01soil Classification Note1!08!2011S2

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1soil classification note1 08.doc 1

2 Soil Classification systems

2.1 Introduction

Soil mechanics is concerned with particulate materials (soils) found in the grounds

that are not cemented and not greatly compressed. These soils usually have a

sedimentary origin, however, they can also occur as the result of rock weatheringwithout any transport of the particles.

The soil particles canhave varying sizes, shapes and mineralogy; these properties

(of soils) are usually interrelated. For instance

- The larger sized particles are generally composed of quartz and feldspars,

minerals that have high strengths and the particles are fairly round.

- The smaller sized particles are generally composed of the clay minerals kaolin,

illite and montmorillonite, minerals that have low strengths and form plate like

particles.

One of the most important aspects of particulate materials is that there are gaps or

voids between the particles. The amount of voids is also influenced by the size,

shape and mineralogy of the particles.

Because of the wide range of particle sizes(grading chart), shapes and mineralogy

in a typical soil, a detailed classification of each soil would be very expensive and

inappropriate for most geotechnical engineering purposes. However, some form of

simple classification system giving information about the engineering properties isrequired on all sites.

GRAVEL -These are particles sizes between the sizes 200 mm and 2 mm. BS testsieve sizes to be used for separation

mmVery coarse gravel 200 - 60

Coarse gravel 60 - 20Medium gravel 20 - 6Fine gravel 6 - 2

SAND-These are particles sizes between the sizes 2.0 mm and 0.06 mm. BS testsieve sizes to be used for separation

mm

Coarse sand 2.0 - 0.6

Medium sand 0.6 - 0.2

Fine sand 0.2 - 0.06

SILT-These areparticles between the sizes 0.06 mm and 0.002 mm.

mmCoarse silt 0.06 - 0.02Medium silt 0.02 - 0.006Fine silt 0.006 - 0.002

CLAY-These areparticles smaller in size than 0.002 mm.

1mm=1000m
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Why is classification of soil necessary? (what is the Purposes)

Usually the soil on site has to be used. Soils differ from other engineering

materials in that, one has very little, if any, control over their physical properties.

The extent and properties of the variation of soils at the site have to be

determined. To give an indication of the engineering property such as stiffness and strength

for preliminary design. Cheap and simple tests are available.

Characteristic of the material (soils):

Soils have variability in physical and engineering properties, which is also dependence

upon localise conditions

a. Main soil types: Gravel, Sand, Silts, Clay, Organic

o Gravels, sands, silts and clayare sub-divided as

o Organicsoils (definition of NZGeotech Society Sect 2.3.5) characteristic

Eg. Peat* is found mostly in wetland or mangrove, marshy or swampy areas

Deposit of partially decomposed organic debris, inherit pest/micro-organism,

Humus (decomposed plant and animal remains and animal waste), release

methane gas

Usually highly saturated with water

Dark, strong odour

Not suitable for engineering works

More value (biological and certain nutrients) to farmers and gardener,

wetlands is an important ecological system, they are home to many rare and

specialised organisms, large-scale removal of peat from bogs will destroy wildlife

habitats as it takes many years for a wetlands to regenerate.

b. PhysicalProperties:

Density (), Unit weight () void ratio (e), porosity (n), degree of saturation (S)

c. Engineeringproperties:

Shear strength (), cohesion (c), angle of internal friction (), compaction,

permeability (k),

*Classification of peat (for info) - source:

Von Post Scalehttp://www.fao.org/docrep/x5872e/x5872e00.htm#Contents

Soil type Description Nature

GravelsCoarse

Coarse,

Medium or Fine grain

Non cohesive

Sands Non cohesive

SiltsFine Fine grain

Cohesive

Clays Cohesive
http://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../THE%20VON%20POST%20SCALE%20OF%20HUMIFICATION.pdfhttp://../THE%20VON%20POST%20SCALE%20OF%20HUMIFICATION.pdfhttp://www.fao.org/docrep/x5872e/x5872e00.htm#Contentshttp://www.fao.org/docrep/x5872e/x5872e00.htm#Contentshttp://www.fao.org/docrep/x5872e/x5872e00.htm#Contentshttp://../THE%20VON%20POST%20SCALE%20OF%20HUMIFICATION.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdf


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Properties of soils a brief summary(Q2/2004)

