Soil Classification

Soil Classification- Purpose

Classifying soils into groups with similar behavior, in terms

of simple indices, that can provide geotechnical engineers

with a general guidance about engineering properties of the

soils through the accumulated experience.

Simple indices

GSD, LL, PI

Classification

system

(Language)

Estimate

Engineering

Properties

Achieve

Engineering

Purposes Use the

accumulated

experience

Communicate

between

engineers

Unified Soil Classification System

(USCS) Origin of USCS:

This system was first developed by Professor Casagrande (1944) for the purpose of airfield construction during World War II. Afterwards, it was modified by him, for the U.S. Bureau of Reclamation, and the U.S. Army Corps of Engineers in 1948 to enable the system to be applicable to dams, foundations, and other structures.

Four major divisions: (1) Coarse-grained (2) Fine-grained (3) Organic soils (4) Peat

Organic Soils

Highly organic soils- Peat (Group symbol Pt )

A sample composed primarily of vegetable tissue in various stages of

decomposition and has a fibrous to amorphous texture, a dark-brown to black

color, and an organic odor should be designated as a highly organic soil and

shall be classified as peat, Pt.

Organic clay or silt ( group symbol OL or OH)

“The soil’s liquid limit (LL) after oven drying is less than 75 %

of its liquid limit before oven drying.” If the above statement is

true, then the first symbol is O.

The second symbol is obtained by locating the values of PI

and LL (not oven dried) in the plasticity chart.

Important distinction between soil particles

based on particle size:

COARSE-GRAINED (COHESIONLESS)

Aggregate inter-lock (stone-to-stone contact)

FINE-GRAINED (COHESIVE)

Cohesive Bond between fine particles

Classification of Soils

Grain Size (USCS)

0.075 mm

No. 200 Sieve

Coarse-Grained Fine-Grained

4.75 mm

No. 4 Sieve

Comparison of the Ranges of Particle Sizes

Unified Soil Classification System

(USCS)

Information required for soil classification according to USCS:

1) Particle Size

– Gravel (G), Sand (S), silt (M), and Clay (C)

2) Particle Size Distribution

– Gradation parameters (Cc and Cu)

3) “Atterberg Limits” of the Fine Portion of the Mix

– Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI)

Definition of Grain Size (USCS)

Boulders Cobbles Gravel Sand Silt and Clay

Coarse Fine Coarse Fine Medium

300 mm 75 mm

19 mm

No.4

4.75 mm

No.10

2.0 mm

No.40

0.425 mm

No.200

0.075 mm

General Guidance

Coarse-grained soils

Gravel Sand

Fine-grained soils

Silt Clay

NO.200

0.075 mm

Grain size distribution

Cu

Cc

Atterburg Limits (PL, LL)

Plasticity chart

50%

NO. 4

4.75 mm

Required tests: Sieve Analysis

Atterberg Limit

50%

Symbols

Soil Symbols:

G: Gravel

S: Sand

M: Silt

C: Clay

O: Organic

Pt: Peat

Plasticity Symbols:

H: High LL (LL>50)

L: Low LL (LL<50)

Gradation Symbols:

W: Well-Graded

P: Poorly-Graded

Example: SW, Well-Graded Sand

SC, Clayey Sand

SM, Silty Sand,

MH, Highly Plastic Silt )(

631

)(

431

sandsfor

CandC

gravelsfor

CandC

soilgradedWell

uc

uc

Particle Size Distribution Terms

P - Poorly graded (uniform sizes)

W - Well graded Good mix of sizes

P - Poorly graded Missing range of sizes

Gradation Parameters

Effective size D10: 0.02 mm

D30: 0.6 mm

D60: 9 mm

Gradation Parameters

Criteria

mm9D

mm6.0D

)sizeeffective(mm02.0D

60

30

10

)(

631

)(

431

sandsfor

CandC

gravelsfor

CandC

soilgradedWell

uc

uc

2)9)(02.0(

)6.0(

)D)(D(

)D(C

curvatureoftCoefficien

45002.0

9

D

DC

uniformityoftCoefficien

2

6010

2

30c

10

60u

Atterburg Indices Plasticity Index PI

For describing the range of

water content over which a

soil acts plastically.

PI = LL – PL

Liquidity Index LI

For scaling the natural

water content of a soil

sample to the Limits.

contentwatertheisw

PLLL

PLw

PI

PLwLI

LI <0 (A), brittle fracture if sheared

0<LI<1 (B), plastic solid if sheared

LI >1 (C), viscous liquid if sheared

Liquid Limit,

LL

Liquid State

Plastic Limit,

PL

Plastic

State

Shrinkage Limit,

SL

Semisolid State

Solid State

PI

A

B

C

Volume Change

(Dv)

Moisture content (%)

PL LL

Shrinkage

limit

Atterberg Limits and Volume Change

Soil

drying

Plasticity Chart

•The A-line generally

separates the more

clay-like materials

from silty materials,

and the organics from

the inorganics.

•The U-line indicates

the upper bound for

general soils.

Note: If the measured

limits of soils are

above the U-line, they

should be rechecked.

