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