Soil typesPermeability

(compacted)Workability Compressibility

Desirability of use

as

foundation soils

Silt (ML) Low Fair Medium LowClayey gravel (CC) Low to very low Good Very low Medium

Peat (Pt) High Poor Very high Very low

Well graded sand

(SW)Medium to high Excellent Very low High

Uniform sand (SP) High Good Low Low

To achieve this, continuous (core) samplesare recovered from boreholes, drilled to a

depth that will depend on the scale and complexity of the project. Observation of the

core enables the different soil layers (horizons) to be determined and thenclassification tests are performed for these different strata. The extent of the

different soil layers can be determined by correlating the results from different

boreholesand this information is used to build a picture of the sub-surface profile.

An indication of the engineering properties is determined on the basis of particle size

(i.e. Gravel, Sand, Silt and Clay). This crude approach is used because the

engineering behaviour of soils with very small particles, usually soils containing clay

minerals, is significantly different from the behaviour of soils with larger particles.

Clays can cause problems because they are relatively compressible when wet, drain

poorly, have low strengths at high water content and can swell in the presence of

water.

Overview of classifications

Coarse Fine Grained Peat

Soil

Gravel (G) or Sand (S) Silt (M) or Inorganic Clay Organic silt or Clay

W = Well GradedP = Poorly graded

U = UniformC = Well Graded with some clayF = Well graded with excess fines

L = Low Plasticity (LL 50%)Note: for soil classification, use only Low and High

Casagrandes Extended Classification System


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Table -1A(refer Moodle)

2.2 Identification, Classification and Description systems

There are several systems for classifying soils. The reason for the many number of

such systems is the use of particular systems for certain types of construction, andthe development of localised systems. However, most of these systems have similar

purposes, i.e.

To determine the suitability of different soils for various purposes such as

highway construction, foundation or dam constructions

To develop correlations with useful soil properties, for example, compressibility

and strength

Classification systems available

- Unified soil classification system (there is also a NZ version, see item 2.2.2)

- Massachusetts Institute of Technology (MIT) classification system

- AASHTO Soil Classification System

- USDA textural soil classification system

- American Society for Testing and Materials (ASTM) System

2.2.1 AASHTO (or PRA) system

The AASHTO Soil Classification System was developed by the American

Association of State Highway and Transportation Officials, and is used as a guide for

the classification of soils and soil-aggregate mixtures for highway construction

purposes.

The classification system was first developed by Hogentogler and Terzaghi in 1929,[1]

but has been revised several times since.

Table1is an example of the PRA or AASHO system (American Association of State

Highway Officials), which ranks soils from 1 to 8 to indicate their suitability as a sub-

grade for pavements.

1. Well graded gravel or sand; may include fines

2. Sands and Gravels with excess fines

3. Fine sands4. Low compressibility silts

5. High compressibility silts

6. Low to medium compressibility clays

7. High compressibility clays

8. Peat, organic soils

2.2.2 Classification Chart (NZ Geotechnical SocietyGuidelines)
http://soil%20meachanic%20sidney%20rosenak%20%281a%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281a%29.pdf/http://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/AASHTO_Soil_Classification_System#cite_note-0http://en.wikipedia.org/wiki/AASHTO_Soil_Classification_System#cite_note-0http://en.wikipedia.org/wiki/AASHTO_Soil_Classification_System#cite_note-0http://../reference/AASHTO%20Soil%20Classification%20System.docxhttp://../reference/AASHTO%20Soil%20Classification%20System.docxhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../5NZGeotech%20Society%20Guideline/GT1%20Soil%20and%20rock%20Classify%20Guideline.pdfhttp://../reference/AASHTO%20Soil%20Classification%20System.docxhttp://en.wikipedia.org/wiki/AASHTO_Soil_Classification_System#cite_note-0http://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://soil%20meachanic%20sidney%20rosenak%20%281a%29.pdf/


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Source: NZ geotechnical society guidelines Sect 2.3.1

Classification Chart (NZ Geotechnical SocietyGuidelines)

Major Divisions Symbols Typical Names

Coar

segrainsoils

morethan35%o

fsoil>0.0

6mm

GRAVELS

More than of

coarse fraction

>2 mm

GW Well graded gravels or gravel-sand mixtures.

