Transcript Rocks - Civil Engineering Society

SOIL CLASSIFICATION
Sarik Salim
Soil Classification System
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Soil classification system is important in
geotechnical engineering because it provide
systematic method of categorizing soil according
to their probable engineering behavior.
By knowing the classification of the soil, an
engineer will have a good idea on how to proceed
with detailed site investigation and laboratory
testing and subsequently with the design of
foundation as well as the engineering situation
both during and after construction.
Common soil classification systems used in
civil engineering practice:
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British soil classification system (BS)
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The Unified soil classification system (USCS)
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The classification system proposed by
AASHTO (American Association of State
Highway and Transportation Officials).
British soil classification system (BS)
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The British soil classification system is shown
in table 1.3 (in your note).
The plasticity chart shown in Figure (1.2).
In this system, the soil is classified by group
symbols composed of main term shown in table
1.4
A group of symbol may consist of two to four
letters for example
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SW means well graded sand
SCL means very clayey sand or clay of low plasticity
Cont’d
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The term fine is used when it is not required
to differentiate between Silts (M) and Clay
(C).
Any cobbles or boulders retained in 63 mm
sieve are removed before the classification
test but the percentage should be determined.
The presence of these particles is indicated in
the classification symbol joined by plus sign,
for example:
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GW+Cb means well graded gravel with cobbles
Example 1
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Refer to the lecture note
The Unified Soil Classification System
(USCS)
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The unified soil classification method is
introduced by Casagrande (1942).
This system is modified slightly and adopted
by ASTM in 1985 (Table 5, in your note).
The USCS categorize the soil into two groups
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Coarse grained soil (gravel & sand)
Fine grained soil (silt & clay)
Coarse grained soil
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< 50 % passing from No 200 sieve.
other considerations in this classification are:
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The percentage of coarse fraction (retained on No
40 sieve).
Coefficient of uniformity (Cu) and coefficient of
curvature (Cc) for the soil with < 12 % passing
No. 200 sieve,
The symbol of the group
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G for gravel
S for Sand
Fine grained soil
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> 50% passing No. 200 sieve
The symbol of the group
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M for inorganic silt
C for inorganic clay
O for silt and clay that contain organic materials.
Pt for peat, muck and other highly organic soils
The fine grained soil is further classified
based on its plasticity index (PI)
cont’d
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Other symbols used in this classification
system:
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W for well graded soil
P for poorly graded soil
L for low plasticity clay
H for high plasticity clay
Symbols such as GM, SM, GC, SC are used
for soil that contain both coarse and fine
particles.
Example 2
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Refer to the lecture note
The AASHTO classification system
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Table 1.6 (in your note) shows the
classification system proposed by AASHTO
(American Association of State Highway and
Transportation Officials).
AASHTO is widely used in the field of
highway.
This system classify the soil into seven main
groups A1-A7.
Cont’d
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Granular soil is classified into groups A-1, A2 and A3, where < 35% of soil particles
passing a No. 200 sieve.
Clay and silt categorized as group A-4 to A-7,
where >= 35% passing No. 200.
AASHTO classification uses grain size
distribution obtained from the Sieve Analysis
and soil plasticity obtained from Atterberg
limit test.
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Gravels are soil fraction that pass the sieve with
diameter of 75mm and retained in sieve No.10(2mm
diameter)
Sand is the fraction of soil passing No.10 (2mm)
sieve and retained in the sieve No.200 (0.075mm).
Silts and clay passes No.200 sieve
Adjective silty is used when fine fraction of soil
have PI (Plasticity Index) <10
While Clayey is used when the fine fraction has P
>10
If boulders (Particle size >75mm) is found in the
soil, then they are not used in the analysis.
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Group index (GI) is used to evaluate the
quality of the soil as sub-grade material.
GI= (F-35)[0.2+0.005(LL-40)]+0.01 (F-15)(PI-10)
 Where,
F the percentage of particles passing sieve NO.200
LL is the liquid limit
PI is the plasticity Index
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Good quality soil for sub grade has low GI
Example 3
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Refer to the lecture note