1. Soil Index Properties and Classification
GLE/CEE 330 Lecture Notes
Soil Mechanics
William J. Likos, Ph.D. Department of Civil and Environmental Engineering University of Wisconsin-Madison

2. Purpose of Soil Classification
Simple indices:
1) Grain Size
2) Atterberg Limits
Classification system (Language)
Estimate engineering properties
Achieve engineering purposes
Use the accumulated experience
Communicate between engineers
(after HKUST)

3. Classification Systems
1) USDA (US Department of Agriculture)
Based on texture (grain size)
Used primarily for land use mapping
2) AASHTO (American Association of State
Highway and Transportation Officials)
Used for roadways (subgrade)
Designation: A1-A8 with subgroups
Sieve analysis and Atterbergs
*3) USCS (Unified Soil Classification System)
Used for general geotechnical work
2-Symbol designation (e.g., SP, ML)
Sieve analysis and Atterbergs

4. Tools for Soil Classification
Coarse-Grained
Gravel
Sand
Sieve Analysis
Grain Size Distribution, Cc, Cu
Percent Clay
(% < 2mm)
Plasticity Chart
Fine-Grained
Silt
Clay
Hydrometer Analysis PI
Liquid Limit
Plastic Limit LL
(Photos: NRCS)

5. Coarse-Grained Soils (Sands/Gravels)
d > mm
Well Rounded
Poorly graded
Medium to Fine
Angular
Poorly graded
Medium to Fine
(Mitchell, 1993)

6. Grain Shape How does soil formation influence grain shape/size?
How does grain shape/size influence engineering behavior?
(Mitchell, 1993)

7. Fine-Grained Soils (Silts/Clays)
d < mm
Silt: 2 mm < d < 75 mm
(glimmers in sun)
Clay: d < 2 mm
AND SPECIFIC
MINERALOGY!!!
(Mitchell, 1993)

8. 1:1 Clay Minerals (e.g., Kaolinite)
Non-swelling
Surface area = m2/g
Gs = 2.60
used in ceramics
(R.L. Kugler)

9. 2:1 Clay Minerals (e.g., Smectite)
Highly Swelling
Surface area = m2/g (Primary)
700 – 800 m2/g (Secondary)
Gs = 2.65 – 2.80
Used for adsorbents, drilling fluids
“Expansive Soils”
(Mitchell, 1993)

10. Expansive Soils In the US
(after Tourtelot, 1973)

11. Tools for Soil Classification
Coarse-Grained
Gravel
Sand
Sieve Analysis
Grain Size Distribution, Cc, Cu
Percent Clay
(% < 2mm)
Plasticity Chart
Fine-Grained
Silt
Clay
Hydrometer Analysis PI
Liquid Limit
Plastic Limit LL
(Photos: NRCS)

12. Laboratory Sieve Shaker
Sieve Analysis
Laboratory Sieve Shaker

13. Grain Size Distribution Curve (GSD)
Poorly Graded
(narrow size range)
% Passing by Mass
Well Graded
(wide size range)
Gap Graded
Grain Diameter (mm) (LOG SCALE!)

14. Grain Size Coefficients, Cc and Cu
(e.g., for well graded soil below, Cu = 400)
Poorly Graded
(narrow size range)
% Passing by Mass
Well Graded
(wide size range)
Gap Graded
D10 = 0.02 mm
D60 = 8 mm
Grain Diameter (mm) (LOG SCALE!)

15. Hydrometer Analysis Stoke’s Law Fdrag = f(d) Fbouy = f(d)
Sedimentation Technique
Used for material passing #200 (75 mm) sieve
Typically used to determine % Clay (d < 2 mm)
dispersing agent
Stoke’s Law
Fdrag = f(d)
Fbouy = f(d)
m = viscosity
rs = solids density
rw = water density
v = settling velocity
Fgrav = f(d)

16. Atterberg Limits
describe consistency of fine-grained soils (<75 mm) as function of water content
solid
semi-solid
plastic
liquid
water
content
Shrinkage
Limit
(SL, %)
Plastic
Limit
(PL, %)
Liquid
Limit
(LL, %)
Plastic Limit
Plasticity Index
PI = LL - PL
used as classification tool
used to assess swelling potential (“fat clays”)
Activity (A) and Liquidity Index (LI)
Liquid Limit

19. Liquid Limit Test
(Casagrande Cup)

20. Liquid Limit Test (Fall Cone Method)
LL = 20 mm penetration
PL = c(2)m

21. Unified Soil Classification System (USCS)
Four major divisions:
Coarse-grained
Fine-grained
Organic soils
Peat
Coarse
Fine
Gravel
Sand
Silt and Clay
Boulders
Cobbles
Coarse
Fine
Coarse
Medium
Fine
No.200
0.075 mm
300 mm
75 mm
No.4
4.75 mm
19 mm
No.10
2.0 mm
No.40
0.425 mm

23. USCS Designation Soil symbols: G: Gravel S: Sand M: Silt C: Clay
O: Organic
Pt: Peat
Liquid limit symbols:
H: High Plasticity (LL>50)
L: Low Plasticity (LL<50)
Gradation symbols:
W: Well-graded
P: Poorly-graded
Use GSD curve to define gradation

24. USCS Designation Examples
2-symbol designation
First symbol is general soil type
Second symbol is adjective describing that soil type
GC = Clayey gravel
SW = Well-graded sand
SC = Clayey sand
SM = Silty sand
CH = High plasticity clay (aka “fat” clay)
CL = Low plasticity clay (aka “lean” clay)
GC-ML = Dual classification

25. Plasticity Chart
(Holtz and Kovacs, 1981) LL PI H L
The A-line separates clays from silts and the organics from the inorganics.
LL = 50 separates low plasticity from high plasticity
The U-line indicates the upper bound for general soils.

26. (Grain size distribution)
USCS Procedures
COARSE
(Grain size distribution)
FINE
(LL, PI)
(Santamarina et al., 2001)

27. Example Passing #200 = 30% (so 70% retained on #200) Passing #4 = 70%SC
(15% gravel)
Clayey sand
with gravel
LL= 33
PI= 12
(Santamarina et al., 2001)
(after HKUST)

28. Organic Soils Highly organic soils - Peat (PT)
A sample composed primarily of vegetable tissue in various stages of decomposition and has a fibrous to amorphous texture, a dark-brown to black color, and an organic odor should be designated as a highly organic soil and shall be classified as peat, PT.
Organic clay or silt (OL or OH):
“The soil’s liquid limit (LL) after oven drying is less than 75 % of its liquid limit before oven drying.” If the above statement is true, then the first symbol is O.
The second symbol is obtained by locating the values of PI and LL (not oven dried) in the plasticity chart.
(after HKUST)

29. Dual Classification
For the following three conditions, a dual symbol should be used.
1) Coarse-grained soils with 5% - 12% fines.
About 7 % fines can change the hydraulic conductivity of the coarse-grained media by orders of magnitude.
The first symbol indicates whether the coarse fraction is well or poorly graded. The second symbol describes the fines. For example: SP-SM, poorly graded sand with silt.
2) Fine-grained soils plotting within the shaded zone of plasticity chart
CL- ML: Silty clay, SC-SM: Silty, clayey sand.
3) Soils containing similar fines and coarse-grained fractions.
possible dual symbols GM-ML
(after HKUST)