Little or no fines.

GP Poorly graded gravels or gravel-sand mixtures.

Little or no fines.

GM Silty gravels, gravel-sand-silt mixtures.

GC Clayey gravels, gravel-sand-clay mixtures.

SANDS

More than of

coarse fraction


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2.2.3 Unified Soil Classification System(USCS chart)andinterpretation

The standard system used worldwide for most major construction projects is known

as the Unified Soil Classification System (USCS).

The system is based on the original system devised by Cassagrande. In 1945,Casagrande proposed a Soil Classification system, which was later modified and

was adopted internationally from 1952 onwards.

Overview: In this system, soils are identified by standard symbols with lab results

determined from Sieve Analysis (Coarse grained) and Atterberg Limits (Fine

grained) tests. The system consists of the following Symbols for classifying soils:

Firstletter G for gravel, S for sand, M for silt,

C for clay, O for organic, Pt for peat

Secondletter W for well graded

P for poorly graded

M for silt as second fraction present

C for clay as second fraction present

L for low plasticity

H for high plasticity

For visual-manual classification use letters L or H only

Double symbols: For borderline* cases, use double symbol e.g. GW - GC

GWWell gradedgravels sand mixture

GCclayedgravel; gravel-sand-clay mixture

Described as: Well graded gravel sand mixture with

clay binder

*USCS specifies % of fine materials less than 0.075mm (#200 sieve size) as 5 to 12%

Third letter In NZ the presence of significant amounts of allophane is of

sufficient importance a letter A is added as a third symbol to

indicate the presence of allophane

Allophane (orAmorphous Clays) -http://www.teara.govt.nz/

are amorphous hydrous alumina, which is a silicate clay minerals found in many soils

derived from volcanic ash, rock flour* They impart a greasy or waxy rather than

sticky consistency to soils.

They show marked irreversible changes in physical properties when dried below

their natural water content due to collapse of their gel like structure and aggregation

into much coarser grain size." Its color varies from white through green, blue, yellow,
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to brown and aspecific gravityof 1.0. Test 3.4 of NZS 4402is used for determination

of allophane content.

*Allophane is a weathering or hydrothermal (heat generated near seabed or volcanic) alteration

product offeldspars(common raw material in the production ofceramicsor household cleaner) and

other primary minerals and has a composition similar tokaolinite. Rock flour is also known asglacial

flour, consists of clay-sized particles of rock, generated by glacial erosion or by artificial grinding to

a similar size. Because the material is very small, it becomes suspended in river water making the

water appear cloudy, which is sometimes known asglacial milk.

http://www.landcareresearch.co.nz/services/laboratories/minlab/researchhighlights.asp

Properties and Uses of Allophane soils

Table 5.1 (paper on stabilising allophane soil)http://www.teara.govt.nz/

USCS system of soils identification proceduresUSCS chart

Step I Decide whether Coarse or Fine grained(1stand 2nd letters of symbol)

A. Coarse Grained Materials

Check if more than half (35% NZGS guidelines) of the material is coarser than the

75m (60m NZGS) sieve, then the soil is classified as coarse, else go to (B)

The following steps are then followed to determine the appropriate 2 letters

symbol

1. Determine the prefix(1st letter of the symbol)

1.1 If more than half of the coarse fraction is sand then use prefix S

1.2 If more than half of the coarse fraction is gravel then use prefix G

Otherwise, the materials should be classified as fine grain:

i.e. M or C. Atterburg limits is required to proceed further (refer (B) Fine

Grained Soils)

2. Determine the suffix(2nd letter of symbol)

This depends on the uniformity coefficient Cu and the coefficient of curvature Cc

obtained from the grading curve; the percentage of fines, and the type of fines.

Follow steps (2a) and (2b)

2a) Determine the percentage of fines, (from sieve analysis)

First find the % of material passing the 75m sieve.

