ISO 14688-2:2004 Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification

Standard Number
BS EN ISO 14688-2:2004
Title
Geotechnical investigation and testing. Identification and classification of soil. Principles for a classification
Publication Date
International Relationships
EN ISO 14688-2:2004, ISO 14688-2:2004
Descriptors
This part of ISO 14688, together with ISO 14688-1, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. The relevant characteristics may vary and therefore, for particular projects or materials, more detailed subdivisions of the descriptive and classification terms may be appropriate.
Keywords
Soil mechanics, Soil science, Soil testing, Soils, Identification methods, Soil classification tests, Classification systems
Title in French
Reconnaissance et essais geotechniques. Denomination, description et classification des sols. Principes pour une classification
Title in German
Geotechnische Erkundung und Untersuchung. Benennung, Beschreibung und Klassifizierung von Boden. Grundlagen von Bodenklassifizierung
Pages
20

CONTENTS

 

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 14688-2 was prepared by Technical Committee ISO/TC 182, Geotechnics, Subcommittee SC 1,

Geotechnical investigation and testing.

ISO 14688 consists of the following parts, under the general title Geotechnical investigation and testing — Identification and classification of soil:

  • Part 1: Identification and description
  • Part 2: Principles for a classification
  • Part 3: Electronic exchange of data on identification and description of soil

1 Scope

This part of ISO 14688, together with ISO 14688-1, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. The relevant characteristics may vary and therefore, for particular projects or materials, more detailed subdivisions of the descriptive and classification terms may be appropriate.

Identification and description of soil are covered by ISO 14688-1.

The classification principles established in this part of ISO 14688 permit soils to be grouped into classes of similar composition and geotechnical properties and, with respect to their suitability for geotechnical engineering purposes, such as

  • foundations,
  • ground improvements,
  • roads,
  • embankments,
  • dams, and
  • drainage systems.

This part of ISO 14688 is applicable to natural soil and similar man-made material in situ and redeposited, but it is not a classification of soil by itself.

Identification and description of rock are covered by ISO 14689-1.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 3310-1, Test sieves — Technical requirements and testing — Part 1: Test sieves of metal wire cloth

ISO 3310-2, Test sieves — Technical requirements and testing — Part 2: Test sieves of perforated metal plate

ISO 14688-1, Geotechnical investigation and testing — Identification and classification of soil — Part 1: Identification and description

ISO 14689-1, Geotechnical investigation and testing — Identification and classification of rock — Part 1: Identification and description

3 Terms and definitions

For the purposes of this document, the terms and definitions of ISO 14688-1 and the following apply.

3.1

soil classification

assignment of soil into soil groups on the basis of certain characteristics, criteria and genesis

3.2

soil group

a particular collection of soils of similar composition and geotechnical properties

3.3

uniformity coefficient

CU

measure of the shape of the grading curve within the range from d10 to d60

CU = d60 /d10

NOTE d10 and d60 are the particle sizes corresponding to the ordinates 10 % and 60 % by mass of the percentage passing.

3.4

coefficient of curvature

CC

measure of the shape of the grading curve within the range from d10, d30 to d60

CC = (d30)2/(d10 · d60)

3.5

water content

w

mass of water which can be removed from the soil, usually by drying, expressed as a percentage of the dry mass

3.6

liquid limit

wL

water content at which a fine soil passes from the liquid to the plastic condition, as determined by the liquid limit test

3.7

plastic limit

wP

water content at which a fine soil becomes too dry to be in a plastic condition, as determined by the plastic limit test

3.8

plasticity index

IP

numerical difference between the liquid limit and plastic limit of a fine soil

IP = wLwP

3.9

liquidity index

IL

numerical difference between the natural water content and the plastic limit expressed as a percentage ratio of the plasticity index

IL = (wwP)/IP

3.10

consistency index

IC

numerical difference between the liquid limit and the natural water content expressed as a percentage ratio of the plasticity index

IC = (wLw)/IP

3.11

density index

ID

coarse soils (sands and gravels) index dependent upon the void ratio (e) and the void ratios corresponding to the minimum density (emax) and the maximum density (emin), as measured in the laboratory

ID = (emaxe)/(emaxemin)

3.12

undrained shear strength

cu

shear resistance of soil in the undrained condition

3.13

void ratio

ratio of the volume of voids to the volume of solids of a soil

3.14

compressibility index

Cc

compressibility index is defined according to the relation

NOTE Δe is the change in void ratio (negative value when Δe decreases) and is the change in void ratio Δe for a relative increase of effective stress from lgσ' to lg(σ' + Δσ').

4 Principles of soil classifications

4.1 General

Soils shall be classified into soil groups on the basis of their nature which is the composition only, irrespective of their water content or compactness, taking into account the following characteristics:

  • particle size distribution (grading);
  • plasticity;
  • organic content;
  • genesis.

NOTE Some principles for soil classification are given in Annex A.

