1. Engineering Classification of Soils

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

4. Soil Texture Wentworth Scale Remove Cobbles and Boulders from
Analysis (>75mm)
“Gravel”
75-2 mm
“Sand” mm
“Silt and Clay”
<0.075 mm

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

10. E. Plasticity Index (PI)
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

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

13. 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

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

16. 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

18. AASHTO Classification for Soils

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

20. Determine the group index

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

22. Example Problem

24. A-7-6(10)

27. II. 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

28. II. Unified System
B. The classification scheme

29. 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)

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

34. “clayey materials”
“silty materials”

39. Classify the following soils by both the AASHTO and Unified Systems,
HOMEWORK:
Classify the following soils by both the AASHTO and Unified Systems,
and give the group index for the AASHTO system.
Sieve Analysis -- % finer than
Soil Sample
#10
#40
#200
Liquid Lmt
Plastic Lmt A 95 79 53 36 21 B
100 78 65 26 C 80 62 35 20 D 90 55 45 28 E 71 60 40

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

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

42. For fine grained soils:

43. For fine grained soils:

44. For coarse-grained soils:
See next slide

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

47. Fine grained soils

48. Fine grained soils

49. Gravelly soils

50. sandy soils