Thenif % fines is

- Less than 5% use W or P as suffix- More than 12% use M or C as suffix
http://en.wikipedia.org/wiki/Specific_gravityhttp://en.wikipedia.org/wiki/Specific_gravityhttp://en.wikipedia.org/wiki/Specific_gravityhttp://en.wikipedia.org/wiki/Weatheringhttp://en.wikipedia.org/wiki/Weatheringhttp://en.wikipedia.org/wiki/Hydrothermalhttp://en.wikipedia.org/wiki/Hydrothermalhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Kaolinitehttp://en.wikipedia.org/wiki/Kaolinitehttp://en.wikipedia.org/wiki/Kaolinitehttp://www.landcareresearch.co.nz/services/laboratories/minlab/researchhighlights.asphttp://www.landcareresearch.co.nz/services/laboratories/minlab/researchhighlights.asphttp://../allophanes%20conference%20paper.pdfhttp://../allophanes%20conference%20paper.pdfhttp://www.teara.govt.nz/http://www.teara.govt.nz/http://2uscs%20soil%20classification%20chart1.pdf/http://2uscs%20soil%20classification%20chart1.pdf/http://2uscs%20soil%20classification%20chart1.pdf/http://2uscs%20soil%20classification%20chart1.pdf/http://www.teara.govt.nz/http://../allophanes%20conference%20paper.pdfhttp://www.landcareresearch.co.nz/services/laboratories/minlab/researchhighlights.asphttp://en.wikipedia.org/wiki/Kaolinitehttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Hydrothermalhttp://en.wikipedia.org/wiki/Weatheringhttp://en.wikipedia.org/wiki/Specific_gravity


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- Between 5% and 12% use Dual symbols. Use S or G from (1) above as

the prefix with either W or P as the 1st suffix andthen with one of M or C

for the 2nd suffix of the dual symbol system (refer examples below).

To determine whether soil is W or P, calculate Cu and Cc and follow the belowsteps:

If W or P is required for the suffix then Cu and Cc must be evaluated

CD

Du

60

10

CD

D Dc

30

2

60 10( )

If prefix is G then suffix is W if Cu > 4 andCc is between 1 and 3

otherwise use P

If prefix is S then suffix is W if Cu > 6 andCc is between 1 and 3

otherwise use P

2b) If M or C is required they have to be determined from the procedure used

for fine grain materials discussed below.

Note that M stands for Silt and C for Clay.

This is determined from whether the soil lies above or below the A-line in

the plasticity chart shown below.

Examples:

For a coarse grain soil which is predominantly sand the following symbols are

possible:

Two letters symbol: SW, SP, SM, SC,

Double symbols: SW-SM, SW-SC, SP-SM, SP-SC

B. Fine grained materials

If more than half (35% NZGS guidelines) of the material is smaller than the 75m(60m NZGS) sieve, the soil is classified as fine grained.

These are classified solely according to the results from the Atterberg Limit

Tests.

Values of the Plasticity Index and Liquid Limit are used to determine a point in the

plasticity chart shown in Figure below.

The classification symbol is determined from the region of the chart in which thepoint lies.


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Examples CH High plasticity clay

CL Low plasticity clay

MH High plasticity silt

ML Low plasticity silt

OH High plasticity organic soil (Rare)Pt Peat

Step II - Describing the soil

The final stage of the classification is to give a description of the soil to go with the 2-

symbol class.

For a coarse grain soilthis should include:

a) the percentages of sand and gravel

b) maximum particle size and angularity

c) surface conditiond) hardness of the coarse grains

e) local or geological name

f) any other relevant information

If the soil is undisturbedmention is also required of

a) stratification and degree of compactness

b) cementation

c) moisture conditions

d) drainage characteristics

Examples:

Describe the following soils using the Unified Soil Classification System table

1) SW-SC: Well gradedClayedSands (or gravely Sands-Clay mixtures)

2) SP-SM: Poorly gradedSiltySands (or gravely Sand Silt mixtures)

3) GW-GM:Well gradedSiltyGravels (or gravels Sand Silt mixtures)

4) GP-GC: Poorly gradedClayedGravels (or gravels Sand Clay mixtures)

We will look further at classification of soil from sieve analysis results together with

the USCS chart to identify coarsegrained materials in the later class. The following

sections study the characteristics of finegrained materials.

Example 1

Example 2
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2.3 Atterberg Limits (i.e. Plastic limits, Liquid limits)

The Atterberg limits (Plastic Limits (PL) and Liquid Limits (LL)) of soil are a basic

measure of the nature of a fine-grainedsoil.

The Atterberg limits are not only used to identify the soil's classification, but it alsoallows for the use of empirical correlations for materials engineering properties.

