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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