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Soil Classification
Sources:
Soil Mechanics – Laboratory Manual,B.M. DAS (Chapters 9)
Soil Classification Systems USCS AASHTO
USCS Classification System
Class Outlines
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
-
Naming Convention
Fine-grained SoilFirst 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
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)
Grain Size Distribution Curves
GRAVEL SAND FINES
Cob
bles
or
Bou
lder
s
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
Fine-grained Soils
Coarse-grained Soils
Example – Soil A
Soil A: D60 = 4.2 mm , D30 = 0.6 mm, D10 = 0.09 mmCu = 46.67Cc = 0.95
Gravel
98-62 = 36%
Sand
62-8 = 54%
Fines = 8%
Example – Soil A (Cont.)Gravel = 36%Sand = 54%Fines = 8%Cu = 46.7Cc = 0.95
LL = 42PL = 31PI = 42-31 = 11
GO TO Plasticity Chart
Example – Soil A (Cont.)
LL = 42PL = 31PI = 42-31 = 11
ML
Example – Soil A (Cont.)
Soil A is then classified as
SP-SM – Poorly-grades sand with silt and gravel