There is a close relationship between these limits and properties of a soil such as

compressibility, permeability, and strength. This is very useful and the procedures

are relatively simple.

Depending on the water content of the soil, soil may appear in four states:

- Solid,

- Semi-solid,

- Plastic and

- Liquid.

Source: Soil Mechanic, MJ Smith

In each state, the consistency and behavior of a soil is different and so are its

engineering properties. Thus, the boundary between each state can be defined

based on a change in the soil's behavior.

The Atterberg limits can be used to distinguish between silt and clay, and it can

distinguish between different types of silts and clays (plasticity chart).

These limits were created by Albert Atterberg (1911), a Swedish chemist. They were

later refined by Arthur Casagrande.

PL: Plastic Limit (>SL)LL: Liquid Limit (>PL)SL: Shrinkage Limit

Note: these are moisture contentsat different stages of soil (or soilconditions)

Fig cFig B

Fig A
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These tests are only used for the fine-grained, i.e. silt and clay, fraction of a soil

(the % passing a 425m sieve).

Three limits are indicated, the definitions are as follow, their determination are

described in section Physical property of soils.

The liquid and plastic limits appear to be fairly arbitrary, but recent research has

suggested they are related to the strength of the soil as shown below.

Liquid Limit (LL)(NZS4402:1986 Test 2.2)

It is the water contentat which a soil passes from the liquid to the plastic state. Two

tests are in use. One uses the Casagrande Liquid Limit Apparatus and the other the

Cone Penetrometer. Tests show that LL >16%

The test procedure is described in NZS 4202:1986 Test 2.2 (Fig. 1 .2C)

Plastic Limit (PL)(NZS4402:1986 Test 2.3)

It is the water contentat which a soil approaches a solid state and becomes too dry

to become plastic. This test involves rolling soil samples into 3 mm diameter threads

and remoulding repeatedly until the threads crack, at which stage the water content

is the plastic limit.

The test procedure is described by Test 2.3 NZS 4202: 1986. (Fig. 1 .2A)

Shrinkage Limit(NZS4402:1986 Test 2.6)

It defines the change of state from asemi-solid to a solid state and is the moisture

content at which the soil remains at constant volume on drying out. i.e. materials

becomes a non-compressible soil.

However, weight of soil continues to decrease till fully dried. Shrinkage limit is less

commonly uses as compare with PL and LL.

Plasticity Index (PI)Plastic Index is the range of water contentwithin which a soil is plastic.

The difference in water content between the liquid and plastic limits is defined as the

"Plasticity index" of the soil. i.e.

PI = LLPL

The finer the soil the greater the water content range and hence the larger the PI.

Soils with a highPI tend to be clay, those with a lower PI tend to be silt, and thosewith aPIof 0 tend to have little or no silt or clay. PIincrease with finer sand.


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The diagrams (Plasticity Chart) below indicate the relationship of the LL, PL and PI.

It follows that the greater the plasticity index, the more plastic and compressible and

the greater the volume change characteristics of the soil.

The plasticity index has proven to be one of the most useful of all soil indices and is

essential to the description of a cohesive soil.

Liquidity Index(IL),

At which soil changes its physical state, it describes the state of a soil at the time of

test.

Where w is the natural water content of soil

IL is a comparison of soils plasticity with its natural water content:

- If IL>1, quick clay, i.e. w=LL soil is at liquidstate

- If IL < 0, desiccated clay, i.e. w < PL driedout, dehydrated, volume

expansion if becomes wet,

- If IL=0 i.e. w=PL soil is at plasticstate

If we take a very soft (or high moisture content) clay specimen and allow it to dry, asthe soil dries its strength and stiffness will increase.

The behaviours of the soil would have relationships similar to that shown in figure

above (drying process figure 14 - consistency limits) and the below stress/strain

relationship.

Figure (a to c) : Stress/Strain curves


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1) The cohesive soil has highest water content, i.e. a suspension of soil particles in

water, the soil behaves as a liquid. (see also figure c)

2) If a shear stress were applied to it, there would be a continual deformation with no

sign of a failure stress value. As the soil is slowly dried out, a point is reachedwhen the soil just begins to exhibit a small shear resistance. (see also figure b)

3) If the stress is removed it is found that the soil has experienced a permanent

deformation, it is acting as a plastic solid and not as a liquid.