PI

H L

Symbols of USCS – Coarse Grained Soils

Class Passing No. 200 Description Secondary symbol

Gravel

(G)

Less than 5%

“Clean”

Well-graded W

Poorly-graded P

More than 12%

“Dirty” Excess of fines C or M

Sand

(S)

Less than 5%

Well-graded W

Poorly-graded P

More than 12% Excess of fines C or M

Symbols of USCS – Fine Grained Soils

Class Primary

symbol

Size

Indication

[A-Line

Position]

Surface

Activity

[B-Line

Position]

Secondary

symbol

CLAY C Above

Low Plasticity

LL < 50%

L

“Lean”

High Plasticity

LL > 50%

H

“Fat”

SILT M Below

Low Plasticity

LL < 50%

L

“Lean”

High Plasticity

LL > 50%

H

“Fat”

Borderline Cases (Dual Symbols)

For the following three conditions, a dual symbol should be used:

1) Coarse-grained soils with 5% - 12% fines: About 7 % fines can change the hydraulic conductivity of the coarse-grained

soils by orders of magnitude.

The first symbol indicates whether the coarse fraction is well or poorly graded. The

second symbol describes the fine content. For example: SP-SM, poorly graded sand

with silt.

2) Fine-grained soils with PI between 4 and 7 and LL between

about 12 and 28:

It is hard to distinguish between the silty and more clay-like materials.

CL-ML: Silty clay, SC-SM: Silty, clay sand.

3) Soil contain similar fines and coarse-grained fractions.

possible dual symbols GM-ML

Plasticity Chart

LL

PI

H L

Procedures for Classification

Coarse-grained

material

Grain size

distribution

Fine-grained

material

LL, PI

( From Santamarina et al., 2001, Similar to Table 2.7 of your text book)

Example #1

Passing No.4 sieve = 70%

LL= 33

PL= 13

PI=LL-PL=33-13=20

Above A-Line

SC

Passing No.200 sieve = 30%

Borderline Cases (Summary)

(From old version of the text book, Holtz and Kovacs, 1981)

Flow chart for classification of coarse-grained soils (<50% passing #200

sieve). (Adapted from ASTM D2487.)

Flow chart for classification of inorganic fine-grained soils (≥50% passing #200 sieve).

(Adapted from ASTM D2487.)

Flow chart for classification of organic fine-grained soils (≥50% passing #200 sieve).

(Adapted from ASTM D2487.)

Assessment of Soil Properties Based on

Group Symbol

Assessment of Soil Properties Based on

Group Symbol (Cont.)

American Association of State Highway and

Transportation Officials System (AASHTO)

Origin of AASHTO: (for road construction)

The AASHTO methodology to classify soils was originally

developed by Hogentogler and Terzaghi in 1929 as the

Bureau of Public Roads Classification System.

The original system was based on the stability

characteristics of soils when used as a road surface or

with a thin asphalt pavement.

There are several revisions since 1929, and the latest

version published in 1945 is essentially the present

AASHTO system.

Definition of Grain Size (AASHTO)

Boulders Gravel Sand Clay

Coarse Fine

75 mm 2 mm

Sieve #10 0.425 mm

Sieve #40

0.075 mm

Sieve #200

305 mm

Cobbles Silt

0.0

02

mm

0.0

01

mm

General Guideline – 8 major groups: A1~ A7 (with several subgroups) and organic soils A8

– The required tests are sieve analysis and Atterberg limits.

– The group index, an empirical formula, is used to further evaluate soils within a group (subgroups).

The original purpose of developing the AASHTO classification system was to provide a systematic method to classify soils for use in highway construction.

A4 ~ A7 A1 ~ A3

Granular Materials

35% pass #200 sieve

Silt-clay Materials

36% pass #200 sieve

Using LL and PI separates silty materials

from clayey materials

Using LL and PI separates silty materials

from clayey materials (only for A2 group)

Group Index (GI)

)10PI)(15F(01.0GI 200

For Group A-2-6 and A-2-7

In general, the rating for a pavement subgrade is

inversely proportional to the group index, GI.

use the second term only

F200: percentage passing through the No.200 sieve

)10)(15(01.0)40(005.02.0)35( 200200 PIFLLFGI

General Equation:

Flow chart for AASHTO Soil Classification

38

Example #2 Passing No.200 = 86%

LL=70, PI=32

Check: LL-30=40 > PI=32

3347.33

)10PI)(15F(01.0

)40LL(005.02.0)35F(GI

200

200

Round off A-7-5(33)

USDA Soil Classification

The US Department of Agriculture (USDA) soil classification method is typically made based on the relative proportions of silt, sand and clay contents of the soil.

Follow any two component percentages and move along the predefined lines to find the nominal name for the soil type.

This classification method is rarely used by geotechnical engineers.

USDA Soil Classification

Example

Based on the sieve analysis of a granular backfill soil, the following percentages of aggregate sizes were identified from the gradation chart. Classify this soil according to the USDA method. Sand: 40% Silt: 40% Clay: 20% Solution: Based on the USDA triangle, the soil is Loam.

County Soil Survey

websoilsurvey.nrcs.usda.gov

El Paso, County