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ECGD 4122 – Foundation Engineering Lecture 2

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  1. Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 2

  2. Revision of Soil Mechanics • Soil Composition • Soil Classification • Groundwater • Stress (Total vs. Effective) • Settlement • Strength

  3. Soil: A 3-Phase Material Air Water Solid

  4. The Mineral Skeleton Solid Particles Volume Voids (air or water)

  5. Air Water Solid Three Phase Diagram Idealization: Three Phase Diagram Mineral Skeleton

  6. Water Solid Fully Saturated Soils Mineral Skeleton Fully Saturated

  7. Dry Soils Air Solid Dry Soil Mineral Skeleton

  8. Air Water Solid Partially Saturated Soils Mineral Skeleton Partly Saturated Soils

  9. Air Water Solid Three Phase System Va Wa~0 Vv Vw Ww WT VT Ws Vs Volume Weight

  10. Weight Relationships • Weight Components: • Weight of Solids = Ws • Weight of Water = Ww • Weight of Air ~ 0

  11. Volumetric Relationships • Volume Components: • Volume of Solids = Vs • Volume of Water = Vw • Volume of Air = Va • Volume of Voids = Va + Vw = Vv

  12. Volumetric Relationships • Volume Components: • Volume of Solids = Vs • Volume of Water = Vw • Volume of Air = Va • Volume of Voids = Va + Vw = Vv

  13. Specific Gravity • Unit weight of Water,w • w = 1.0 g/cm3 (strictly accurate at 4° C) • w = 62.4 pcf • w = 9.81 kN/m3

  14. Specific Gravity, Gs • Iron 7.86 • Aluminum 2.55-2.80 • Lead 11.34 • Mercury 13.55 • Granite 2.69 • Marble 2.69 • Quartz 2.60 • Feldspar 2.54-2.62

  15. Specific Gravity, Gs

  16. Example: Volumetric Ratios • Determine void ratio, porosity and degree of saturation of a soil core sample Data: • Weight of soil sample = 1013g • Vol. of soil sample = 585.0cm3 • Specific Gravity, Gs = 2.65 • Dry weight of soil = 904.0g

  17. 134.9cm3 243.9cm3 109.0cm3 1013.0g 585.0cm3 341.1cm3 904.0g Example Air Wa~0 W =1.00 Water 109.0g Solid s =2.65 Volumes Weights

  18. Air 134.9cm3 W =1.00 Water 243.9cm3 109.0cm3 585.0cm3 Solid s =2.65 341.1cm3 Volumes Example

  19. Soil Unit weight (lb/ft3 or kN/m3) • Bulk (or Total) Unit weight = WT / VT • Dry unit weight d = Ws / VT • Buoyant (submerged) unit weight b = - w

  20. Typical Unit weights

  21. Fine-Grained vs. Coarse-Grained Soils • U.S. Standard Sieve - No. 200 • 0.0029 inches • 0.074 mm • “No. 200” means...

  22. Sieve Analysis (Mechanical Analysis) • This procedure is suitable for coarse grained soils • e.g. No.10 sieve …. has 10 apertures per linear inch

  23. Hydrometer Analysis • Also called Sedimentation Analysis • Stoke’s Law

  24. GrainSize Distribution Curves

  25. Soil Plasticity • Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity. • Albert Atterberg, Swedish Soil Scientist (1846-1916)…..series of tests for evaluating soil plasticity • Arthur Casagrande adopted these tests for geotechnical engineering purposes

  26. liquid (pea soup) Liquid limit Plasticity Index plastic (pea nut butter) Plastic limit semi-solid (cheese) Shrinkage limit solid (hard candy) Atterberg Limits • Consistency of fine-grained soil varies in proportion to the water content

  27. Liquid Limit (LL or wL) • Empirical Definition • The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device

  28. Engineering Characterization of Soils Soil Properties that Control its Engineering Behavior Particle Size coarse-grained fine-grained • Soil Plasticity Particle/Grain Size Distribution Particle Shape

  29. Clay Morphology • Scanning Electron Microscope (SEM) • Shows that clay particles consist of stacks of plate-like layers

  30. Soil Consistency Limits • Albert Atterberg (1846-1916) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911) • Arthur Casagrande (1902-1981) ……Adopted these tests for geotechnical engineering purposes

  31. Arthur Casagrande (1902-1981) • Joined Karl Terzaghi at MIT in 1926 as his graduate student • Research project funded by Bureau of Public Roads • After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard • Soil Plasticity and Soil Classification (1932)

  32. Casagrande Apparatus

  33. Casagrande Apparatus

  34. Casagrande Apparatus

  35. Liquid Limit Determination

  36. Plastic Limit (PL, wP) • The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches

  37. Plastic Limit (PL, wP)

  38. Plasticity Index ( PI, IP ) • PI = LL – PL or IP=wL-wP • Note: These are water contents, but the percentage sign is not typically shown.

  39. Plasticity Chart

  40. USCS Classification Chart

  41. USCS Classification Chart

  42. Plasticity Chart

  43. Groundwater U = porewater pressure = wZw

  44. Stresses in Soil Masses P X X Area = A  = P/A Soil Unit Assume the soil is fully saturated, all voids are filled with water.

  45. Effective Stress • From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil. • Therefore, we may define effective stress = total stress minus pore pressure ′ =  - u where, ′ = effective stress  = total stress u = pore pressure

  46. Effective Stress ′ =  - u • The effective stress is the force carried by the soil skeleton divided by the total area of the surface. • The effective stress controls certain aspects of soil behavior, notably, compression & strength.

  47. Effective Stress Calculations ′z =  iHi - u where, H = layer thickness sat = saturated unit weight U = pore pressure = w Zw When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.

  48. Geostatic Stresses

  49. Compressibility & Settlement • Settlement requirements often control the design of foundations • This chapter provides a general overview of principles involved in settlement analysis • The subject will be dealt with in greater detail in Chapter 7.

  50. Increase in Vertical Effective Stress • Due to a Placement of a fill • Due to an external load