Civil Engineering - Texas Tech University

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

Cobble

s o

r B

ould

ers

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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