4) As further moisture is driven from the soil, resistance to large shearing stresses

becomes possible. Eventually the soil simply fractures with no plastic deformation,

i.e. it acts as a brittle solid. (see figure a)

The Atterberg Limits and relationships derived from them are simple measures of the

water absorbing ability of soilscontaining clay minerals.

For example, if clay has a very high LI and LL it is capable of absorbing large

amounts of water, and for instance would be unsuitable for the base of a pavement.

The LL and PL are also related to the soil strength.

Notethat only the fraction finer than 425m is tested in the Atterberg Limits Tests.

If this fraction is only small (that is, the soil contains significant amounts of sand or

gravel) it might be expected that the soil would have better properties.

While this is true to some extent it is important to also realise that the soil behaviour

is controlled by the finest 10 - 20 % of the particles

Plasticity ChartCasagrande classification

The Plasticity Chart is a plot of Liquid Limit (horizontal axis) and Plasticity Index

(Vertical axis) as shown below.

For a particular soil, the PL and LL can be obtained after Attergerg limit tests, which

(these limits) are then plotted on the plasticity Chart enabling the soil to be classified

with respect to the "A Line". Soils of the same geological origin usually plot on the

plasticity chart as straight lines parallel to the A line.

The A-Line is an empirical boundary established after many observations on soil

tests. In general, clay plots above the A line while silt plots below A line

A-Line separates Inorganic clays (soil above A-line) eg. CL or CH from the Inorganicsilts; organic silts or organic clay (soil below A-line), eg. OH, MH, OL and ML.


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U-line (Upper Limit)

Another line, known as the "U" line, can be plotted using the equation of the U-line:

PI=0.9(LL-8) WherePIis Plasticity Index and LL is liquid limit

U-line can be used as a check for correctness of lab data (research show that U lineis the upper limit for most natural soils), data that are above u-line should be rejected.

Staying within these ranges is recommended.

"Fat" or plastic clays plot above the A-line. Organic soils, silts and clays containing a large

portion of "rock flour" (finely ground non-clay minerals) plot below it.

Low PL

High PL

ML

Intermediate

Cohesionless

LL35LL20

Compressibility andpermeability increase,Strength decrease

Permeability decrease, strength increaseCompressibility about same


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Soils Classifications, Properties and Applicability

ReferTable 1Band1C(Moodle) for further details classification of various types of

soils and indication of their applicability. For example, from Table 1B, the clay and

silt content of fine soils at low, medium and high plasticity are as follow:

Low Plasticity (silts):LL< 35% with Less than 20% of clay,

Not gritty between the fingers, exhibits dilatancy, this soil groups include OL, ML, CL

Medium (Intermediate) Plasticity:

35% < LL< 50% with 20% to 40% clay,

Shows slight shrinkage on drying, this soil group include MI, CI and OI

High Plasticity (clay):

LL> 50% with More than 40% clay,

Considerable shrinkage on drying, highly compressible, this soil group include MH,

CH and OH

Example: Different between OH and MH in theUSCS chart(Q3/2008)

Criteria OH MH

Description

(USCS)

Organic silt

Organic clay of high plasticity;

Inorganic silt;

Micaceous or diatomaceous fine sandy or

silt soils; elastic silt

Colour Dark brown/black Light brownTexture Fibrous Rough

Odour smell when heated No odour when heated

Definitions - Micaceous or diatomaceous: A naturally occurring soft, chalk likesedimentaryrockthat is easily crumbled into a fine white to off-white powder. This powderhas anabrasivefeel, similar topumicepowder, and is very light, due to its highporosity. Thetypical chemical composition of diatomaceous earth is 86% silica, 5% sodium, 3%magnesiumand 2%iron.

http://en.wikipedia.org/wiki/Diatomaceous_earthhttp://www.merriam-webster.com/dictionary/micaceous

LimitationsClassifications

It is a tool that helps engineer identify and make preliminary assessment of

engineering behaviors, but it has some limitations, as mentioned by Casagrande

(1948):

