Engineering Classification of Soils

1. Engineering Classification of Soils

3. Overview • Two Systems of Classification 1. Pedological Classifications (soil weathering, texture, chemistry, profile thickness, etc.) 2. Engineering Classifications • soil texture • degree ofplasticity (Atterberg Limits)

5. B. Overview of Mechanics As water content increases, the shear strength decreases

6. It’s all about shear strength

7. C. Liquid Limit • Soil is practically a liquid • Shows minimal shear strength • Defined as the moisture content required to close a distance of 0.5 inch along the bottom of a groove after 25 blows of the liquid limit device. animation

8. C. Liquid Limit D. Plastic Limit • Water content at which the soil is a plastic • Less water content than liquid limit • Wide range of shear strengths at plastic limit • Defined as the moisture content % at which the soil begins to crumble when rolled into 1/8” diameter threads animation

9. C. Liquid Limit D. Plastic Limit E. Plasticity Index (PI) • Difference between Liquid Limit and Plastic Limit • Important measure of plastic behavior

10. C. Liquid Limit In general…. PI Degree of Plasticity 0 Nonplastic 1-5 Slightly plastic 5-10 Low plasticity 10-20 Medium plasticity 20-40 High plasticity 40+ Very high plasticity (from Burmister, 1949) D. Plastic Limit E. Plasticity Index (PI) • Difference between Liquid Limit and Plastic Limit • Important measure of plastic behavior

11. I. Unified System • Overview A. Arthur Casagrande (USAF) proposed for the construction of Airfields B. Basis -Over half of material retained on #200 sieve, use textural characteristics -Over half of material passes the #200 sieve, use plasticity-compressibility characteristics

12. II. Unified System B. The classification scheme

13. II. Unified System C. The procedure • Determine the percent passing through the #200 sieve (boundary between sand and silt/clay). • If less than 50% passes, then it is a coarse grained soil (gravel and sand) • If greater than 50% passes, then it is a fine grained soil (silt and clay)

14. “clayey materials” “silty materials”

16. Uniformity Coefficient = D60/D10, where we use the % finer by weight (% passing through) for the values

19. “clayey materials” “silty materials”

24. HOMEWORK: Classify the following soils by both the AASHTO and Unified Systems, and give the group index for the AASHTO system.

25. Alternate method for classifying soils using Unified Method…..(bonus information!)

26. For fine grained soils: Where R = ‘retained’ F = ‘falling through’

27. For fine grained soils:

28. For fine grained soils:

29. For coarse-grained soils: See next slide

30. Uniformity Coefficient = D60/D10, where we use the % finer by weight (% passing through) for the values

32. Fine grained soils

33. Fine grained soils

34. Gravelly soils

35. sandy soils

36. Procedure for AASHTO Classification(American Association of StateHighway and Transportation Officials) Developed in 1929 as the Public Road Administration Classification System Modified by the Highway Research Board (1945)

37. Procedure for AASHTO Classification • Determine the percentage of soil passing the #200 sieve • Determine the subgroups • For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40, and 200 sieves, AND • Determine the liquid limit and plasticity index • THEN, determine soil group or subgroup from Table 9.1

38. Cobble Gravel Very Coarse to Med Sand Fine/Very Fine Sand Silt/Clay 3 “ #10 #40 #200 • For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40, and 200 sieves. #10 #200 #40

40. Procedure for AASHTO Classification • Determine the percentage of soil passing the #200 sieve • Determine the subgroups • For fine-grained soils (silt & clay), determine the liquid limit and plasticity index • Determine soil group or subgroup from Table 9.2

42. AASHTO Classification for Soils

43. Determine the Group Index (usually reflects the relative strength of the material, where low values have the greatest shear strength)

44. Determine the group index

45. Determine the group index fine {e.g.: A-7-5(9)}

46. Example Problem

48. A-7-6(10)