Soil Classification

ChihChih--Ping LinPing LinNational National ChiaoChiao TungTung Univ.Univ.cplin@mail.nctu.edu.twcplin@mail.nctu.edu.tw

Soil Mechanics −

Outline

1.1. PurposePurpose

2.2. Classification SystemsClassification Systems

3.3. The Unified Soil Classification System The Unified Soil Classification System (USCS)(USCS)

4.4. American Association of State Highway and American Association of State Highway and Transportation Officials System (AASHTO)Transportation Officials System (AASHTO)

PurposeClassifying soils into groups with similar behavior, Classifying soils into groups with similar behavior,

in terms of in terms of simplesimple indices, can provide geotechnical indices, can provide geotechnical engineers a general guidance about engineering engineers a general guidance about engineering properties of the soils through the properties of the soils through the accumulated accumulated experienceexperience..

Simple indices

GSD, LL, PI

Classification system

(Language)

Estimate engineering properties

Achieve engineering

purposesUse the

accumulated experience

Communicate between

engineers

Classification Systems

Two commonly used systems:Two commonly used systems:

Unified Soil Classification System (USCS).Unified Soil Classification System (USCS).

American Association of State Highway and American Association of State Highway and Transportation Officials (AASHTO) SystemTransportation Officials (AASHTO) System

Unified Soil Classification SystemOrigin of USCS:

This system was first developed by Professor A. Casagrande(1948) for the purpose of airfield construction during World WarII. Afterwards, it was modified by Professor Casagrande, the U.S. Bureau of Reclamation, and the U.S. Army Corps of Engineers to enable the system to be applicable to dams, foundations, and other construction (Holtz and Kovacs, 1981). ASTM D2487

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

Group Symbol Group Symbol –– Group NameGroup Namee.g. SM e.g. SM –– Silty sand with gravelSilty sand with gravel

Definition of Grain Size

Boulders CobblesGravel Sand Silt and

ClayCoarse Fine Coarse FineMedium

300 mm 75 mm

19 mm

No.4

4.75 mmNo.10

2.0 mm

No.40

0.425 mm

No.200

0.075 mm

No specific grain size-use Atterberg limits

Group SymbolsSoil symbols:Soil symbols:

G: GravelG: Gravel

S: SandS: Sand

M: SiltM: Silt

C: ClayC: Clay

O: OrganicO: Organic

Pt: PeatPt: Peat

Gradation symbols:Gradation symbols:

W: WellW: Well--gradedgraded

P: PoorlyP: Poorly--gradedgraded

Fine ComponentFine Component

C: ClayeyC: Clayey

M: SiltyM: Silty

Liquid limit symbols:Liquid limit symbols:

H: High LL (LL>50)H: High LL (LL>50)

L: Low LL (LL<50)L: Low LL (LL<50)

Example:

SW, Well-graded sand

SC, Clayey sand

SM, Silty sand,

MH, Elastic silt

)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

≥<<

≥<<−

Group Names

The group symbol was enhanced by addition The group symbol was enhanced by addition of several group names for each group symbol.of several group names for each group symbol.e.g. SM e.g. SM –– Silty sand with gravelSilty sand with gravel

Noun = Primary componentNoun = Primary component

Adjective = Secondary component or further explanation of Adjective = Secondary component or further explanation of first componentfirst component

““with with …”…” =Tertiary component=Tertiary component

Adjective Noun “with .”

General Guidance

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt ClayNO.200

0.075 mm

2. Gradation orFine component

Grain size distribution(Cu and Cc)

2. CompressibilityPL, LL(Plasticity chart)

50 %

NO. 4

4.75 mm

50%1. MajorComponent

Required tests: Sieve analysis and Atterberg limits (P#40)

Initial Classification

FineFine--grained soilgrained soilP#200 > 50%P#200 > 50%

CoarseCoarse--grained soilgrained soilP#200 < 50%P#200 < 50%

Highly Organic SoilHighly Organic SoilPrimarily organic materialPrimarily organic materialDark brown, dark gray, or black colorDark brown, dark gray, or black colorOrganic odorOrganic odorSoft consistencySoft consistency

If P#200 > 5%, conduct Atterberg limits tests on the soil passing #40

Conduct sieve analysis test

Classification of fine-grained soils

(Holtz and Kovacs, 1981)

LL

PI

HL

• The A-line generally separates the more claylike 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 on the left of U-line, they should be rechecked.

Plasticity Chart

Classification of coarse-grained soils

P#200<50%

G(P#4<50%)

P#200

<5% 5-12% >12%

GSD(W,P)

PlasticityChart(M,C)

Both

S(P#4<50%)

P#200

<5% 5-12% >12%

GSD(W,P)

PlasticityChart(M,C)

Both

)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

≥<<

≥<<−

Organic Soils

Highly organic soilsHighly organic soils-- Peat (Group symbol PT)Peat (Group symbol PT)−− A sample composed primarily of vegetable tissue in various stageA sample composed primarily of vegetable tissue in various stages of s of

decomposition and has a fibrous to amorphous texture, a darkdecomposition and has a fibrous to amorphous texture, a dark--brown brown to black color, and an organic odor should be designated as a hito black color, and an organic odor should be designated as a highly ghly organic soil and shall be classified as peat, PT.organic soil and shall be classified as peat, PT.

