1. Soils and their Classification

2. Soils - What are they? - Sedimentary origins (usually)
Particulate materials
- Sedimentary origins (usually)
- Residual
Wide range of particle sizes
- larger particles: quartz, feldspar
- very small particles: clay minerals
Voids between particles

3. 4

4. 2

5. 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.

6. 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
notes a,b,c

7. 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
notes a,b,c

8. 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

9. 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

10. 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

11. Grading curves
W Well graded

12. Grading curves
W Well graded
U Uniform

13. Grading curves
W Well graded
U Uniform
P Poorly graded

14. Grading curves W Well graded U Uniform P Poorly graded
C Well graded with some clay

15. Grading curves W Well graded U Uniform P Poorly graded
C Well graded with some clay
F Well graded with an excess of fines

16. 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.

17. 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.

18. 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

19. Example: equal amounts sand/silt/clay

20. Example: equal amounts sand/silt/clay

21. Example: equal amounts sand/silt/clay

22. Atterberg Limits
Particle size is not that useful for fine grained soils

23. Atterberg Limits
Particle size is not that useful for fine grained soils
Figure 4 Moisture content versus volume relation during drying SL PL LL

24. 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 SL PL LL

25. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit
LL - Liquid limit

26. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit
LL - Liquid limit
Plasticity Index = LL - PL = PI or Ip

27. Atterberg Limits SL - Shrinkage Limit PL - Plastic Limit
LL - Liquid limit
Plasticity Index = LL - PL = PI or Ip
Liquidity Index = (m - PL)/Ip = LI

28. 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

29. Unified Soil Classification
Each soil is given a 2 letter classification (e.g. SW). The following procedure is used.

30. 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)

31. 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

32. 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

33. 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

34. Unified Soil Classification
To determine if W or P, calculate Cu and Cc
x% of the soil has particles smaller than Dx

35. Unified Soil Classification
To determine W or P, calculate Cu and Cc
x% of the soil has particles smaller than Dx

36. Unified Soil Classification
To determine W or P, calculate Cu and Cc
x% of the soil has particles smaller than Dx

37. Unified Soil Classification
To determine W or P, calculate Cu and Cc
x% of the soil has particles smaller than Dx
D90 = 3 mm

38. 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

39. 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

40. Unified Soil Classification
Fine grained soils (> 50% finer than 75 mm)
Both letters determined from plasticity chart

42. Example
Example

43. Example %fines (% finer than 75 mm) = 11% - Dual symbols required

44. Example %fines (% finer than 75 mm) = 11% - Dual symbols required
D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

45. Example Particle size fractions: Gravel 17% Sand 73% Silt and Clay 10%

46. 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 = = 6

47. Example

48. 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 = = 6
From Plasticity Chart point lies below A-line
Suffix2 = M

49. 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 = = 6
From Plasticity Chart point lies below A-line
Suffix2 = M
Dual Symbols are SW-SM

50. 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 = = 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