ISO 14688-1:2002 Geotechnical investigation and testing - Identification and classification of soil - Part 1: Identification and description

4.1 General

Subclauses 4.2 to 4.10 give soil characteristics that generally permit soil to be identified with adequate accuracy for general (or preliminary) characterization. A more accurate identification and classification based on grading, plasticity or organic content can be achieved by laboratory tests. In addition to identifying soils, the condition in which a soil is encountered, any particular secondary constituents, other features of a soil, such as carbonate content, particle shape, surface roughness of particles, odour, any common names and the geological classification should all be indicated. For the identification and description, methods and additional tests shall be carried out according to clause 5. The identification and description of soils generally follows the flow chart in Figure 1.

Figure 1 — Flow chart for the identification and description of soils

4.2 Particle size

Particle size is the fundamental basis for designating mineral soils using particle fractions to distinguish the soil mechanical behaviour. Table 1 shows the terms to be used for each soil fraction and its sub-fractions, together with the corresponding chosen range of particle sizes.

Basic soils are soils with uniform grading (i.e. they consist of particles of only one size range) as specified in Table 1 (e.g. gravel Gr, fine sand FSa, coarse silt CSi). The first letter of the abbreviation of the soil fraction is a capital.

Table1 — Particle size fractions

4.3 Composite soils

4.3.1 General

Most soils are composite and consist of principal and secondary fractions. They are designated by a noun (main term) describing the principal fraction and by one or more adjectives (qualifying terms) describing the secondary fractions (e.g. sandy gravel saGr, gravelly clay grCl).

Composite soil terms shall be written with small letters.

Interlayered soils can be written in small underlined letters following the principal soil fraction (e.g. gravelly clay interbedded with sand grClsa).

4.3.2 Principal fraction

The principal fraction in terms of mass determines the engineering properties of the soil. It may be given in capital letters for clarity.

In the case of very coarse soils, the principal fraction is the relevant very coarse soil fraction predominating in terms of mass. The very coarse fraction should be separated from the sample before identifying the fine and coarse fractions.

In the case of coarse soils, the principal fraction is the relevant coarse soil fraction predominating in terms of mass. Composite coarse soils include a fines fraction (silt and/or clay) that does not determine the engineering properties of the soil.

NOTE 1 The fines fraction is not regarded as determining the characteristics of a composite soil if the soil exhibits no, or only very low, dry strength in the test described in 5.6 or if, when tested in accordance with 5.8, it exhibits a very low plasticity.

In both cases, the name shall be based on the subfraction into which coarse soil is identified, cf. 4.2 e.g. gravel, sand, medium gravel, fine sand.

In the case of fine soils, the principal fraction is the relevant fine soil type determining the engineering properties of the soil.

In the case of composite fine soils, the fines fraction determines the engineering properties of the soil.

NOTE 2 The fines fraction is regarded as determining the characteristics of a composite soil if this is of at least medium dry strength as determined in the test described in 5.6, or exhibits a certain degree of plasticity as determined in the test described in 5.8.

In both cases, the soil shall be termed either “clay” or “silt”, depending on the plasticity of the fines fraction and not on the grading. The identification shall be based on 5.6, 5.7, 5.8 and 5.9.

NOTE 3 The minimum size of soil sample required for accurate identification increases with maximum particle size.

4.3.3 Secondary fractions

Secondary and further fractions do not determine but will affect the engineering properties of the soil.

The secondary fractions as adjectives shall be placed with the term describing the principal fraction in the order of their relevance, as shown in the following examples:

• sandy gravel (saGr),
• coarse sandy fine gravel (csaFGr),
• medium sandy silt (msaSi),
• fine gravely coarse sand (fgrCSa),
• silty fine sand (siFSa),
• fine gravelly, coarse sandy silt (fgrcsaSi),
• medium sandy clay (msaCl).

If coarse secondary fractions are present in a particularly small or particularly large proportion, the term “slightly” or “very” shall precede the qualifying term.

If it is a fine soil, the properties of which are determined by fines fractions, a soil can be identified as “silt” or “clay” by checking for the presence of fine secondary constituents on the basis of its plasticity properties using the tests described in 5.6, 5.7, 5.8 and 5.9.

If, in the case of coarse soils, two soil fractions are present in approximately equal proportions, an oblique shall be placed between the relevant terms, e.g. gravel/sand (Gr/Sa)or fine/medium sand (FSa/MSa).

4.4 Plasticity

Soils that are subjected to the test described in 5.8 and permit their consistency limits to be determined, may be identified as exhibiting plastic properties.

NOTE Such soils are also referred to as cohesive soils.

Assessment of the plasticity and identification of a soil as either silt or clay shall be carried out by testing as specified in 5.6, 5.7, 5.8 and 5.9. This permits the soil to be qualified as being of

• low plasticity, or
• high plasticity.

An exact determination can only be made by establishing the liquid limit wL and the plastic limit, wP, in laboratory tests.

