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Soil Classification
Soil Classification- Purpose
Classifying soils into groups with similar behavior, in terms
of simple indices, that can provide geotechnical engineers
with a general guidance about engineering properties of the
soils through the accumulated experience.
Simple indices
GSD, LL, PI
Classification
system
(Language)
Estimate
Engineering
Properties
Achieve
Engineering
Purposes Use the
accumulated
experience
Communicate
between
engineers
Unified Soil Classification System
(USCS) Origin of USCS:
This system was first developed by Professor Casagrande (1944) for the purpose of airfield construction during World War II. Afterwards, it was modified by him, for the U.S. Bureau of Reclamation, and the U.S. Army Corps of Engineers in 1948 to enable the system to be applicable to dams, foundations, and other structures.
Four major divisions: (1) Coarse-grained (2) Fine-grained (3) Organic soils (4) Peat
Organic Soils
Highly organic soils- Peat (Group symbol 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 ( group symbol 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.
Important distinction between soil particles
based on particle size:
COARSE-GRAINED (COHESIONLESS)
Aggregate inter-lock (stone-to-stone contact)
FINE-GRAINED (COHESIVE)
Cohesive Bond between fine particles
Classification of Soils
Grain Size (USCS)
0.075 mm
No. 200 Sieve
Coarse-Grained Fine-Grained
4.75 mm
No. 4 Sieve
Comparison of the Ranges of Particle Sizes
Unified Soil Classification System
(USCS)
Information required for soil classification according to USCS:
1) Particle Size
– Gravel (G), Sand (S), silt (M), and Clay (C)
2) Particle Size Distribution
– Gradation parameters (Cc and Cu)
3) “Atterberg Limits” of the Fine Portion of the Mix
– Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI)
Definition of Grain Size (USCS)
Boulders Cobbles Gravel Sand Silt and Clay
Coarse Fine Coarse Fine Medium
300 mm 75 mm
19 mm
No.4
4.75 mm
No.10
2.0 mm
No.40
0.425 mm
No.200
0.075 mm
General Guidance
Coarse-grained soils
Gravel Sand
Fine-grained soils
Silt Clay
NO.200
0.075 mm
Grain size distribution
Cu
Cc
Atterburg Limits (PL, LL)
Plasticity chart
50%
NO. 4
4.75 mm
Required tests: Sieve Analysis
Atterberg Limit
50%
Symbols
Soil Symbols:
G: Gravel
S: Sand
M: Silt
C: Clay
O: Organic
Pt: Peat
Plasticity Symbols:
H: High LL (LL>50)
L: Low LL (LL<50)
Gradation Symbols:
W: Well-Graded
P: Poorly-Graded
Example: SW, Well-Graded Sand
SC, Clayey Sand
SM, Silty Sand,
MH, Highly Plastic Silt )(
631
)(
431
sandsfor
CandC
gravelsfor
CandC
soilgradedWell
uc
uc
Particle Size Distribution Terms
P - Poorly graded (uniform sizes)
W - Well graded Good mix of sizes
P - Poorly graded Missing range of sizes
Gradation Parameters
Effective size D10: 0.02 mm
D30: 0.6 mm
D60: 9 mm
Gradation Parameters
Criteria
mm9D
mm6.0D
)sizeeffective(mm02.0D
60
30
10
)(
631
)(
431
sandsfor
CandC
gravelsfor
CandC
soilgradedWell
uc
uc
2)9)(02.0(
)6.0(
)D)(D(
)D(C
curvatureoftCoefficien
45002.0
9
D
DC
uniformityoftCoefficien
2
6010
2
30c
10
60u
Atterburg Indices Plasticity Index PI
For describing the range of
water content over which a
soil acts plastically.
PI = LL – PL
Liquidity Index LI
For scaling the natural
water content of a soil
sample to the Limits.
contentwatertheisw
PLLL
PLw
PI
PLwLI
LI <0 (A), brittle fracture if sheared
0<LI<1 (B), plastic solid if sheared
LI >1 (C), viscous liquid if sheared
Liquid Limit,
LL
Liquid State
Plastic Limit,
PL
Plastic
State
Shrinkage Limit,
SL
Semisolid State
Solid State
PI
A
B
C
Volume Change
(Dv)
Moisture content (%)
PL LL
Shrinkage
limit
Atterberg Limits and Volume Change
Soil
drying
Plasticity Chart
•The A-line generally
separates the more
clay-like materials
from silty materials,
and the organics from
the inorganics.
•The U-line indicates
the upper bound for
general soils.
Note: If the measured
limits of soils are
above the U-line, they
should be rechecked.