It is not possible to classify all soils into a small number of groups such that the

relation of each soil to the many divergent problems of applied soil mechanics can

be adequately presented

Identifying the proper classification is just a starting points, one should not expect too

much from a classifications system, instead, we should combine it with the use

other test results, understanding soil behaviors with engineering judgment andexperience.
http://soil%20meachanic%20sidney%20rosenak%20%281b%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281b%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281b%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281c%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281c%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281c%29.pdf/http://3uscs%20soil%20classification%20chart2.doc/http://3uscs%20soil%20classification%20chart2.doc/http://3uscs%20soil%20classification%20chart2.doc/http://en.wikipedia.org/wiki/Chalkhttp://en.wikipedia.org/wiki/Chalkhttp://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Abrasivehttp://en.wikipedia.org/wiki/Abrasivehttp://en.wikipedia.org/wiki/Abrasivehttp://en.wikipedia.org/wiki/Pumicehttp://en.wikipedia.org/wiki/Pumicehttp://en.wikipedia.org/wiki/Pumicehttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Sodiumhttp://en.wikipedia.org/wiki/Sodiumhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Diatomaceous_earthhttp://en.wikipedia.org/wiki/Diatomaceous_earthhttp://www.merriam-webster.com/dictionary/micaceoushttp://www.merriam-webster.com/dictionary/micaceoushttp://www.merriam-webster.com/dictionary/micaceoushttp://en.wikipedia.org/wiki/Diatomaceous_earthhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Sodiumhttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Pumicehttp://en.wikipedia.org/wiki/Abrasivehttp://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Chalkhttp://3uscs%20soil%20classification%20chart2.doc/http://soil%20meachanic%20sidney%20rosenak%20%281c%29.pdf/http://soil%20meachanic%20sidney%20rosenak%20%281b%29.pdf/


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Example: Classification tests have been performed on a soil sample. The following resultswere obtained from sieve analysis and Atterberg limits tests. Determine the USCSclassification and symbols. The Classification tests results is as follow: Atterberg limits test results:

Liquid limit LL = 32, Plastic Limit, PL =26,

Plasticity Index Ip = 6 (i.e. =32 -26) Sieve Analysis, a grading curve can be plotted (such as below).

1) The following were obtained from thegrading curve:-The % of differentparticle size fractions(to determine G or S), i.e. Particle size fractions:

Gravel 17%Sand 73%, 0.06mm to 2mm

>50% larger than #200 sieve>50% smaller than #4 sieve (4.75mm), coarsegrained, i.e. Sand

Silt (7%) & Clay (3%) total 10% fine contenti.e. 5 to 12% fine, borderline case,dual symbols, PL and LL tests required

Of the coarse fraction sand (about 73%) is the major fraction, hence, determines Prefix isS(i.e. SAND) according to NZGSs guide

-From grading curve obtain D10, D30, D60 (to determine W or P)D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm,calculate Cu= 12.5 >6, Cc= 1.38, (i.e. between 1 to 3) and hence determine the Suffix1 =W

2) From the Atterberg Limits test given, determine the soils Plasticity chart locationLL = 32, PL = 26, Plasticity Index Ip = 6,

From Plasticity Chart, point lies below A-line, and hence Suffix2= ML i.e. low plasticityinorganic silts

3) Write the Symbols and describe the soil:

SW-ML : Well gradedinorganic low plasticitySiltySandwith little fines ORWell gradedSAND,somegravel withminorsilt andtrace ofclay

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

%

Finer

83%

10%

3%

sandsilt gravelclay
http://../week%202%20-%20sieve%20analysis%20&%20lab/1class%20example/example%20ans/Example%20Figure%2013ans.dochttp://../week%202%20-%20sieve%20analysis%20&%20lab/1class%20example/example%20ans/Example%20Figure%2013ans.dochttp://../Example.docxhttp://../Example.docxhttp://../Example.docxhttp://../1%20intro%20soil%20formation%20and%20soil%20profile/0introduction,%20soil%20profile%20and%20soil%20groups/0introduction%202010%20R00.dochttp://../1%20intro%20soil%20formation%20and%20soil%20profile/0introduction,%20soil%20profile%20and%20soil%20groups/0introduction%202010%20R00.dochttp://../1%20intro%20soil%20formation%20and%20soil%20profile/0introduction,%20soil%20profile%20and%20soil%20groups/0introduction%202010%20R00.dochttp://../1%20intro%20soil%20formation%20and%20soil%20profile/0introduction,%20soil%20profile%20and%20soil%20groups/0introduction%202010%20R00.dochttp://../Example.docxhttp://../week%202%20-%20sieve%20analysis%20&%20lab/1class%20example/example%20ans/Example%20Figure%2013ans.doc


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2.4 Soil Texture Triangle (USDA system)

To describe the relative proportions of different grain sizes of mineral particles in a

soil.