4.5 Organic content

Small quantities of dispersed organic matter in a soil can produce a distinctive odour (see 5.11) and colours. The intensity of odour and colour indicates the proportion of organic matter and should be described.

4.6 Peat and other organic soils

The designation for soils consisting principally of organic matter are summarized in Table 2. Peats are generally of low density and have a distinctive odour (see 5.11).

Peat is identified and described according to the degree of decomposition, this being determined in the wet state by squeezing (see Table 5), and according to its fibre content.

In the case of organic soils with mineral constituents, these should be described by qualifying terms, e.g. fine sandy peat.

Table 2 — Identification and description of organic soil

4.7 Volcanic soils

Generally, particles of volcanic soils are vesicular and the density of the soils is relatively low. The soils possess a characteristic colour depending on the property of their host magma or rock. Volcanic soil as a principal fraction is identified and described according to the particle size, structure and colour (see Table 3).

Table 3 — Identification and description of volcanic soils

4.8 Discontinuities and bedding

The term discontinuity is used to describe surfaces in the soil which separate soils of different types or form planes of weakness within the soil. Most discontinuities in soil fall into one of two broad groups. These are as follows.

• a) “Depositional” discontinuities, which result from the way in which the soil was deposited or formed, such as bedding planes, which are commonly parallel but may show sedimentary structures such as cross-bedding or graded bedding. Such features may not be mechanical breaks, and shall be described by the thickness of units between the bedding planes.
• b) “Mechanical” discontinuities, which include mechanical breaks in the soil as a consequence of shrinkage, ice unloading or tectonic stress. Fissures, faults and shears are examples of such discontinuities and are most common in over-consolidated soils. Fissures and shears may also be a consequence of previous landsliding in a soil.

Discontinuities may significantly affect the engineering behaviour of the soil and their frequency of occurrence is expressed by noting their spacing; quantification of the range of actual spacings can also be given. If discontinuities occur, they may be described by methods according to ISO 14689.

4.9 Interbedding and mixed soils

Interbedding is a sequence of different soil layers of variable thickness and extent which are summarized for practical reasons (thin laminae, rapid change). The properties of single layers shall be described; also very thin layers shall be considered. The bedding may be mixed by processes (roots, burrows, cryoturbination), so that mixed soils develop e.g. solifluction soils.

4.10 Origin of deposit

The description of the soil should be concluded, where possible, with the geological origin of the soil, usually in brackets. This also indicates some properties and mineral composition before test results become available.

5.1 Determination of particle size distribution

To determine the particle size distribution, the soil sample shall be spread out on a flat surface or on the palm of the hand. The particle sizes of the sample shall be compared with those of a grading standard comprising sections containing material of different particle size ranges according to Table 1.

As the individual particles of silt and clay are not visible to the naked eye, the methods given in 5.4, 5.6, 5.7 and 5.9 shall be used in determining the characteristics of such soil.

5.2 Determination of particle shape

In the case of coarse fractions, the particle shape is described by reference to the angularity of the particles (which indicates the degree of rounding at edges and corners), their general form and their surface characteristics. Terms to be used for these aspects, usually only applied to gravel size or larger, are given in Table 4. It is usual practice to estimate mean angularity or roundness using a standard set of charts.

Table 4 — Terms for the designation of particle shape

5.3 Determination of mineral composition

The mineral composition of the individual particles of a soil is identified according to geological science. The names of the minerals present, together with any coatings, shall be included in the description of the soil. When making a field examination of the coarse fraction, a hand lens is often necessary.

NOTE Gravel particles are usually rock fragments, e.g. sandstone, limestone, flint. Sand and finer particles are generally individual mineral particles; quartz, mica, feldspar and clay minerals. Gravel and sand particles may be coated with mineral matter, including calcite, or iron oxide. Crystals, for example gypsum in clay and pyrite in chalk, may be present.

5.4 Determination of fines content

For identifying composite soils, the fines fraction of a small sample quantity shall be washed out and the coarse residue described on the basis of the particle size and shape, type of material and any special constituents. The duration and thoroughness of the washout process and examination of the product indicate the type and the percentage of fines.

5.5 Determination of soil colours

The colour of a soil, although dependent upon local conditions, will often characterize the composition of the material content and its distribution. Colour facilitates the distinction between mineral and organic soils.

It is important to identify the colour of a freshly cut surface in full daylight because some soils change their colour very quickly in air. An example of this is fine soil containing iron oxide compounds which, in the fresh-water saturated condition, often has an olive green colour but which rapidly oxidizes to red on exposure to air. Colour changes such as those due to oxidation or desiccation should always be recorded.

A colour chart provides a useful aid, particularly to improve the consistency between descriptions by different persons.

5.6 Determination of dry strength

The dry strength provides information on the plasticity of a soil and thus on its behaviour and identification as silt or clay.

