ISO 14688-2:2004 Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification
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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
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
- ground improvements,
- 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.
assignment of soil into soil groups on the basis of certain characteristics, criteria and genesis
a particular collection of soils of similar composition and geotechnical properties
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.
coefficient of curvature
measure of the shape of the grading curve within the range from d10, d30 to d60
CC = (d30)2/(d10 · d60)
mass of water which can be removed from the soil, usually by drying, expressed as a percentage of the dry mass
water content at which a fine soil passes from the liquid to the plastic condition, as determined by the liquid limit test
water content at which a fine soil becomes too dry to be in a plastic condition, as determined by the plastic limit test
numerical difference between the liquid limit and plastic limit of a fine soil
IP = wL – wP
numerical difference between the natural water content and the plastic limit expressed as a percentage ratio of the plasticity index
IL = (w – wP)/IP
numerical difference between the liquid limit and the natural water content expressed as a percentage ratio of the plasticity index
IC = (wL – w)/IP
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 = (emax – e)/(emax – emin)
undrained shear strength
shear resistance of soil in the undrained condition
ratio of the volume of voids to the volume of solids of a soil
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
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);
- organic content;
NOTE Some principles for soil classification are given in Annex A.
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)|
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
(≤ 2 mm) % of dry mass
|Low-organic||2 to 6|
|Medium-organic||6 to 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
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
|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.
|Undrained shear strength of clays||Undrained shear strength
|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
|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;
- compressibility index CC;
- swelling index;
- carbonate index.
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
> 63 mm
|Bo||xBo||Requires special consideration|
> 2 mm
> 0,063 mm
|Gr||cosaGr||Particle size (grading) Shape of grading curve Relative density Permeability|
|saGr, grSa||sasiGr, grsiSa|
|Sa||siGr, clGr||siSa, clSa, saclGr|
|orSa||(Mineralogy) (Particle shape)|
|Wet soils sticks together||fine||low plasticity dilatant
|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||Cases requiring special consideration should be classified according to national or project requirements|
|Soil||Principal||Secondary or tertiary component|
|Gravel||Gr||gr||Gr(gr) and Sa(sa) can be subdivided into fine F(f), medium M(m) or coarse C(c)|
|any combination of components|
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.
- 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
|Sand||20 to 40
|Silt + clay
|5 to 15
15 to 40
10 to 20
20 to 40
 ISO 22476-1, Geotechnical investigation and testing — Field testing — Part 1: Electrical cone and piezocone penetration tests
 ISO 22476-2, Geotechnical investigation and testing — Field testing — Part 2: Dynamic probing
 ISO 22476-3, Geotechnical investigation and testing — Field testing — Part 3: Standard penetration test
 ISO 22476-4, Geotechnical investigation and testing — Field testing — Part 4: Menard pressuremeter test
 ISO 22476-6, Geotechnical investigation and testing — Field testing — Part 6: Self-boring pressuremeter test
 ISO 22476-8, Geotechnical investigation and testing — Field testing — Part 8: Full displacement pressuremeter test
 EN 1997-2, Eurocode 7: Geotechnical design — Part 2: Design assisted by laboratory testing