33 Description of soils

33.1 The scope of soil description

The soil should be described in terms of material and mass characteristics, that is in terms of nature, state and structure.

• Nature of the soil grains: their particle size grading, shape and texture or, if appropriate, their plasticity, together with special features such as organic or carbonate content. The nature of a soil does not usually change during civil engineering works. Nature of the soil can be described on disturbed samples.
• State of the soil grains: their packing, water content, degree of saturation, strength or relative density, and stiffness. The state of a soil usually changes during civil engineering works; the description of the state of the soil requires undisturbed samples or exposures.
• Structure: all the features of a soil that are removed by reconstitution, i.e. fabric or microfabric features, such as bedding, discontinuities or cementing. Structure is often destroyed by large distortions, and so can be observed only in the field on natural or artificial exposures or, to some extent, in an undisturbed (Quality Class 1 or 2) sample.

Inorganic soils should be described as very coarse, coarse or fine. Table 7 shows the recommendations which are developed in more detail in the text.

NOTE 1 Low density soils, as shown in BS EN ISO 14688-1:2002, Figure 1 can also include other inorganic soils such as loess and some chemically weathered residual soils.

NOTE 2 Very coarse soils are boulders and cobbles whereas coarse soils are gravels and sands. When saturated and unconfined, they cannot sustain negative pore pressures, so these soils do not have any undrained strength or apparent cohesion. Fine soils are clays and silts. When saturated they can sustain suctions in unconfined tests and so have an apparent cohesion. If well-graded soils contain a sufficient proportion of fine grains to fill the spaces between the coarse grains, they are described as fine soils; if well-graded soils contain insufficient fine grains to fill the spaces between the coarse grains, they are described as coarse soils. These distinctions based on grain size apply equally to natural soils and man placed and made materials.

NOTE 3 The terms coarse and fine are often loosely referred to as granular and cohesive. Use of the latter terms in this context is discouraged.

Any description should acknowledge the quality of the sample by not over-describing the nature, state or structure, e.g. the state of a soil may not be described on a sample in which the water content is not representative.

The geological unit should be named where known as identified in the desk study, but might need updating as required by the ground investigation. Geological unit names should be used consistently throughout the investigation.

33.2 The basis of soil description

COMMENTARY ON 33.2

A soil's characteristics are based on the particle size grading of the coarser particles and the plasticity of the finer particles; these play a major role in determining the engineering properties of the soil and form the basis of the soil's description.

A first appraisal of the physical properties relevant in the engineering context should be made from the visual description of the soil's nature and composition, assisted by a few simple hand tests (see Clause 34). Soils that stick together when wet and can be rolled into a thread that supports the soil's own weight (i.e. they have cohesion and plasticity) are matrix supported and should be described as fine soils (see BS EN ISO 14688-1:2002, Figure 1). Soils that do not exhibit these properties are clast supported and should be described as coarse soils.

NOTE 1 The boundary between fine and coarse soils is on the basis of behaviour, not weight percentage.

The principal soil type should first be determined as defined in Table 7, followed by description of the secondary and tertiary fractions and other features such as bedding, colour and particle shape.

NOTE 2 The descriptive process is summarized in Figure 6 (see BS EN ISO 14688-1:2002, Figure 1).

In a soil description, the main characteristics should be given using the following standard word sequence, as applicable (following the order of the rows in Table 7). The main description may be followed, where appropriate, by further details for clarity.

• a) mass characteristics comprising state and structure (see 33.3):
  • 1) relative density/consistency;
  • 2) discontinuities;
  • 3) bedding;
• b) material characteristics comprising nature and state (see 33.4);
  • 1) colour;
  • 2) composite soil types: particle grading and composition; shape and size;
  • 3) tertiary constituents either before or after the principal soil type as appropriate;
  • 4) principal soil type (name in capitals, e.g. SAND), based on grading and plasticity shape;
• c) stratum name: geological formation, age and type of deposit (see 33.5); classification (optional).

EXAMPLES:

• Firm closely fissured yellowish brown CLAY (LONDON CLAY FORMATION).
• Medium dense light greyish brown sandy slightly clayey subrounded fine to coarse GRAVEL of various lithologies with low cobble content. Cobbles are subrounded of strong sandstone (RIVER TERRACE DEPOSIT).
• Greenish brown gravelly fine to coarse slightly glauconitic SAND. Gravel is rounded fine and medium of black flint (BLACKHEATH MEMBER).
• Firm to stiff brown slightly sandy slightly gravelly CLAY with occasional lenses (5 mm by 15 mm to 15 mm by 50 mm) of yellow silty sand. Gravel is subangular to subrounded fine and medium of various lithologies (GLACIAL TILL).

Materials in interstratified beds may be described as follows:

• Thinly interbedded yellow fine SAND and soft grey CLAY (ALLUVIUM).

Any additional information on the secondary or tertiary constituents should be placed at the end of the main description after a full stop, to keep the standard main description concise and unambiguous.