • The moisture/water in the voids of a soil mass can occur in a variety of forms. Depending upon the form of occurrence they are given different names e.g., • Hygroscopic Moisture • Film Moisture • Capillary Moisture • Chemically Bound Moisture

  24. Different forms of moisture • Hygroscopic Moisture: • Also known as adsorbed moisture, contact moisture or surface bound moisture. • This form of soil moisture exists as a very thin film of moisture surrounding the surfaces of individual soil particles and is held by the forces of adhesion. • It depends upon temperature and humidity. • It is not affected by gravitational forces, capillary forces and air drying at ordinary ordinary temperature. • The approximate values of hygroscopic moisture for various soils are as under: 1- Sand 1-2 % 2- Silt 7-9 % 3- Clay 17-20 %

  25. Different forms of moisture • Film Moisture: • The moisture film attached to the soil particles, above the layer of hygroscopic moisture film, is known is film moisture. • It is held by the molecular forces and is not affected by gravity. • The amount of film moisture depends on the specific surface i.e., higher the specific surface higher will be the film moisture and vice versa.

  26. Different forms of moisture • Capillary Moisture: • The moisture which in held within the voids of capillary size. The capillary moisture is continuously connected to the groundwater table. • Capillary water can be removed from the soil by drainage

  27. Different forms of moisture • Chemically Bound Moisture: • Moisture contained chemically within the mineral particles and can be removed only by chemical processes of the substance when the crystalline structure of the mineral breaks. • Chemically bound moisture is not important for common soil engineering problems and therefore is not determined.

  28. 11. Particle Size Distribution • The percentage of various particle sizes present in a soil is known as particle size distribution or gradation. • Particle size analysis is made by sieving or by sedimentation. • Sieving method – when particle size >.074 mm • Sedimentation method – when particle size < .074mm

  29. 11. Particle Size Distribution • The sieves normally required are as follows:

  30. 11. Particle Size Distribution • Significance: • Engineering classification of soils. • Selection of the most suitable soil for construction of roads, airfields, levees, dams and other embankments. • To predict the seepage through soil (although permeability tests are more generally used) • To predict the susceptibility to frost action. • Selection of most suitable filter material.

  31. 11. Particle Size Distribution • The gradation curve: • A gradation curve is drawn by plotting the percentage finer (%age passing) on ordinate against the particle sizes on abscissa. • The gradation curves indicate the type of soil, and provide very important information related to the properties and behavior of soil

  32. 11. Particle Size Distribution • The gradation curves have great importance in civil engineering and are extensively used for the following purposes. • Determination of Effective Grain (Particle) Size. • Determination of Uniformity co-efficient. • Determination of co-efficient of Curvature. • Determination of percentage of different soil types in a soil sample e.g., sand, silt, clay. • Determination of percentage larger or finer than a given size. • Classification of soil. • Design of filters. • Concrete mix design.

  33. 11. Particle Size Distribution • Well-Graded Soil: • A soil containing an assortment of particles with a wide range of sizes. • A well-graded soil has following merits: 1. Higher shear strength 2. Higher density 3. Reduced Compressibility 4. Higher stability 5. Higher Bearing Capacity 6. Low permeability well graded uniformly graded Ideal packing, due to particles Loose packing, as smaller ranging from large to small particles to fill voids are sizes missing

  34. 11. Particle Size Distribution • Uniformly-Graded Soil: • A uniformly graded soil is defined as a soil containing particles having a limited range of sizes (Almost the same sizes) • Poorly-Graded Soil: • A poorly graded soil is defined as a soil containing particles of varying sizes with intermediate particle sizes missing. • Such soils give lower density and lower strength. • The gradation curve of a poorly graded soil show steps indicating an excess of certain particle sizes, and a deficiency of others

  35. 11. Particle Size Distribution • The gradation curves: • well graded soil b) uniformly graded soil • poorly graded soil.

  36. 11. Particle Size Distribution • Co-efficient of uniformity: • When the value of Cu is less than 4, the soil is generally considered as uniformly graded. • A higher value of Cu represents a wide range of particle sizes and the soil is termed as well graded.

  37. 11. Particle Size Distribution • Co-efficient of curvature: • It is also known as coefficient of gradation (Cg) or Co-efficient of Concavity. • Cc = 1, represents that all the soil particles have the same size, and the soil is uniformly graded. • Cc between 0.2 and 2.0 indicate well graded or poorly graded soil.

  38. 12. Relative Density (Dr) • The term relative density (also called density index, ID) is used to express the state of compactness of a granular soil. • The following relationship between the void ratio values is termed as the relative density.

  39. 12. Relative Density (Dr) • The range of values for relative densities (Dr) and the commonly referred state of compaction for granular soil.

  40. 13. Atterbergor Consistency Limits • The consistency of a soil means its physical state with respect to the moisture content present that time. • Consistency states are: • Solid state • Semi solid state • Plastic state • Liquid state.

  41. 13. Atterbergor Consistency Limits • Boundaries of the above four states are: • Shrinkage Limit: It is the moisture content at which a soil changes from solid state to semi-solid state. • Plastic Limit: It is the moisture content at which a soil changes from semi-solid state to plastic state. • Liquid Limit: It is the moisture content at which a soil changes from plastic state to liquid state.

  42. 13. Atterbergor Consistency Limits • Shrinkage Limit • It is that moisture content at which a reduction in moisture will not cause a decrease in the total volume of soil mass, but an increase in moisture will result in an increase in volume of soil mass. • At Shrinkage Limit The Degree Of Saturation is 100%. • At certain point during drying process, air begins to enter the soil mass and the volume decrease becomes appreciably less than the volume of water lost. • The shrinkage limit is not given much importance since it is not used in soil classification.

  43. 13. Atterbergor Consistency Limits • Shrinkage Limit • Concept of surface tension forces and induced compressive stresses(a) Particle separated due to thick moisture film(b) Meniscus contracting due to drying process(c) Meniscus tending to tear off (d) Meniscus fully torn off allowing air entry

  44. 13. Atterbergor Consistency Limits • Relationship between volume and moisture content:

  45. The soils which show higher shrinkage upon drying also swell more upon wetting and are known as expansive soils. Expansive soils are very dense and hard in dry state due to very high shrinkage stresses Shrinkage cracks at Rawal lake which dried due to drought

  46. 13. Atterbergor Consistency Limits • Plastic Limit • The moisture content at which a soil can be rolled into threads of 1/8” (3.2mm) diameter without cracking and crumbling. • Threads thinner than 1/8” (3.2 mm) diameter are possible, if the moisture is higher than the plastic limit. • And if the moisture is less than plastic limit the thread will crumble before reaching the required diameter of 1/8” (3.2 mm).

  47. 13. Atterbergor Consistency Limits • Plastic Limit

  48. 13. Atterbergor Consistency Limits • Liquid Limit • The moisture content at which 25 blows of Cassagrande apparatus closes a standard groove cut in the soil paste along a distance of 12.7 mm (0.5 in). • The moisture content which gives a penetration depth of 20mm of the standard cone (fall cone test) into the soil, when the cone is released for 5 seconds.

  49. 13. Atterbergor Consistency Limits • Liquid Limit

  50. 13. Atterbergor Consistency Limits • Plasticity Index • Plasticity Index indicates the range of moisture through which a cohesive soil behaves as a plastic material • It is the numerical difference between liquid and plastic limits. It is expressed as: