Classification os soil

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CE 353 Geotechnical Engineering
Lecture Outline:
1. Classification Systems
2. American Association of State
Highway and Transportation
Officials System (AASHTO)
3. The Unified Soil Classification
System (USCS)
Classification of Soil
Textbook: Braja M. Das, "Principles of Geotechnical Engineering", 7th E. (Chapter 5).
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Dr M. Touahmia

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Classification Systems
• Soils in nature rarely exist separately as gravel, sand, silt, clay or organic
matter, but are usually found as mixtures with varying proportions of these
components.
• Classifying soils into groups with similar behavior, in terms of simple indices,
can provide geotechnical engineers a general guidance about engineering
properties of the soils through the accumulated experience.
• Two commonly used systems for Classifying soils based on particle
distribution and Atterberg limits:
1. AASHTO System: American Association of State Highway and Transportation
Officials.
2. USCS: Unified Soil Classification System.

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AASHTO Soil Classification System
Origin:
• AASHTO system of soil classification was developed by Hogentogler and
Terzaghi in 1929 as the Public Road Administration classification system. It
has undergone several revisions, with the present version proposed by the
Committee on Classification of Materials for Subgrades and Granular Type
Roads of the Highway Research Board in 1945 (ASTM designation D-3282;
AASHTO method M145).
• The system is based on the following three soil properties:
1. Particle-size distribution (AASHTO T-11 and AASHTO T-27 test)
2. Liquid Limit (AASHTO T-89 test).
3. Plasticity Index (AASHTO T-90 test).

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• Key Elements:
1. Grain Size:
• Gravel: Fraction passing 75mm sieve and retained on #10 (2mm) US sieve
• Sand: Fraction passing #10 sieve and retained #200 sieve
• Silt and Clay: Fraction passing #200 sieve
2. Plasticity:
• Term silty is applied when fine fractions have a PI < 10
• Term clayey is applied when fine fractions have PI > 11
3. Groups: (see Tables)
• Soils are classified into eight groups, A-1 through A-8.
• The major groups A-1, A-2, and A-3 represent the coarse grained soils.
• The A-4, A-5, A-6, and A-7 represent fine grained soils.
• The A-8 are identified by visual inspection.

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• The ranges of the LL and PI for groups A-2 , A-4, A-5 , A-6 and A-7:

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• Group Index (GI):
• To evaluate the quality of a soil as a highway subgrade material, one must
incorporate a number called the Group Index (GI) with the groups and
subgroups of the soil. This index is written in parentheses after the group or
subgroup designation. The group index is given by the equation:
where:
F200: % passing #200 sieves expressed as whole number
LL: liquid limit of soil
PI: Plasticity Index of soil
       101501.040005.02.035 200200
 PIFLLFGI
The first term is
determined by the LL
The second term is
determined by the PI

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It may be noted that:
• The higher the value of GI the weaker will be the soil and vice versa. Thus,
quality of performance of a soil as a subgrade material is inversely
proportional to GI.
• A soil having GI of zero is considered as the best.
• If the equation gives negative value for GI, consider it zero.
• Always round off the GI to nearest whole number.
• GI = 0 for soils of groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3.
• For groups A-2-6 and A-2-7 use partial GI for PI only:
  101501.0 200
 PIFGI

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• Classification of Highway Subgrade Materials:

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• Classification of Highway Subgrade Materials:

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DESCRIPTION OF GROUPS & SUBGROUPS:
• Group A-1: The typical material of this group is a well-graded mixture of
stone fragments or gravels, coarse sand, fine sand, and a non-plastic or
slightly plastic soil binder. This group also includes stone fragments, gravels,
coarse sand, volcanic cinders etc, without a well-graded binder of fine
material.
• Subgroup A-1-a: includes those materials consisting predominantly of stone
fragments or gravel, either with or without a well-graded binder of fine
material.
• Subgroup A-1-b: includes those materials consisting predominantly of coarse
sand with or without a well-graded soil binder.

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• Group A-3: The typical material of this group is fine beach sand or fine
desert blown sand without silty or clayey fines or with a small amount of
non-plastic silt. This group includes also stream-deposited mixtures of poorly
graded fine sand and limited amounts of coarse sand and gravel.
• Group A-2: This group includes a wide variety of “granular” materials, which
are at the borderline between the materials falling in groups A-1 and A-3
and the silty-clay materials of group A-4 through A-7. It include any materials
not more than 35% of which passes a #200 sieve and which cannot be
classified as A-1 or A-3 because of having fines content or plasticity, or both,
in excess of the limitations for those groups.

