33.4 Material characteristics of soils

COMMENTARY ON 33.4

Material characteristics refer to those characteristics that can be described from visual and manual examination of either disturbed or undisturbed samples, and include soil name, colour, particle shape, particle grading and particle composition.

33.4.1 Range of application

Material characteristics of soils should be described in accordance with BS EN ISO 14688-1.

33.4.2 Colour

The colour should be described using the terms given in Table 7. The colour description should be the overall impression of the damp soil. Strata with more than one distinct colour may be described, for example, as mottled, but strata with more than three distinct colours should normally be described as multicoloured. Where a soil is multicoloured, the colour assemblage should be recorded where this is important to identifying the geological unit to which the soil belongs, e.g. within the Lambeth Group. Colour changes due to oxidation or desiccation, for example, should be noted.

For a large majority of descriptions, a simple approach to colour description should be used (see Figure 7.3 of Norbury, 2010 [42]). For more detailed descriptions, colour charts such as those based on the Munsell colour system may be used (see Munsell soil color charts [46] and the Geological Society of America's Rock color chart [47]).

NOTE Consistency of colour description is usually more important than absolute accuracy, and the use of reference samples or colour charts is useful in this regard.

33.4.3 Particle shape, grading and composition

Where appropriate, particle shape should be described by reference to the general form of the particles, their angularity (which indicates the degree of rounding at edges and corners) and their surface characteristics. The angularity and form terms are illustrated in Figure 7.

The following terms should be used where appropriate.

• a) Angularity:
  • very angular;
  • angular;
  • subangular;
  • subrounded;
  • rounded;
  • well rounded;
• b) Form:
  • cubic;
  • flat (or tabular);
  • elongate;
• c) Surface texture:
  • rough;
  • smooth.

Angularity terms should be applied to particles of gravel size or larger; form may be described for extreme particle shapes. Any layering or preferred orientation of the particles should be recorded.

NOTE 1 Occasionally the use of these terms might be of greater importance and used more fully, such as in an aggregate assessment study. "Flat" is preferred to the term "flaky", used in BS EN 933-3, where the shapes are illustrated. The surfaces of particles might be described, for example, as etched, pitted, honeycombed or polished.

The distribution of particle sizes within sands and gravels should be described using the terms in Table 7, stating the predominant size fractions present, e.g. "fine and medium GRAVEL" or "fine to coarse SAND".

NOTE 2 The absence of these adjectives means that fine, medium and coarse fractions are all present in roughly equal proportions; use of the conjunctions "and" or "to" allow differentiation between predominant fractions and a range of sizes.

NOTE 3 The composition of particles visible to the naked eye or with a hand lens may be described. Gravel particles are usually rock fragments, e.g. sandstone, limestone, flint. Sand and finer particles are normally individual mineral grains, e.g. quartz, mica, feldspar.

NOTE 4 The composition (mineralogy) of sand is to be reported, such as quartz or shells.

NOTE 5 Gravel and sand particles might be coated with mineral matter, including calcite, limonite and other iron oxides as well as black coating of sulfidic minerals. Crystals, for example gypsum in clay, might be present.

If particles are weathered, showing, for instance, cracking, concentric layering or discoloration, these conditions should be described.

a) High sphericity — very angular	b) Low sphericity — very angular
c) High sphericity — angular	d) Low sphericity — angular
e) High sphericity — sub-angular	f) Low sphericity — sub-angular
g) High sphericity — sub-rounded	h) Low sphericity — sub-rounded
i) High sphericity — rounded	j) Low sphericity — rounded
k) High sphericity — well rounded	I) Low sphericity — well rounded

33.4.4 Soil name (Principal soil type and secondary constituents)

33.4.4.1 General

COMMENTARY ON 33.4.4.1

The soil name is based on the particle size grading for coarse soils and the plasticity for fine soils. These characteristics are used because they can be measured readily with reasonable precision, and estimated with sufficient accuracy for descriptive purposes. They give a general indication of the probable engineering characteristics of the soil at any particular water content. Figure 6 is a key to the naming and description of soils by hand and eye. Where a soil (omitting any boulders or cobbles) "sticks together when wet and remoulds" it is described as a fine soil ("CLAY" or "SILT" dependent on its plasticity). When it does not stick together and remould, it is described as a coarse soil ("SAND" or "GRAVEL" dependent on its particle size grading). The description of soil containing boulders or cobbles is discussed in 33.4.4.2.

The principal soil type which dominates the engineering behaviour and the secondary constituents which modify that behaviour should be described. The principal soil name should be based on particle size distribution of the coarse fraction and/or the plasticity of the fine fraction as determined by the Atterberg Limits (see Section 8).

The basic soil types and their sub-divisions should be described on the basis of the range of their particle sizes and plasticity as shown in Table 7.

Stratum descriptions presented on exploratory hole and other logs should be reviewed against available laboratory test results, e.g. Atterberg Limits and particle size distributions. Where discrepancies arise due to "miscalibration" of the logger then adjustments should be made; differences due to local material variation or test results for non-representative samples require separate consideration and possible comment on the log or in the report. Consistency descriptions of fine soils should be reviewed against comparable strength results, but generally would not be amended.

NOTE The naming of soils falling entirely within sands, gravels, cobbles or boulders is straightforward, as the particles are visible to the naked eye (see 35.2). A common difficulty arises in that the proportions of each soil fraction are percentages by weight, requiring adjustment from the percentage by volume seen by the eye. The naming of soils falling entirely within either clays or silts is less straightforward, relying on simple hand tests (see 35.3). Most natural soils are composed of more than one soil type (see 33.4.4.2 to 33.4.4.5). In composite soil types, cobbles and boulders are treated separately and are discounted in assessing proportions of the other components.