4.2 Fractions

Soil is a mixture of materials of different particle size, which are grouped into fractions as specified in ISO 14688-1.

Classification of coarse and very coarse soils is to be based on the particle size distribution alone (see 4.3 and Table 1).

Table 1 — Classification of very coarse soil
Fraction Percent by mass Term
Boulders < 5 low boulder content
5 to 20 medium boulder content
> 20 high boulder content
Cobbles < 10 low cobble content
10 to 20 medium cobble content
> 20 high cobble content

NOTE The classification of very coarse soils requires a very large sample. It is not possible to recover representative samples from boreholes to use this classification.

In the case of soils composed of both fine and coarse material, classification is to be based on both plasticity and particle size distribution (see 4.3 and 4.4).

4.3 Particle size distribution (grading)

The particle sizes and their distribution in a soil are determined by mechanical analysis carried out as follows:

  • the separation of the coarser fractions by sieving on a series of standard sieves according to ISO 3310-1 and ISO 3310-2;
  • the determination of the finer fractions by an accepted process (e.g. sedimentation, optical methods).

NOTE An example of how this can be done is given in Annex B.

The results of the sieving and sedimentation process are plotted as a grading curve.

When designating the coarse fractions, a distinction may be drawn between well graded, poorly graded and gap-graded particle size distributions. In this connection the coefficient of curvature (CC) and the uniformity coefficient (CU) provide quantitative means for describing the shape of the grading curve. If certain grain sizes are absent, the term gap-graded is used. The median d50 of the grading curve, together with CU and CC may also be used to indicate the particle size grading (see Table 2).

Table 2 — Shape of grading curve
Shape of grading curve CU CC
Multi-graded > 15 1 < CC < 3
Medium-graded 6 to 15 < 1
Even-graded < 6 < 1
Gap-graded Usually high Any (usually < 0,5)

4.4 Plasticity

The fine fractions of soil, represented by clay and silt and containing clay minerals (see also ISO 14688-1), both alone or in mixtures with coarser material, are usually classified according to their plasticity characteristics. This is carried out on the basis of laboratory tests to determine the liquid limit wL and plastic limit wP.

The degree of plasticity of fine soils should be classified using the following terms:

  • a) non-plastic;
  • b) low plasticity;
  • c) intermediate plasticity;
  • d) high plasticity.

4.5 Organic content

When soils with organic constituents are classified according to their organic content (see Table 3), a distinction is to be made between organic soils and mineral soils with an organic content.

Table 3 — Classification of soils with organic constituents
Soil Organic content
(≤ 2 mm) % of dry mass
Low-organic 2 to 6
Medium-organic 6 to 20
High-organic > 20

Classification of coarse and composite organic soils accumulated in situ is based on the type of organic matter and that of organic soils, on the genetic origin and the degree of decomposition of the organic constituents.

5 Other principles suitable for soil classification

5.1 General

There is a variety of quantifying terms which can be used to describe soils which include density, undrained shear strength and consistency index.

5.2 Correlations of density terms for sands and gravels

The terms used for the classification of density index ID are very loose, loose, medium dense, dense and very dense (see Table 4). Density index can be related to the results of field tests (see, for example, EN 1997-2).

Such field tests are, for example, Dynamic Probing (DP) according to ISO 22476-2, Standard Penetration Test (SPT) according to ISO 22476-3, cone penetration tests (CPT) according to ISO 22476-1 and pressuremeter tests (PMT) according to ISO 22476-4, ISO 22476-6 and ISO 22476-8. These documents are being prepared.

Table 4 — Correlations to classify density terms
Term Density index
ID
%
Very loose 0 to 15
Loose 15 to 35
Medium dense 35 to 65
Dense 65 to 85
Very dense 85 to 100

5.3 Undrained shear strength of fine soils

The terms to be used for the designation of the undrained shear strength according to the results of laboratory and field tests are given in Table 5.

Table 5 — Undrained shear strength of fine soils
Undrained shear strength of clays Undrained shear strength
cu
kPa
Extremely low < 10
Very low 10 to 20
Low 20 to 40
Medium 40 to 75
High 75 to 150
Very high 150 to 300
Extremely higha > 300
a Materials with shear strength greater than 300 kPa may behave as weak rocks and should be described as rocks according to ISO 14689-1.

NOTE When making an immediate (field) examination, the strength is estimated by manual tests or measured by a simple field test, e.g. pocket penetrometer or small vane apparatus.

Fine soils may also be classified according to their sensitivity, the ratio between the undisturbed and remoulded undrained shear strengths. The sensitivity is low (< 8), medium (8 – 30) or high (> 30); soils with sensitivities > 50 are described as quick clays.

5.4 Consistency index

Terms to be used for the designation of the consistency index (IC) of silts and clays where appropriate, are given in Table 6.