PI
H L
Symbols of USCS – Coarse Grained Soils
Class Passing No. 200 Description Secondary symbol
Gravel
(G)
Less than 5%
“Clean”
Well-graded W
Poorly-graded P
More than 12%
“Dirty” Excess of fines C or M
Sand
(S)
Less than 5%
Well-graded W
Poorly-graded P
More than 12% Excess of fines C or M
Symbols of USCS – Fine Grained Soils
Class Primary
symbol
Size
Indication
[A-Line
Position]
Surface
Activity
[B-Line
Position]
Secondary
symbol
CLAY C Above
Low Plasticity
LL < 50%
L
“Lean”
High Plasticity
LL > 50%
H
“Fat”
SILT M Below
Low Plasticity
LL < 50%
L
“Lean”
High Plasticity
LL > 50%
H
“Fat”
Borderline Cases (Dual Symbols)
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
soils by orders of magnitude.
The first symbol indicates whether the coarse fraction is well or poorly graded. The
second symbol describes the fine content. For example: SP-SM, poorly graded sand
with silt.
2) Fine-grained soils with PI between 4 and 7 and LL between
about 12 and 28:
It is hard to distinguish between the silty and more clay-like materials.
CL-ML: Silty clay, SC-SM: Silty, clay sand.
3) Soil contain similar fines and coarse-grained fractions.
possible dual symbols GM-ML
Plasticity Chart
LL
PI
H L
Procedures for Classification
Coarse-grained
material
Grain size
distribution
Fine-grained
material
LL, PI
( From Santamarina et al., 2001, Similar to Table 2.7 of your text book)
Example #1
Passing No.4 sieve = 70%
LL= 33
PL= 13
PI=LL-PL=33-13=20
Above A-Line
SC
Passing No.200 sieve = 30%
Borderline Cases (Summary)
(From old version of the text book, Holtz and Kovacs, 1981)
Flow chart for classification of coarse-grained soils (<50% passing #200
sieve). (Adapted from ASTM D2487.)
Flow chart for classification of inorganic fine-grained soils (≥50% passing #200 sieve).
(Adapted from ASTM D2487.)
Flow chart for classification of organic fine-grained soils (≥50% passing #200 sieve).
(Adapted from ASTM D2487.)
Assessment of Soil Properties Based on
Group Symbol
Assessment of Soil Properties Based on
Group Symbol (Cont.)
American Association of State Highway and
Transportation Officials System (AASHTO)
Origin of AASHTO: (for road construction)
The AASHTO methodology to classify soils was originally
developed by Hogentogler and Terzaghi in 1929 as the
Bureau of Public Roads Classification System.
The original system was based on the stability
characteristics of soils when used as a road surface or
with a thin asphalt pavement.
There are several revisions since 1929, and the latest
version published in 1945 is essentially the present
AASHTO system.
Definition of Grain Size (AASHTO)
Boulders Gravel Sand Clay
Coarse Fine
75 mm 2 mm
Sieve #10 0.425 mm
Sieve #40
0.075 mm
Sieve #200
305 mm
Cobbles Silt
0.0
02
mm
0.0
01
mm
General Guideline – 8 major groups: A1~ A7 (with several subgroups) and organic soils A8
– The required tests are sieve analysis and Atterberg limits.
– The group index, an empirical formula, is used to further evaluate soils within a group (subgroups).
The original purpose of developing the AASHTO classification system was to provide a systematic method to classify soils for use in highway construction.
A4 ~ A7 A1 ~ A3
Granular Materials
35% pass #200 sieve
Silt-clay Materials
36% pass #200 sieve
Using LL and PI separates silty materials
from clayey materials
Using LL and PI separates silty materials
from clayey materials (only for A2 group)
Group Index (GI)
)10PI)(15F(01.0GI 200
For Group A-2-6 and A-2-7
In general, the rating for a pavement subgrade is
inversely proportional to the group index, GI.
use the second term only
F200: percentage passing through the No.200 sieve
)10)(15(01.0)40(005.02.0)35( 200200 PIFLLFGI
General Equation:
Flow chart for AASHTO Soil Classification
38
Example #2 Passing No.200 = 86%
LL=70, PI=32
Check: LL-30=40 > PI=32
3347.33
)10PI)(15F(01.0
)40LL(005.02.0)35F(GI
200
200
Round off A-7-5(33)
USDA Soil Classification
The US Department of Agriculture (USDA) soil classification method is typically made based on the relative proportions of silt, sand and clay contents of the soil.
Follow any two component percentages and move along the predefined lines to find the nominal name for the soil type.
This classification method is rarely used by geotechnical engineers.
USDA Soil Classification
Example
Based on the sieve analysis of a granular backfill soil, the following percentages of aggregate sizes were identified from the gradation chart. Classify this soil according to the USDA method. Sand: 40% Silt: 40% Clay: 20% Solution: Based on the USDA triangle, the soil is Loam.
County Soil Survey
websoilsurvey.nrcs.usda.gov
El Paso, County