1. Soil Classification Sources:
Soil Mechanics – Laboratory Manual,B.M. DAS (Chapters 9)

2. Class Outlines Soil Classification Systems USCS Classification System
AASHTO
USCS Classification System

3. 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

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

5. 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 ( mm) : Sand
< # 200: Fines
First, Find % passing # 200
If (5%) or more of soil passes # 200 sieve, then conduct Atterberg Limits test (LL & PL)

6. Grain Size Distribution Curves
GRAVEL
SAND
FINES
Cobbles or Boulders

7. 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

8. Fine-grained Soils

9. Coarse-grained Soils

10. Example – Soil A Gravel Sand Fines = 8% Cu = 46.67 Cc = 0.95
98-62 = 36%
Sand
62-8 = 54%
Fines = 8%
Cu = Cc = 0.95
Soil A: D60 = 4.2 mm , D30 = 0.6 mm, D10 = 0.09 mm

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

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

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