Table 6 — Consistency index IC of silts and clays
Consistency of silts and clays Consistency index
IC
Very soft < 0,25
Soft 0,25 to 0,50
Firm 0,50 to 0,75
Stiff 0,75 to 1,00
Very stiff > 1,00

These subdivisions may be approximate, particularly in materials of low plasticity. Also, the strength of a clay may not be constant at a given consistency index. The liquidity index may be used as an alternative.

5.5 Other suitable parameters

Some other parameters may be used for soil classification for specific purposes such as:

  • dry density;
  • clay activity;
  • mineralogical nature,
  • saturation index;
  • permeability;
  • compressibility index CC;
  • swelling index;
  • carbonate index.

Annex A

(informative)

Principles of soil classifications

The most common approach to classification is to divide the soils on the basis of particle size grading and plasticity. The division is made on the relative size fractions present for the coarser soil fractions, determined on the whole sample, and on the plasticity of the finer fractions (e.g. Table A.1).

The principles for establishing classifications to suit particular geological conditions or engineering problems are given in this standard.

Extensions or amplification of these principles at national or project level are possible and an example is given in Table A.1.

Particular classifications would normally be expected to quantify the boundaries or rules for categorization.

Table A.1 — Principles of a classification of soils
Criterion Soil group Quantification Denomination into groups of similiar properties Further subdivision as appropriate by
Wet soil does not stick together very coarse most particles
> 200 mm
 
most particles
> 63 mm
Bo xBo  
  boCo coBo
Co saCo, grCo sagrCo
Requires special consideration
coarse most particles
> 2 mm
 
most particles
> 0,063 mm
Gr   cosaGr
  coGr  
  saGr, grSa sasiGr, grsiSa
Sa siGr, clGr siSa, clSa, saclGr
  orSa  
Particle size (grading) Shape of grading curve Relative density Permeability
(Mineralogy) (Particle shape)
Wet soils sticks together fine low plasticity dilatant
 
plastic
non-dilatant
Si saSi sagrSi
    saclSi
  clSi, siCl  
Cl   sagrCl
  orSi, orCl  
Plasticity
Water content
Strength, sensitivity
Compressibility, stiffness
(Clay mineralogy)
Dark colour, low density organic  
Or saOr, siOr clOr
Requires special consideration
Not naturally Made ground deposited
Mg xMg
Man-made material Requires special consideration
Relaid natural materials As for natural soils
Key to symbols
SoilBoulders
Cobbles
Gravel
Sand
Silt
Clay
Organic
Made ground
 
Principal
Bo
Co
Gr
Sa
Si
Cl
Or
Mg
 
Secondary or tertiary component
bo
co
gr
sa
si
cl
or

x
 
 
Gr(gr) and Sa(sa) can be subdivided into fine F(f), medium M(m) or coarse C(c)
 
 
 
 
any combination of components
Cases requiring special consideration should be classified according to national or project requirements

Annex B

(informative)

Example of a classification of soils, based on grading alone

Figure B.1 and Table B.1 give an example of a possible soil classification, based on grading alone.

Key

  • 1 gravel content (2 mm to 63 mm)
  • 2 sand content (0,063 mm to 2 mm)
  • 3 fines content (< 0,063 mm)
  • 4 clay content in % of mass of coarse and fine soil (grain size < 63 mm)
  • 5 fine soils (silt and clay)
  • 6 mixed-grained soils (silty or clayey gravel and sand)
  • 7 coarse soils (gravel and sand)
  • S soil

See also Table A.1.

Figure B.1 — Classification of soils, based on grading alone
Table B.1 — Guiding values for the division of mineral soils on a basis of the contents of various fractions
Fraction Content of fraction in wt % of material
≤ 63 mm
Content of fraction in wt % of material
≤ 0,063 mm
Name of soil
Modifying term Main term
Gravel 20 to 40
> 40
  gravelly gravel
Sand 20 to 40
> 40
  sandy sand
Silt + clay
(fine soil)
5 to 15
 
15 to 40
 
> 40
< 20
≥ 20
< 20
≥ 20
< 10
10 to 20
20 to 40
> 40
slightly silty
slightly clayey
silty
clayey
 
clayey
silty
 
 
 
 
silt
silt
clay
clay

Bibliography

[1] ISO 22476-1, Geotechnical investigation and testing — Field testing — Part 1: Electrical cone and piezocone penetration tests

[2] ISO 22476-2, Geotechnical investigation and testing — Field testing — Part 2: Dynamic probing

[3] ISO 22476-3, Geotechnical investigation and testing — Field testing — Part 3: Standard penetration test

[4] ISO 22476-4, Geotechnical investigation and testing — Field testing — Part 4: Menard pressuremeter test

[5] ISO 22476-6, Geotechnical investigation and testing — Field testing — Part 6: Self-boring pressuremeter test

[6] ISO 22476-8, Geotechnical investigation and testing — Field testing — Part 8: Full displacement pressuremeter test

[7] EN 1997-2, Eurocode 7: Geotechnical design — Part 2: Design assisted by laboratory testing