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• Group A-4: The typical material of this group is a non-plastic or moderately
plastic silty soil 75% or more of which usually passes the #200 sieve. The
group also includes mixture of fine silty soil and up to 64% of sand and gravel
retained on the #200 sieve.
• Group A-5:The typical material of this group is similar to that described
under Group A-4, but it may be highly elastic, as indicated by high liquid
limit.
• GroupA-6: The typical material of this group is a plastic clay soil 75% or more
of which usually passes the #200 sieve. The group also includes mixtures of
fine clayey soil and up to 64% of sand and gravel retained on the #200 sieve.
Materials of this group usually have high volume change between wet and
dry states.

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• Group A-7: The typical material of this group is similar to that described
under Group A-6, but it has the high liquid limits characteristics of the A-5
group and may be elastic as well as subject to high volume change.
• Subgroup A-7-5: includes those materials which have moderate plasticity
indexes in relation to liquid limit and which may be highly elastic as well as
subject to considerable volume change.
• Subgroup A-7-6: includes those materials which have high plasticity indexes
in relation to liquid limit and which are subject to extremely high volume
change.
• Group A-8: The typical material of this group is peat and muck soil ordinarily
found in obviously unstable, swampy areas. Characterized by:
- low density - high compressibility - high water content and - high organic
matter content.

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Unified Soil Classification System, USCS
• Origin: The Unified Soil Classification system was first developed by
Professor A. Casagrande in 1942 for the purpose of airfield construction
during world War II. Afterwards, it was expended and revised in cooperation
with the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers so
that it applies not only to airfields but also to embankments, dams,
foundations, and other engineering features. In 1969 the American Society
for Testing and Materials (ASTM) adopted the USCS as a standard method
for classification for engineering purposes (ASTM Test Designation D-2487).
• The USCS is based on the recognition of the type and predominance of the
constituents considering grain-size, gradation, plasticity and compressibility.
It classifies soils into Four major categories:
1. Coarse-grained
2. Fine-grained
3. Organic soils
4. Peat

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Procedures for Classification:
• From sieve analysis and the grain-size distribution curve determine the
percent passing as the following:
• First, Find % passing # 200.
• If 5% or more of the soil passes the # 200 sieve, then conduct Atterberg
Limits (LL & PL).
• If the soil is fine-grained (≥50% passes #200) follow the guidelines for fine-
grained soils.
• If the soil is coarse-grained (<50% passes #200) follow the guidelines for
coarse-grained soils: Find % Gravel & Sand, calculate Cu & Cc, LL, PL &PI

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SYMBOLS
Soil symbols:
• G: Gravel
• S: Sand
• M: Silt
• C: Clay
• O: Organic
• Pt: Peat
Liquid limit symbols:
• H: High LL (LL>50)
• L: Low LL (LL<50)
Gradation symbols:
• W: Well-graded
• P: Poorly-graded
)sandsfor(
6Cand3C1
)gravelsfor(
4Cand3C1
soilgradedWell
uc
uc



GROUP SYMBOLS
• The group symbols for coarse-grained gravelly
soils are: GW, GP, GM, GC, GC-GM, GW-GM,
GW-GC, GP-GM, and GP-GC.
• The group symbols for fine-grained soils are:
CL, ML, OL, CH, MH, OH, CL-ML, and Pt.
Example:
SW, Well-graded sand
SC, Clayey sand
SM, Silty sand,
MH, Elastic silt

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Classification and Symbols

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Laboratory Classification Criteria

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Plasticity Chart :

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Plasticity Chart :

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ORGANIC SOILS:
1. Organic clay or silt( group symbol OL or OH):
• If The soil’s liquid limit (LL) after oven drying is less than 75 % of its liquid
limit before oven drying 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.
2. 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.

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Borderline Cases (Dual Symbols): For the following three conditions,
a dual symbol should be used:
1. Coarse-grained soils with 5% - 12% fines:
• The first symbol indicates whether the coarse fraction is well or poorly
graded. The second symbol describe the contained fines. For example: SP-
SM, poorly graded sand with silt.
2. Fine-grained soils with limits within the shaded zone. (PI between 4 and 7
and LL between about 12 and 25):
• It is hard to distinguish between the silty and more claylike materials.
• CL-ML: Silty clay, SC-SM: Silty, clayed sand.
3. Soil contain similar fines and coarse-grained fractions:
• possible dual symbols GM-ML.

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Borderline Cases:

Classification os soil

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