33.4.4.2 Deposits containing boulder-size and cobble-size particles

When the "sample" is considered representative, such as might be the case in very large bulk samples from excavations or in excavated or exposed faces, then these deposits should be described as shown in Table 12.

The term large boulder does not have an upper size limit, so dimensions should be given wherever available.

Mixtures of very coarse soil particles should be described as in Table 13.

Very coarse soils with secondary finer material (coarse and fine soil) should be described as in Table 14.

NOTE The description of "finer material" is made in accordance with 33.4.2 to 33.4.6, ignoring the very coarse fraction; the principal soil type name of the finer material can also be given in capital letters, e.g. COBBLES with some sandy CLAY.

Finer material (coarse and fine soil) with a secondary very coarse fraction should be described as in Table 15.

Percentages are approximate visual estimates in a field description and should only be taken as a subjective guide.

NOTE Representative sampling of soil mixtures containing very coarse soils is not possible in normal boreholes, and is very difficult even in conventional trial pits; a representative sample of a soil including boulders would need to weigh more than a tonne (see BS 1377-2); the need for a representative size of sample applies to soil description in the field as much as to laboratory testing. Cobbles or boulders are often noted only in passing on the driller's records, but frequently have a much greater significance to the engineering works (particularly piling or excavation).

The location of individual cobbles and boulders should be noted on the log, even when it is considered appropriate to include the very coarse soil as part of the main description. Where possible, the characteristics of such cobbles and boulders should be described using the terms in Clause 36 and their size in mm can also usefully be reported.

33.4.4.3 Deposits containing gravel-size and sand-size particles

COMMENTARY ON 33.4.4.3

As noted in 33.4.4.1, a coarse soil (omitting any boulders or cobbles) does not stick together and is described as a "SAND" or "GRAVEL", depending on which of the constituents predominates by weight.

The terms in Table 16 should be used to describe the composition of the coarse fraction; percentages are by weight of the whole material less boulders and cobbles, and are approximate estimates in a field description.

The appropriate adjectives should be used before the principal soil type. Further details should be provided at the end of the main description, after a full stop for clarity, e.g. "Medium dense brown very gravelly coarse SAND. Gravel is subangular fine and medium of sandstone and mudstone".

33.4.4.4 Deposits containing silt-size and clay-size particles

COMMENTARY ON 33.4.4.4

Most fine soils are mixtures of clay- and silt-size particles; these can include silt-size aggregates of clay minerals and clay-size particles such as quartz. Soils formed solely of coarse silt are not common in nature, although more common as man-made deposits (e.g. tailings, washings and discards).

Fine soil should be described as either a "SILT" or a "CLAY" depending on the plastic properties.

The field distinction between SILT and CLAY should be made using the hand tests in 34.3. The use of secondary fine descriptors in a fine soil is permitted for materials that show behaviour that is borderline between those showing clay-like and silt-like behaviour, hence "silty CLAY" or "clayey SILT", but this should be applied only where the secondary fraction is important, as these terms are qualitative only. Further subdivision such as into slightly clayey or very silty is not appropriate and should not be used.

NOTE 1 The description of plasticity can be carried out using terms provided in BS EN ISO 14688-1 (low or high plasticity). The definitions provided are based on the plastic limit test, and are thus different from the classification based on the plasticity chart which uses liquid limit (BS EN ISO 14688-2:2004, 4.4). Care and clarity is needed if the descriptive terms are used.

Soils formed solely of coarse silt might not demonstrate plasticity, but should still be described as silt rather than fine sand, if the grains cannot be seen with the naked eye.

NOTE 2 The distinction between clay and silt is often taken to be the A line on the plasticity chart, with clays plotting above and silts below (see Figure 8) and might not agree with field tests (see Child, 1986 [48] and Smart, 1986 [49]). The effects of clay mineralogy and organic content are significant.

33.4.4.5 Deposits containing mixtures of fine soil and coarse soil

The terms in Table 17 should be used to describe those common soils that include a range of soil fractions. The appropriate quantifying terms should be used before the principal soil type. The word order for secondary fractions before the principal soil term should be increasing proportion when there are two coarse soil secondary constituents, or coarse and then fine if one of each. If any of the secondary constituent sizes need qualifying adjectives, these should be added in separate sentences after the main description; this commonly applies to the gravel or, less frequently, the sand fractions. Additional detail should be added in these sentences as appropriate, e.g. "Gravelly very clayey fine to medium SAND with low cobble content. Gravel and cobbles are rounded fine and medium of quartzite". If multiple sentences are used, the proportion of all constituents should be provided in the first sentence.

NOTE 1 If the secondary fraction is fine, the terms "silt" and "clay" are mutually exclusive and which of the terms silty or clayey is used is based solely on the plastic properties of the fine fraction (see 33.4.4.4), e.g. "gravelly clayey fine SAND". On the other hand, the terms "sandy" and "gravelly" may both be used, in which case the percentages are assessed separately, e.g. "slightly gravelly slightly sandy CLAY.' means that the soil contains up to 35% sand and up to 35% gravel. Soils that exhibit cohesion but have a high proportion of coarse particles with the fine soil matrix, such as many glacial deposits, are difficult to describe accurately.

NOTE 2 Descriptions with cumulative proportions of the various fractions, excluding cobbles and boulders, exceeding 100% are incorrect. However, the sum of the soil fractions in descriptions of soils composed of very coarse particles and finer material could apparently exceed 100%. This is because the description of the finer material and the very coarse soils are apportioned separately.