OOrganic clay or silt( group symbol OL or OH):rganic clay or silt( group symbol OL or OH):−− ““The soilThe soil’’s liquid limit (LL) after oven drying is less than 75 % of its s liquid limit (LL) after oven drying is less than 75 % of its

liquid limit before oven drying.liquid limit before oven drying.”” If the above statement is true, then If the above statement is true, then the first symbol is O.the first symbol is O.

−− The second symbol is obtained by locating the values of PI and LThe second symbol is obtained by locating the values of PI and LL L (not oven dried) in the plasticity chart. (not oven dried) in the plasticity chart.

Summary of Soil Classification

Coarse-grained material

Grain size distribution

Fine-grained material

LL, PI

(Santamarina et al., 2001)

Highly

(Santamarina et al., 2001)

Highly

Example

Passing No.200 sieve 30 %

Passing No.4 sieve 70 %

LL= 33

PI= 12

PI= 0.73(LL-20), A-line

PI=0.73(33-20)=9.49

SC (≥15% gravel)

Clayey sand with gravel

Borderline Cases (Dual Symbols)For the following three conditions, a dual symbol should be For the following three conditions, a dual symbol should be

used.used.

CoarseCoarse--grained soils with 5% grained soils with 5% -- 12% fines.12% fines.−− About 7 % fines can change the hydraulic conductivity of the About 7 % fines can change the hydraulic conductivity of the

coarsecoarse--grained media by orders of magnitudegrained media by orders of magnitude..

−− The first symbol indicates whether the coarse fraction is well oThe first symbol indicates whether the coarse fraction is well or r poorly graded. The second symbol describe the contained fines. Fpoorly graded. The second symbol describe the contained fines. For or example: SPexample: SP--SM, poorly graded sand with silt.SM, poorly graded sand with silt.

FineFine--grained soils with limits within the shaded zone. (PI grained soils with limits within the shaded zone. (PI between 4 and 7 and LL between about 12 and 25).between 4 and 7 and LL between about 12 and 25).−− It is hard to distinguish between the silty and more claylike maIt is hard to distinguish between the silty and more claylike materials.terials.

−− CLCL--ML: Silty clay, SCML: Silty clay, SC--SM: Silty, clayed sand.SM: Silty, clayed sand.

Soil contain similar fines and coarseSoil contain similar fines and coarse--grained fractions.grained fractions.−− possible dual symbols GMpossible dual symbols GM--MLML

Borderline Cases (Summary)

(Holtz and Kovacs, 1981)

American Association of State Highway and Transportation Officials system

Origin of AASHTO: (For road construction)

•This system was originally developed by Hogentogler and Terzaghi in 1929 as the Public Roads Classification System. Afterwards, there are several revisions. The present AASHTO (1978) system is primarily based on the version in 1945. (Holtz and Kovacs, 1981)

•Rated soils according to their suitability for support roadway pavement

•Assign a group classification and a group index to the soilA-1 ………A-8 0 ………20

best soil … worst soil good soil … poor soil

Definition of Grain Size

Boulders Gravel Sand Silt-Clay

Coarse Fine

75 mm No.4

4.75 mmNo.40

0.425 mm

No.200

0.075 mm

No specific grain size-use Atterberglimits

General GuidanceThe required tests are sieve analysis and The required tests are sieve analysis and AtterbergAtterberg limits.limits.8 major groups: A1~ A7 (with several subgroups) and 8 major groups: A1~ A7 (with several subgroups) and organic soils A8organic soils A8

The group index, an empirical formula, is used to further The group index, an empirical formula, is used to further evaluate soils within a group (subgroups).evaluate soils within a group (subgroups).The original purpose of this classification system is used for The original purpose of this classification system is used for road construction (road construction (subgradesubgrade rating).rating).

A4 ~ A7A1 ~ A3

Granular Materials

≤ 35% pass No. 200 sieve

Silt-clay Materials

≥ 36% pass No. 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)

Classification

Das, 1998

Classification (Cont.)

Das, 1998

Note:

The first group from the left to fit the test data is the correct AASHTO classification.

Group Index

[ ])10PI)(15F(01.0

)40LL(005.02.0)35F(GI

200

200

−−+−+−=

)10PI)(15F(01.0GI 200 −−=For Group A-2-6 and A-2-7

The first term is determined by the LL

The second term is determined by the PI

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

ExamplePassing No.200 86%

LL=70, PI=32

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)