Classification of SOIL V1.00 Sept2010

Classification v1.00 September 2010

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CLASSIFICATION OF SOILS (IMPORTANT STATEMENT: These lecture notes are based upon BS5930:1999 and

BS1377:1990 but, where appropriate, reference is made to Eurocode 7 related documents. These are namely BS EN ISO 14688-1:2002, BS EN ISO 14688-2:2004 and BS EN ISO 14689-

1:2003). During this transitional stage as the full recommendations of the Eurocode are being

implemented during 2010 students are advised to be aware that published text books are likely to

make little reference to the Eurocode 7 (ie. EN 1997) and there are some major differences in the

way that soils are described. Students should be aware that the final National Annex to BS EN

1997 was published on 31st December 2009 and that sections of BS5930 are currently being re-

written to comply fully with the Eurocode). See: http://www.eurocodes.co.uk

Purpose of soil classification

1. Provides a concise and systematic method for designating various types of soil.

2. Enables useful engineering conclusions to be made about soil properties.

3. Provides a common language for the transmission of information.

4. Permits the precise presentation of boring records and test results.

Object of soil classification

Is to provide a soil NAME and symbol, e.g. GRAVEL is G, based on the results of simple and quick to perform (therefore economic) key tests;

1. Particle size distribution (P.S.D.) or sieve analysis.

2. Plastic properties; Liquid limit test

Plastic limit test Soil is initially classified into either coarse or fine soil on the basis of particle size.

Coarse soil (Granular)

Physical characteristics and appearance are influenced by the distribution of particle sizes within the soil, i.e.>0.063mm (1/16mm)

A granular soil is classified according to its Particle Size Distribution.

Fine soil (Cohesive) Physical characteristics and appearance influenced by cohesion and plastic properties (plasticity) associated with mineral composition and water content.

The fine soil is sub-grouped according to its plasticity.

CLASSIFICATION OF COARSE SOIL

Is classified on the sand (symbol S) and gravel (symbol G) content plus the amount of fines present.

Particle size distribution by sieve analysis

This test involves passing soil through a series of sieves of decreasing mesh size and recording the weight of soil retained on each. Soil which passes through the finest sieve

(63 microns or 0.063mm) collects in a base tray and is also weighed.


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The results are processed and plotted on a semi-log chart of cumulative percentage passing (y-axis) verses log of particle size in mm (x-axis).

NB Where a soil contains fines, a relatively small proportion may cause the soil to form a cohesive mass and dominate the properties of the soil. It is considered that a soil

containing 35% fines or more will behave as a cohesive soil (ie silt or clay). The fines content is critical and is indicated with an extra symbol;

Symbol % Fines

Terminology Fines content

G 0 5

Clean gravel Little influence

S Clean sand

G-F

5 15 Gravel with some fines

Some influence S-F Sand with some fines

GF

15 - 35 Gravel with much fines

Considerable influence SF Sand with much fines

Grading The shape of the Particle Size Distribution curve indicates the range of particle sizes within a soil. Coarse soils are sub-grouped on whether a soil is well graded or poorly

graded.

A well graded (symbol W) soil has approximately equal proportions of particles sizes and the curve is usually smooth. Note the Till is a well graded soil and the well graded

gravel GW in the PSD chart below. A poorly graded (symbol P) soil may contain a high proportion of material within a

limited particle size band or bands. Poorly graded soil may be further sub-divided into uniform soil and gap graded soil:

A poorly graded soil (Pu, uniform or closely graded) has a major proportion of the particles lying between narrow size limits. As shown by the Estuary Sand, SPuF,

in the PSD chart below.

Gap graded soil has particles of both large and small sizes but with a relatively small proportion of particles with intermediate size, see clayey gravely sand, SPg in the PSD chart below.

How the laboratory test data is processed and plotted will be explained during the lecture.


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Soil description Four elements can be deduced on the basis of the grading curve:

Grading Secondary

constituent

Sub-divisions Soil name

(IN CAPS)

Well graded sandy fine and medium GRAVEL

Other physical characteristics can also be included, for example overall colour, strength,

presence of root inclusions, layering/banding or organic odours. Some examples of soil descriptions are:

Dense, reddish-brown, sub-angular, well graded, gravelly SAND

Firm, grey, laminated CLAY of low plasticity with occasional silt partings 0.5-2.0mm

Dense, brown, heterogeneous, well graded, very silty SAND and GRAVEL with

some COBBLES: Till

Stiff, brown, closely fissured CLAY of high plasticity: London Clay Spongy, dark brown, fibrous PEAT

NOTE: A soil description is NOT a soil classification.