To establish the dry strength, a soil sample shall be dried. Its resistance to being crumbled or powdered between the fingers is a measure of the dry strength of the soil, which is governed by the type and percentage of fines. The following strengths are distinguished:

• a) low dry strength: dried soil disintegrates under light to moderate finger pressure;
• b) medium dry strength: dried soil disintegrates only under substantial finger pressure into pieces which still show cohesion
• c) high dry strength: dried soil can no longer be made to disintegrate by finger pressure but can only be broken.

NOTE A low dry strength characterizes silt. A high dry strength characterizes clay, if not caused by cementation. A mixture of clay and silt has a medium dry strength in general.

5.7 Determination of dilatancy

The behaviour of cohesive soils when shaken indicates the content of silt and clay.

A moistened sample of 10 mm to 20 mm size shall be shaken from hand to hand. The sample becomes shiny by the appearance of water on the surface of the sample. When the sample is pressed with the fingers, the water disappears. The content of silt and clay can be estimated according to the time taken for water to appear and disappear during shaking and pressing.

NOTE The water appears and disappears rapidly for silt. Shaking and pressing has no effect on the sample condition of clay. The slower the appearance of water on the sample surface, the lower the content of silt and the higher the content of clay.

5.8 Determination of plasticity

To establish the plasticity (toughness), a moistened soil sample shall be rolled on a smooth surface to produce threads about 3 mm in diameter moulded together, and rolled again until it cannot be rolled out any longer because of loss of water, but only moulded. The plastic limit has been reached.

• a) Low plasticity: a sample having cohesion but cannot be rolled to threads of about 3 mm in diameter.
• b) High plasticity: the sample can be rolled to thin threads.

NOTE A low plasticity characterizes a high content of silt, while a high plasticity corresponds to a high clay content.

5.9 Determination of sand, silt and clay content of soils

To check a soil for the presence of sand, silt and clay, a small soil sample shall be rubbed between the fingers, if necessary under water. The proportion of the sand fraction can be determined from the degree to which the material feels gritty. Coarse silt may also feel gritty but the individual particles cannot be seen with the naked eye.

NOTE A clayey soil feels soapy and sticks to the fingers and cannot be removed without washing, even when in the dry state. Silty soils feel smooth to the touch, the dry soil particles that stick to the fingers can be easily blown away or removed by clapping the hands.

To check a soil for the presence of clay or silt, a sample in its naturally moist state shall be cut with a knife. A shiny cut surface indicates the presence of clay, while a dull sample surface is characteristic of silt or of clayey sandy silt of low plasticity. For a rapid assessment, the sample surface may be scored with a fingernail or smoothed.

5.10 Determination of carbonate content

The carbonate content is determined by the application of droplets of dilute hydrochloric acid (HCl) (3:1 or 10 %). The following characteristics could be distinguished:

• a) carbonate-free (O) if the addition of HCl produces no effervescence;
• b) calcareous (+) if the addition of HCl produces clear, but not sustained, effervescence;
• c) highly calcareous (++) if the addition of HCl produces strong and sustained effervescence.

It should be noted that, in wet or moist clayey soils, the effervescence usually occurs with some delay.

NOTE A high dry strength is often the result of carbonate acting as a cementing agent.

WARNING — Hydrochloric acid can produce poisonous gases when applied to chemically contaminated soils.

5.11 Methods for identification and description of organic soil

The odour of a soil gives an indication of whether it is of inorganic or organic nature. Fresh, moist organic soils generally have a mouldy odour which can be intensified by heating a moist sample. Putrefying, rotten organic components in soil can be recognized by their odour typical of hydrogen sulfide, which can be intensified by pouring dilute hydrochloric acid on the sample. Dry inorganic clays have an earthy odour after being moistened.

5.12 Determination of the degree of decomposition of peat

The degree of decomposition of peat can be established by squeezing a wet sample in the hand (see Table 5). If squeezing is not effective because the peat is too dry, the peat shall be assessed on the basis of its appearance, substantial fractions of well-maintained plant remains being recognizable in undecomposed to moderately decomposed peat and no plant remains in highly or completely decomposed peat.

Table 5 — Degree of decomposition of wet peat as determined by squeezing

5.13 Methods of identification and description of volcanic soils

Soil located in the distribution area of volcanic soil can be identified as volcanic by the existence of pumice and scoria. Another method is to measure the volume of volcanic glass obtained by washing out the soils. If a more accurate identification is requested, it will be necessary to analyse the physical and chemical properties of the mineral constituents of the soil.

5.14 Determination of consistency

The consistency of a cohesive soil shall be determined in a manual test, permitting the following identification and description:

• a) a soil is to be identified as very soft if it exudes between the fingers when squeezed in the hand;
• b) a soil is to be identified as soft if it can be moulded by light finger pressure;
• c) a soil is to be identified as firm if it cannot be moulded by the fingers, but rolled in the hand to 3 mm thick threads without breaking or crumbling;
• d) a soil is to be identified as stiff if it crumbles and breaks when rolled to 3 mm thick threads but is still sufficiently moist to be moulded to a lump again;
• e) a soil is to be identified as very stiff if it 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.

These subdivisions may be approximate, particularly in materials of low plasticity.