Class 3 (b) Soil Classification ( Geotechnical Engineering )

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CE 3121: Geotechnical Engineering Laboratory
Class 3 (b)
Soil Classification
Sources:
Soil Mechanics – Laboratory Manual,B.M. DAS (Chapters 9)

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 Soil Classification Systems
 USCS
 AASHTO
 USDA
 USCS Classification System
Class Outlines

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USCS Classification System
 Originally developed for the United States
Army
 The method is standardized in ASTM D 2487
as “Unified Soil Classification System (USCS)”
 USCS is the most common soil classification
system among geotechnical engineers
 A typical USCS classification would be:
SM Silty sand with gravel
Group
Symbol
Group
Name
-

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Naming Convention
 Fine-grained Soil
First Letter Second Letter
M – Silt L – Low plastic
C – Clay H – High plastic
O – Organic
 Coarse-grained Soil
First Letter Second Letter
S – Sand P – Poorly graded
G – Gravel W – Well graded
M – Silty
C - Clayey

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Classification of Soils
 From sieve analysis and the grain-size
distribution curve determine the percent
passing as the following:
 > 3 inch – Cobble or Boulders
 3 inch - # 4 (76.2 – 4.75 mm) : Gravel
 # 4 - # 200 (4.75 - 0.075 mm) : Sand
 < # 200: Fines
 First, Find % passing # 200
 If (5%) or more of soil passes # 200 sieve,
then conduct Atterberg Limits test (LL & PL)

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Grain Size Distribution Curves
GRAVEL SAND FINES
CobblesorBoulders

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Classification of Soils
 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
 Calculate LL, PL and PI

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Fine-grained Soils

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Coarse-grained Soils

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Example – Soil A
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%

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Example – Soil A (Cont.)
Grave = 36%
Sand = 54%
Fines = 8%
Cu = 46.7
Cc = 0.95
LL = 42
PL = 31
PI = 42-31 = 11
GO TO
Plasticity Chart

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Example – Soil A (Cont.)
LL = 42
PL = 31
PI = 42-31 = 11
ML

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Example – Soil A (Cont.)
 Soil A is then classified as
SP-SM – Poorly-grades sand with silt and
gravel

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In your report
 Use data from Sieve analysis in Lab 2 and
classify the soil
 Assuming the % passing No. 200 Sieve in
55%, and using data from Lab 3 classify the
soil
 Attach the Plasticity Chart