Analysis of particle size distribution curves

Analyse each of the curves A,B and C shown below using the table provided;


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Soil Compositions: Soil A: Gravel..% Sand..% Fines..%

Soil B: Gravel..% Sand..% Fines..%

Soil C: Gravel..% Sand..% Silt..% Clay..% Soil classifications:

Soil A: ______ Soil B: ______ Soil C: ______

Soil Descriptions Soil A: _____________________________________

Soil B: _____________________________________

(Soil C requires further classification of the fines faction according to plasticity)

Class example 1

The results of a sieve analysis on two soils (Soil A, initial mass = 341g and Soil

B, initial mass = 410g) are given below:-

SOIL A SOIL B

Sieve Size (mm) Mass Retained (g) Mass Retained (g)

50.0 0.0 0.0

37.5 0.0 12.3

28.0 0.0 12.3

20.0 0.0 12.3

14.0 9.8 24.6

10.0 6.6 20.5

6.3 19.7 41.0

5.0 29.5 12.3

3.35 114.8 41.0

2.0 65.6 49.2

1.18 36.1 24.6

0.60 16.4 45.1

0.425 19.7 16.4

0.30 6.6 12.3

0.212 3.3 16.4

0.150 6.5 8.2

0.063 3.3 16.4

Tray 3.1 45.1

For soil samples A and B; a) Determine cumulative percentage passing and plot the particle size

distribution on the chart provided.

b) Determine the percentage of gravel, sand and fines. c) Provide a soil classification NAME and symbol.


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CLASSIFICATION OF FINE SOIL The classification of fine soil (F) is based on the property of soil plasticity. Plasticity is a

function of a fine soils capacity to absorb water and remain in a cohesive state.

There are two sub-groups in fine soil;

Sub-group Symbol Particle size

(mm) Silt M 0.06 0.002 Clay C


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Tests to determine liquid and plastic limits

i) Plastic Limit

The moisture content is determined at which a thread of soil can be rolled without breaking until it is only 3mm in diameter.

Mix a small amount of clay with distilled water to form a pat about the size of a marble. Roll out the pat on a glass plate until a thread is formed. The thread should be gently

rolled until the diameter is about 3 mm. If a smooth un-broken thread is obtained, roll up the thread into a ball and gently mark in the palm of the hand and repeat the process until the clay just begins to crumble when the

3 mm diameter is rolled. Then carry out normal moisture content determination. Repeat a total of 3 times and average the results.

ii) Liquid Limit (Preferred method using the cone penetrometer) Details of the apparatus are shown below. The soil to be tested is air dried and

thoroughly mixed. At least 200g of the soil are sieved through a 425 micron () sieve and placed on a glass sheet. The soil is then mixed with distilled water into a paste.

N.B. 1 micron = 0.001mm

A metal cup, approximately 55 mm in diameter and 40 mm deep is filled with paste and the surface struck off level. The cone

is next placed at the centre of the smoothed soil surface and level with it. The

cone is released so that it penetrates into the soil and the amount of penetration, over a time period of 5 seconds, is measured.

The test is now repeated by lifting the cone clear, cleaning it and filling up the

depression in the soils surface by adding a little more of the wet soil.

If the difference between the two measured penetrations is less than 0.5 mm

then the tests are considered valid. The average penetration is noted and a moisture content determination (detailed in

prctical laboratory work) is carried out on the soil tested.

The procedure is repeated at least four times with increasing water contents. The amount of water used throughout should be such that the penetration obtained lie within a range of 15 to 25 mm.

To obtain the liquid limit the variation of cone penetration (plotted vertically) to moisture

content (plotted horizontally) is drawn out (both scales being natural). The best straight line is drawn through the experimental points and the liquid limit is taken to be the moisture content corresponding to a cone penetration of 20 mm

(expressed as a whole number).