Soil particles are groupedaccording to their size into what are called soil separates.These separates are typically named clay, silt, and sand.

Soiltexture classification is based on the fractions of soil separates present in a soil.

The soil texture triangleis a diagram often used to figure out soil textures.

A continuum (i.e. a range of sizes and proportions) exists between sandy and clayey

textured soils.

The term "texture" refers to the size of the individual soil particles and has nothing to

do with the amount of organic matter present in the soil.

Figure: Courtesy of the Soil Science Society of America

It is rare that you find a soil texture composed entirely of a specific soil separate.

Nearly all soils are mixtures of the three soil separates mentioned above.

1

2

3

By calculation

Silt % =100%-70%-10%=20%

By reading off from chart - (step 3)

At intersect of (1) and (2), draw parallel

line to clay% which meets at silt%

Soil Texture class: Clay

70% clay

10% sand


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Important observationsfrom figure above are that:

- Any soil containing more than 50% of clay sized particles would be classified as

clay, whereas sand and silt require 80% of the particles to be in that size range.

- Also any soil having more than 20% clay would have some clay like properties.

- Only a relatively small proportion of clay is necessary to bring about clayeyproperties to a soil. Small amounts of sand and silt have a lesser influence on

how soil behaves.

- Soils that have varying proportions of the three soil separates are called Loam

soils.

- Equal proportion of separates doesn't mean equal influence on soil properties.

- The % of soil separates always sums to 100% in designating a soil texture.

Procedure to establish soil textures: - (read anti clockwise)

- First find the appropriate clay % along the left side of the triangle and

read parallel to the triangle's base.

- Next, find the sand % along the triangle's baseby reading the lines parallel to the

triangle side labelled as "% silt."

- At this point you have two optionsto derive the% silt:

a) Summing the clay and sand percentages and subtracting from 100% OR

b) find the % silt directly from the triangle by finding the % silt along the right side

of the triangle and reading the lines parallel to the "% clay" side of the

triangle.

- The name of the compartment in which these lines intersect indicates the

textural class of thesoil sample.

Example:

soil % sand % silt % clay

1 10 20 70

2 30 40 30

3 What is the % sand, silt, clay of a sandy loam?

4 What is the % sand, silt, clay of a silty clay loam?

http://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htm

12 3
http://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htmhttp://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htmhttp://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htm


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Summary-

Characteristic of soils

Sand is gritty to the touch and the individual grains or particles can be seen with the

naked eye. It is the largest of the three size classes of soil particles. A soil in which

sand predominates is classified, logically enough, as a sand-textured soil or simply a

sandy soil. Sandy soils are coarse in texture.

Silt is smooth and slippery to the touch when wet, and the individual particles are

much smaller than those of sand. These individual particles can only be seen with

the aid of a microscope. Silt-textured or silty soils contain relatively large amounts of

silt.

Clay is sticky and plastic-like to handle when wet. The individual particles are

extremely small and can only be seen with the aid of an electron microscope. As you

might guess, clay-textured, or clay soils, are rich in clay and fine in texture.

The three main soil texture classifications, then, are sandy, silty and clayey.

Sandy soils are coarse-textured, clayey soils are fine-textured, and silty soils

intermediate in texture. "Loam" is another soil texture classification.

A loam soil contains considerable amounts of sand, silt and clay. It is the preferred

texture for horticulture because of its ease of workability. Textures between theseclassifications are also often used in describing soil types, e.g., sandy loam or clay

loam soils.

You can roughly estimate the approximate amount of sand, silt and clay in a soil by a

simple method called "manual texturing."

The texture is determined by the feel of the moist sample when rubbed between the

thumb and forefinger.

If the soil sample is predominantly sand, it will feel very gritty.

If it is predominantly silt, it will feel smooth or slippery to the touch. And if it is

predominantly clay, it will feel sticky.

01soil Classification Note1!08!2011S2

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