The document provides lecture notes on geotechnical engineering, focusing on two primary soil classification systems: the Unified Soil Classification System (USCS) and the AASHTO system. It details the categories of soils based on grain size and plasticity, along with flow charts and practice problems for classifying soils. The content emphasizes the importance of these systems in categorizing soils for engineering applications.

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Geotechnical Engineering–I [CE-221]
BSc Civil Engineering – 4th Semester
by
Dr. Muhammad Irfan
Assistant Professor
Civil Engg. Dept. – UET Lahore
Email: mirfan1@msn.com
Lecture Handouts: https://groups.google.com/d/forum/2016session-geotech-i
Lecture # 11
27-Feb-2018

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Two commonly used classification system:
1. Unified Soil Classification System (USCS)
 preferred by Geotechnical engineers
2. American Association of State Highway and
Transportation Officials (AASHTO) System
 preferred by Transportation engineers
SOIL CLASSIFICATION

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→ Developed by Casagrande in 1942 for US Army
→ System based on particles < 3 in
→ Uses grain size distribution and plasticity of fines
Three major categories:
 coarse-grained soils
 fine-grained soils
 organic soils
P200 < 50% → Coarse grained soil
P200 ≥ 50% → Fine grained soil
UNIFIED SOIL CLASSIFICATION
SYSTEM (USCS) (ASTM D 2487)

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Gravel
Sand
Silt
Clay
Organic
Peat
Liquid limit symbols:
H:
L:
SW
SC
CL
MH
)sandsfor(
6Cand3C1
)gravelsfor(
4Cand3C1
soilgradedWell
uc
uc



USCS – Naming Convention
SM Silty sandGroup Symbol
Group Name
Well-graded sand
Clayey sand
Low plasticity Clay
High Plastic silt
Soil symbols:
G:
S:
M:
C:
O:
Pt:
High LL (LL>50)
Low LL (LL<50)
Gradation symbols:
W:
P:
Well-graded
Poorly-graded

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Definition of Grain Size
Boulders Cobbles
Gravel Sand
Silt and
Clay
Coarse Fine Coarse FineMedium
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
Use
Atterberg limits
UNIFIED SOIL CLASSIFICATION
SYSTEM (USCS)
UNIFIED SOIL CLASSIFICATION
SYSTEM (USCS)

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FLOW CHART –
Coarse and Fine Grained
Passing No. 200 sieve
> 50% passes
No. 200 sieve
< 50% passes
No. 200 sieve
Fine-grained
soils
Coarse-grained
soils

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FIG. 1 Flow chart for Classifying Fine-Grained Soil (50% or More Passes No. 200 Sieve)
Fine Grained Soils

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Group Symbol Group Name
FIG. 1b Flow chart for Classifying Organic Fine-Grained Soil (50% or More Passes No. 200 Sieve)
Fine Grained Organic Soils

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FIG. 2 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)
Coarse Grained Soils

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Used for fine grained soils to determine whether silt (M) or clay (C)
Below A-line is silt – use symbol M
Above A-line is clay – use symbol C
LL > 50  High plasticity
LL< 50  Low plasticity
PLASTICITY/A-LINE CHART

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Soil A
Fines = 52% > 50% => Fine-grained soil
PRACTICE PROBLEM #1

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LL = 30
PI = 8
CL
PRACTICE PROBLEM #1

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LL = 30
PI = 8
CL

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Soil A
Fines = 52% > 50% => Fine-grained soil
Gravel = 100 – 80 = 20%
Sand = 80 – 52 = 28%
PRACTICE PROBLEM #1

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CL – Sandy lean clay with gravel
Soil A
Gravel = 20%
Sand = 28%
Fines = 52%
LL = 30
PI = 8
CL

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Fines
= 8%
PRACTICE PROBLEM #2
Soil A
Fines = 8% < 50% => Coarse grained soil

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18
FIG. 3 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)

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Gravel
= 98-62
= 36%
Sand
= 62-8
= 54%
Fines
= 8%
PRACTICE PROBLEM #2

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20
FIG. 3 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)
Gravel = 36%
Sand = 54%
Fines = 8%

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Soil A: D60 = 4.2 mm , D30 = 0.6 mm, D10 = 0.09 mm
Cu = 46.67
Cc = 0.95
Gravel
= 98-62
= 36%
Sand
= 62-8
= 54%
Fines
= 8%
PRACTICE PROBLEM #2