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(Although, the above method is preferred for determining the Liquid Limit, the Casagrande method is also widely used - see BS 1377)

Use of the A-Line classification chart for fine soil This classification chart plots Plasticity Index (%) against Liquid Limit (%), see below. The main aim is to determine if fine soil is either silt or clay. Clay plots above the A-line

and silt below. Generally high Liquid Limit values correlate with high the plasticity. Silts and organic soils have a low Plasticity Index (ie. a small range of moisture content

over which they are plastic) compared to their Liquid Limit. Clay soils have a high Plasticity Index in relation to their Liquid Limit. Clay minerals

have the capacity to take in moisture and still retain some cohesion.

To classify a fine soil according to the ALine chart:

i) Determine Liquid and Plastic Limits, and therefore Plasticity Index.

ii) Using values of PI and LL, plot the soil on the chart.

iii) Observe which segment the soil comes into (CL, CI, ML etc) iv) Write down the soil name, CLAY for C soils, SILT for M soils. v) Follow the name with the plasticity ie. CI = CLAY of intermediate

plasticity.

Class example 2

Liquid Limit and Plastic Limit tests were carried out on a cohesive soil and the

following results were recorded: -


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Plastic Limit Test Test No. 1 2 3 4

Mass of container (g) 16.06 15.72 16.15 Mass of container + wet soil (g) 32.70 31.71 32.79

Mass of container + dry soil (g) 29.33 28.44 29.44

Liquid Limit Test Mass of container (g) 15.74 12.76 14.62 13.88

Mass of container + wet soil (g) 41.57 40.76 43.06 41.46 Mass of container + dry soil (g) 33.09 30.26 30.42 27.97

Cone Penetration (mm) 15.0 17.0 21.0 24.0

Determine the following: -

i. Plastic limit and liquid limit for the soil.

ii. Plasticity index for the soil. iii. Classification for the soil.

[PL=25%; LL = 76%; PI = 50%; CLAY, CV]

Particle size distribution of fine soils by sedimentation test

For material passing the 63 (0.063mm) sieve, particle size distribution is determined by observing the sedimentation characteristics of the particles as they settle out of

suspension in water (BS 1377).

This is NOT a soil classification test as laboratory conditions are critical; it takes up to several days and is therefore, expensive. It does, however, give the distribution of particles below the smallest sieve size, from 0.063mm to 0.002mm, and can be used to

classify soil.


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Table 1 British Soil Classification System for Engineering Purposes

SOIL GROUPS SUB-GROUPS

and in laboratory identification

GRAVEL and SAND may be qualified sandy GRAVEL and gravely SAND where appropriate

Group Symbol

Sub-group symbol

Fines (% less than 0.06 mm)

Liquid limit

CO

AR

SE S

OILS

less t

han

35

% o

f th

e m

ate

ria

l is

fin

er t

han

0.0

6 m

m

GRAVELS

More

than 5

0%

of coars

e

mate

rial is

of gra

vel siz

e

(coars

er

than 2

mm

) Slightly silty or Clayey GRAVEL

GW G

GP

GW GPu GPg

0 to 5

Silty GRAVEL

Clayey GRAVEL

G-M

G-F G-C

GWM GPM

GWC GPC

5 to 15

Very silty GRAVEL

Very clayey GRAVEL

GM

GF GC

GML, etc

GCL GCI

GCH GCV GCE

15 to 35

SAN

DS

More

than 5

0%

of coars

e

mate

rial is

of sand s

ize

(fin

er

than 2

mm

)

Slightly silty or clayey SAND

SW S

SP

SW SPu SPg

0 to 5

Silty SAND

Clayey SAND

S-M S-F

S-C

SWM SPM

SWC SPC

15 to 35

Very silty SAND Very clayey SAND

SM SF

SC

SML etc SCL SCI SCH

SCV SCE

15 to 35

FIN

E S

OILS

mo

re t

han

35

% o

f th

e m

ate

ria

l is

fin

er t

han

0.0

6 m

m

Gra

velly o

r sandy

SIL

TS a

nd C

LAYS

35%

to 6

5%

fin

es

Gravelly SILT Gravelly CLAY

MG FG

CG

MLG etc CLG CIG

CHG CVG CEG

< 35 35 to 70

50 to 70 70 to 90 > 90

Sandy SILT

Sandy CLAY

MS FS

CS

MLS etc

CLS etc

SIL

TS a

nd

CLAYS 6

5%

to

100%

fin

es

SILT (M SOIL) CLAY

M F

C

ML etc CL CI CH

CV

CE

< 35 35 to 50 50 to 70

70 to 90

> 90

ORGANIC SOILS Description letter O suffixed to say group or subgroup symbol

Organic matter in significant amount

e.g. MHO organic silt of high LL PEAT Pt consists predominantly of plant remains (fibrous or amorphous) Primary Letter Secondary letter G Gravel W Well graded S Sand P Poorly graded

M Silt M With non-plastic fines C Clay C With plastic fines O Organic Soil L Of low plasticity (LL50)


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Rapid methods of classification

Rapid methods of classification are appropriate for identification in the field, or where laboratory facilities are not available. A combination of sound judgment (based on experience) and simple tests is used in conjunction with Tables 2 to 4 below. If soil

sub-group symbols are quoted they should be written enclosed in brackets to indicate that they are based on a rapid (as opposed to laboratory) method.

Particle size

Gravel sizes (>2 mm) are apparent visually; sands (


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Dry strength

If a pat of moist soil is dried, preferably in an oven, it will shrink and harden according to the silt or clay content. Its dry strength may be estimated by attempting to break the pat with the fingers. A high dry strength indicates a clay of high plasticity, whereas

a crumbly powdery dry pat indicates a silt of low plasticity.

Penetration resistance (sands and gravels)

At the surface or in trial pits, a spade, a pick or a small wooden peg driven into the soil, will give an indication of compactness (see Table 4).

Soil structure

From observations in trial pits or at other exposures, the main structural characteristics may be determined; such as the spacing between beds or laminations, whether or not

different materials are inter-bedded and whether or not the soil is fissured (Table 4).


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TABLE 2 (NOTE: In table below 60mm is being replaced by 63mm, 6mm by 6.3mm, etc)

Field identification and description of soils (Part of Table 13: BS 5930 :1999)

Basic soil type

Particle size

mm

Visual identification Particle nature and plasticity

Very

co

arse

so

il

BOULDERS 200

Only seen complete in pits or exposures

Particle shape:

Angular

Sub-angular Sub-rounded

Rounded

Flat

Elongate

Texture:

Rough Smooth Polished

COBBLES 60

Often difficult to recover from bore holes

Co

arse s

oils

(over

65%

sand a

nd g

ravel siz

es)

GRAVELS

Course

20

Easily visible to naked eye;

particle shape can be described; grading can be described

Well graded: wide range of grain sizes well distributed.

Poorly graded: May be uniform: size of most particles lies between narrow limits; or gap graded: an

intermediate size of particle is

markedly under- represented.

Medium

6

Fine 2

SANDS

Course

0.6

Visible to naked eye: very little or no cohesion when dry; grading can be described. Well graded: wide range of grain sizes, well distributed. Poorly graded: not

well graded. (May be uniform: size

of most particles lies between narrow

limits; or gap graded: an intermediate size of particle is markedly under- represented.)

Medium

0.2

Fine

0.06

Fin

e s

oils

(over

35%

silt

and c

lay s

izes) SILTS

Coarse

0.02

Only coarse silt is barely visible to

naked eye; exhibits little plasticity and marked dilatancy; slightly granular or silky to the touch. Disintegrates in water; lumps dry quickly; possess cohesion but can be powdered easily between fingers.

Non-plastic or

low plasticity

Medium

0.006

Fine 0.002

CLAYS

Dry lumps can be broken but not powdered between the fingers; they also disintegrate under water but more (Lean clay) slowly than

silt; smooth to the touch; exhibits plasticity but no dilatancy sticks to the fingers and dries slowly; shrinks appreciable on drying usually showing cracks.

Intermediate and high plasticity clays show these properties

plasticity to a moderate and high degree respectively.

Intermediate plasticity

(lean clay)

High plasticity

(fat clay)

Org

an

ic s

oils

ORGANIC

CLAY, SILT OR SAND

Varies Contains substantial amounts of

organic vegetable matter.

PEATS Varies Predominantly plant remains usually dark brown or black in colour, often with distinctive smell, low bulk density. Can contain disseminated or discrete mineral

soils.