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22
FIG. 3 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)
Gravel = 36%
Sand = 54%
Fines = 8%
Cu = 46.7
Cc = 0.95

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SOIL-A
LL = 42
PL = 31
PI = 42-31 = 11
ML
PRACTICE PROBLEM #2

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24
FIG. 3 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)
Gravel = 36%
Sand = 54%
Fines = 8%
Cu = 46.7
Cc = 0.95
Soil A is classified as [SP-SM: Poorly-graded sand with silt and gravel]
LL = 42
PL = 31
PI = 42-31 = 11
ML

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Classify the following soils Using Unified Classification
System.
PRACTICE PROBLEM #3
Soil SIEVE LL PI
No. 4 No. 200
(Cumulative % passing)
A 92 48 30 10
B 99 76 60 32
C 80 35 24 2

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FIG. 3 Flow chart for Classifying Coarse-Grained Soils (50% or More Retained on No. 200 Sieve)
Coarse Grained Soils

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Used for fine grained soils to determine whether silt (M) or clay (C)
Below A-line is silt – use symbol M
Above A-line is clay – use symbol C
LL > 50  High plasticity
LL< 50  Low plasticity
PLASTICITY/A-LINE CHART

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Two commonly used classification system:
1. Unified Soil Classification System (USCS)
 preferred by Geotechnical engineers
2. American Association of State Highway and
Transportation Officials (AASHTO) System
 preferred by Transportation engineers
SOIL CLASSIFICATION

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→ Classification procedure standardized by ASTM D3282 and
AASHTO M145.
→ Soil rating for subgrade construction.
→ Soils classified into 8 groups (+ several subgroups);
A-1 (best) to A-8 (worst).
→ Organic soils (peat and muck) assigned group A-8.
→ Required tests are sieve analysis and Atterberg limits.
AASHTO Soil Classification

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A4 ~ A7A1 ~ A3
Granular Materials Silt-clay Materials
LL and PI separates silty materials from
clayey materials
LL and PI separates silty materials from
clayey materials (only for A2 group)
AASHTO Soil Classification
(ASTM D3282; AASHTO M145)
A-1 (0)
Group Number Group Index
Example
 35% pass No. 200 sieve > 35% pass No. 200 sieve

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       101501.040005.02.035 200200  PIFLLFGI
Partial GI determined from LL Partial GI determined from PI
where
F200 = percentage passing through the No. 200 sieve
LL = Liquid limit, and PI = Plasticity index
 GI is rounded off to nearest whole number
(e.g., GI = 3.4 is rounded off to 3; GI = 3.5 is rounded off to 4)
 GI = 0, if the equation yields negative value.
GI → Empirical formula used to further evaluate soils within a
group (subgroups)
GROUP INDEX (GI)

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 GI is shown in parentheses after the group symbol,
e.g. A-2-6(3), A-6(12), A-7-5(17)
  101501.0 200  PIFGI
GROUP INDEX (GI)
 Smaller the GI, better is the soil.
 GI of soils belonging to groups A-1-a, A-1-b, A-2-4, A-
2-5, and A-3 is always zero.
 GI for soils belonging to groups A-2-6 and A-2-7 is
computed by using the partial formula.

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Boulders Gravel
Sand Silt and
Clay
75 mm
Coarse
No.10
2.0 mm
No.40
0.425 mm
No.200
0.075
mm
AASHTO Particle Size
Fine

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AASHTO vs USCS
Particle Size Comparison

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AASHTO Classification Chart
Note:
First group from the left to fit test data is the correct AASHTO classification.
(Coarse-grained Soils)

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AASHTO Classification Chart
Note:
First group from the left to fit test data is the correct AASHTO classification.
(Fine-grained Soils)

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Range of LL and PI for soils in groups A-2, A-4, A-5, A-6, and A-7.
AASHTO Plasticity Chart

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CONCLUDED
REFERENCE MATERIAL
An Introduction to Geotechnical Engineering (2nd Edition)
By R. D. Holtz, W. D. Kovacs and T. C. Sheahan
Chapter #3
Principles of Geotechnical Engineering – (7th Edition)
Braja M. Das
Chapter #5