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TABLE 3 Description of composite soil types

Predominantly coarse-grained e.g. SAND or GRAVEL

Descriptive term % clay or silt

Slightly silty or slightly clayey 0 - 5

Clayey or silty 5 10

Very clayey or very silty 10 35

Predominantly fine-grained e.g. CLAY or SILT

Descriptive term % sand or gravel

Sandy or gravelly 35 65

(non used) 0 - 35

TABLE 4 Description of structural or mass characteristics

Descriptive term Field identification

Coarse-grained soils

Loose Easily excavated; 50mm sq wooden peg easily driven in.

Dense Pick required for excavation; 50mm wooden peg hard to drive

Slightly cemented Excavated lumps hold together when abraded

Homogeneous Essentially one type

Heterogeneous Mixture of types

Stratified Alternating layers

Weathered Signs of weakening; concentric layers

Fine-grained soils

Very soft Exudes between fingers when squeezed

Soft Moulded with light finger pressure

Firm Moulded with strong finger pressure

Stiff Cannot be moulded with fingers; can be indented by thumb

Very stiff Cannot be indented by thumb

Hard Can just be indented by thumbnail

Fissured Breaks into polyhedral fragments

Intact Not fissured

Stratified Alternating layers

Homogeneous Essentially one type

Weathered Crumbly or columnar structure

Organic soils (peat)

Firm Fibres already compressed together

Spongy Very compressible open structure

Plastic Can be moulded in the hand and smears the fingers


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TABLE 5 Particle Size Fractions (BS EN ISO 14688-1:2002 Table 1)

Soil fractions Sub-fractions Symbols Particle sizes mm

Very coarse soil Large boulder Boulder Cobble

LBo Bo Co

>630 >200 to 630 >63 to 200

Coarse soil Gravel Coarse gravel

Medium gravel Fine gravel

Gr CGr

MGr FGr

>2,0 to 6,3 >20 to 63

>6,3 to 20 >2,0 to 6,3

Sand Coarse sand

Medium sand Fine sand

Sa CSa

MSa FSa

>0,063 to 2,0 >0,63 to 2,0

>0,2 to 0,63 >0,063 to 0,2

Fine soil Silt

Coarse silt Medium silt

Fine silt

Si

CSi MSi

FSi

>0,002 to 0,063

>0,02 to 0,063 >0,0063 to 0,02

>0,002 to 0,0063

Clay Cl


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TABLE 7 Cohesive Soil Consistency (BS EN ISO 14688-1:2002 Para 5.14)

Very soft Exudes between fingers when squeezed in the hand

Soft Can be moulded by light finger pressure

Firm Cannot be moulded by the fingers, but rolled in the hand to 3mm thick

threads without breaking or crumbling

Stiff Crumbles and breaks when rolled to 3mm threads but is still sufficiently moist to be moulded to a lump again

Very stiff Has dried out and is mostly light coloured. It can no longer be moulded but crumbles under pressure. It can be indented by the thumbnail.

NOTE: These terms are NO LONGER related to the Shear Strength of a soil. Many current

text books will refer to the old way of description as follows from BS5930:1990; Very Soft = 150kN/m2 Hard = >300kN/m2

TABLE 8 Undrained shear strength of fine soils (BS EN ISO 14688-2:2004 Table 5)

Undrained shear strength of clays

Undrained shear strength cu

kPa (ie. kN/m2)

old BS5930 term

Extremely low 300 Hard

Materials with shear strength greater than 300kPa may

behave as weak rocks and should be described as rocks according to BS EN ISO 14689-1:2003.

TABLE 9 Consistency index (Ic) of silts and clays (BS EN ISO 14688-2:2004

Table 6)

Consistency of silts and clays Consistency index Ic

Very soft 1,00

Consistency index, IC, is defined as

LL - w

LL - PL


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TABLE 10 Correlations to classify density terms (BS EN ISO 14688-2:2004

Table 4)

Term Density index

ID ( % )

BS5930 SPT N Value

Very loose 0 to 15 0 to 4

Loose 15 to 35 4 to 10

Medium dense 35 to 65 10 to 30

Dense 65 to 85 30 to 50

Very dense 85 to 100 >50

NOTE: You MUST define whether the terms very loose, loose, etc are based upon Density Index (ID) as per BS EN ISO 14688 OR from SPT N Values from BS5930. Density index, ID, is defined as

emax - e

emax - emin

Classification of SOIL V1.00 Sept2010

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